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Encyclopedia Britannica - Main :: TAV-THE |
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TECHNIQUE OF PHOTOGRAPHY Gelatin Emulsions. The following is an outline of two representative processes. All operations should be conducted in light which can act but very slightly on the sensitive salts employed, and this is more necessary with this process than with others on account of the extreme ease with which the equilibrium of the molecules is upset in giving rise to the molecule which is developable. The light to work with is gaslight or candlelight passing through a sheet of Chance's stained red glass backed by orange paper. Stained red glass allows but few chemically effective rays to pass through it, whilst the orange paper diffuses the light. If daylight be employed, it is as well to have a double thickness of orange paper. The following should be weighed out: i. Potassium iodide 5 grs. 2. Potassium bromide 135 ,. 3. Nelson's No. i photographic gelatin . 30 4. Silver nitrate 175 ,, Autotype or other hard gelatin . . ioo 5' Nelson's No. I gelatin . . . . loo Nos. 3 and 5 are rapidly covered with water or washed for a few seconds under the tap to get rid of any dust. No. 2 is dissolved in 11 oz. of water, and a little tincture of iodine added till it assumes a light sherry colour. No. I is dissolved in 6o minims of water. No. 4 is dissolved in z oz. of water, and No. 3 is allowed to swell up in i oz. of water, and is then dissolved by heat. All the flasks containing these solutions are placed in water at 150 F. and carried into the " dark room, as the orange-lighted chamber is ordinarily called; Nos. 3 and 4 are then mixed together in a jar or flask, and No. 2 added drop by drop till half its bulk is gone, when No. i is added to the remainder, and the double solution is dropped in as before. When all is added there ought to be formed an emulsion which is very ruddy when examined by gaslight, or orange by daylight. The flask containing the emulsion is next placed in boiling water, which is kept in a state of ebullition for about three-quarters of an hour. It is then ready, when the contents of the flask have cooled down to about Too F., for the addition of No. 5, which should in the interval be placed in 2 oz. of water to swell and finally be dissolved. The gelatin emulsion thus formed is placed in a cool place to set, after which it is turned into a piece of coarse canvas or mosquito netting made into a bag. By squeezing, threads of gelatin containing the sensitive salt can be made to fall into cold water; by this means the soluble salts are extracted. This is readily done in two or three hours by frequently changing the water, or by allowing running water to flow over the emulsion-threads. The gelatin is next drained by straining canvas over a jar and turning out the threads on to it, after which it is placed in a flask, and warmed till it dissolves; half an ounce of alcohol being added. Finally it is filtered through chamois leather or swansdown calico. In this state it is ready for the plates. The other method of forming the emulsion is with ammonia. The same quantities as before are weighed out, but the solutions of Nos. 2 and 3 are first mixed together and No. 4 is dissolved in 1 oz. of water, and strong ammonia of specific gravity 88o added to it till the oxide first precipitated is just redissolved. This solution is then dropped into Nos. 2 and 3 as previously described, and finally No. T is added. In this case no boiling is required; but to secure rapidity it is as well that the emulsion should be kept an hour at a temperature of about 90 F., after which half the total, quantity of No. 5 is added. When set the emulsion is washed, drained, and redissolved as before; but in order to give tenacity to the gelatin the remainder of No. 5 is added before the addition of the alcohol, and before filtering. Coating the Plates.Glass plates are best cleaned with nitric acid, rinsed, and then treated with potash solution, rinsed again, and dried with a clean cloth. They are then ready for receiving the emulsion, which, after being warmed to about 12o F., is poured on them to cover well the surface. This being done, the plates are placed on a level shelf and allowed to stay there till the gelatin is thoroughly set; they are then put in a drying cupboard, through which a current of warm air is made to pass. It should be remarked that the warmth is only necessary to enable the air to take up the moisture from the plates. They ought to dry in about twelve hours, and they are ready for use. Exposure.With a good emulsion and on a bright day the exposure of a plate to a landscape, with a lens whose aperture is one-sixteenth that of the focal distance, should not be more than one-half to one-fifth of a second. This time depends, of course, on the nature of the view; if there be foliage in the immediate foreground it will be longer. In the portrait-studio, under the same circumstances, an exposure with a portrait lens may be from half a second to four or five seconds. Development of the Plate.To develop the image either a ferrous oxalate solution or alkaline pyrogallic acid may be used. No chemical restrainer such as potassium bromide is necessary, since the gelatin itself acts as a physical restrainer. If the alkaline developer be used, the following may be taken as a good standard : (Pyrogallol 50 grs. 1. j( Citric acid 10 Water . . t oz. 2 Potassium bromide to grs. Water I oz. 3 Ammonia, 88o I dr. Water 9 One dram of each of these is taken and the mixture made up to 2 OZ. with water. The plate is placed in a dish and the above poured over it without stoppage, whereupon the image gradually appears and, if the exposure has been properly timed, gains sufficient density for printing purposes. It is fixed in a solution of hyposulphite of soda, as in the other processes already described, and then thoroughly washed for two or three hours to eliminate all the soluble salt. This long washing is necessary on account of the nature of the gelatin. Intensifying the Negative.Sometimes it is necessary to intensify the negative, which can be done in a variety of ways with mercury salts. An excellent plan, introduced by Chapman Jones, is to use a saturated solution of mercuric chloride in water. After thorough washing the negative is treated with ferrous oxalate. This process can be repeated till sufficient density is attained. With most other methods with mercury the image is apt to become yellow and to fade; with this apparently it is not. Varnishing the Negative.The negative is often protected by receiving first a film of plain collodion and then a coat of shellac or other photographic varnish. This protects the gelatin from moisture and also from becoming stained with the silver nitrate owing to contact with the sensitive paper used in silver printing. Another varnish is a solution of celloidin in amyl acetate. This is an excellent protection against damp. Printing Processes. The first printing process may be said to be that of Fox Talbot (see above), which has continued to be generally employed (with the addition of albumen to give a surface to the printan addition first made, we believe, by Fox Talbot). Paper for printing is prepared by mixing 150 parts of ammonium chloride with 240 parts of spirits of wine and 2000 parts of water, though the proportions may vary. These ingredients are dissolved, and the whites of fifteen fairly-sized eggs are added and the whole beaten up to a froth. In hot weather it is advisable to add a drop of carbolic acid to prevent decomposition. The albumen is allowed two or three days to settle, when it is filtered through a sponge placed in a funnel, or through two or three thicknesses of fine muslin, and transferred to a flat dish. The paper is cut of convenient size and allowed to float on the solution for about a minute, when it is taken off and dried in a warm room. For dead prints, on which colouring is to take place, plain salted paper is useful. It can be made of the following proportionsgo parts of ammonium chloride, too parts of sodium citrate, to parts of gelatin, 5000 parts of distilled water. The gelatin is first dissolved in hot water and the remaining components are added. It is next filtered, and the paper allowed to float on it for three minutes, then withdrawn and dried. Sensitizing Bath.To sensitize the paper it is floated on a 10% solution of silver nitrate for three minutes. It is then hung up and allowed to dry, after which it is ready for use. To print the image the paper is placed in a printing frame over a negative and exposed to light. It is allowed to print till such time as the image appears rather darker than it should finally appear. Toning and Fixing the Print.The next operation is to tone and fix the print. In the earlier days this was accomplished by means of a bath of sel d'ora mixture of hyposulphite of soda and gold chloride. This gilded the darkened parts of the print which light had reduced to the semi-metallic state: and on the removal of the chloride by means of hyposulphite an image composed of metallic silver, an organic salt of silver and gold was left behind. There was a suspicion, however, that part of the coloration was due to a combination of sulphur with the silver, not that pure silver sulphide is in any degree fugitive, but the sulphuretted organic salt of silver seems to be liable to change. This gave place to a method of alkaline toning, or rather, we should say, of neutral toning, by employing gold chloride with a salt, such as the carbonate or acetate of soda, chloride of lime, borax, &c. By this means there was no danger of sulphurization during the toning, to which the method by sel d'or was prone owing to the decomposition of the hyposulphite. The substances which can be employed in toning seem to be those in which an alkaline base is combined with a weak acid, the latter being readily displaced by a stronger acid, such as nitric acid, which must exist in the paper after printing. This branch of photography owes much to the Rev. T. F. Hardwich, he having carried on extensive researches in connexion with it during 1854 and subsequent years. A. Davanne and A. Girard, a little later, also investigated the matter with fruitful results. The following may be taken as two typical toning-baths:Gold chloride 1 part. Sodium carbonate to parts. Water 5000 (a) Borax loo } Water 4000 (~) S Gold chloride 1 part. l j Water 4000 parts In the latter (a) and (0) are mixed in equal parts immediately before use. Each of these is better used only once. A third bath is: Gold chloride 2 parts. Chloride of lime 2 Chalk 40 Water 8000 , These are mixed together, the water being warmed. When cool the solution is ready for use. In toning prints there is a distinct difference in the modus operandi according to the toning-bath employed. Thus in the first two baths the print must be thoroughly washed in water to remove all free silver nitrate, that salt forming no part in the chemical reactions. On the other hand, where free chlorine is used, the presence of free silver nitrate or some active chlorine absorbent is a necessity. In 1872 Abney showed that with such a toning-bath free. silver nitrate might be eliminated, and if the print were immersed in a solution of a salt such as lead nitrate the toning action proceeded rapidly and without causing any fading of the image whilst toning, which was not the case when the free silver nitrate was totally removed and no other chlorine absorbent substituted. This was an important factor, and one which had been overlooked. In the third bath the free silver nitrate should only be partially removed by washing. The print, having been partially washed or thoroughly washed, as the case may be, is immersed in the toning-bath till the image attains a purple or bluish tone, after which it is ready for fixing. The solution used for this purpose is a 20% solution of hyposulphite of soda, to which it is best to add a dew drops of ammonia in order to render it alkaline. About ten minutes suffice to effect the conversion of the chloride into hyposulphite of silver, which is soluble in hyposulphite of soda and can be removed by washing. The organic salts of silver seem, however, to form a different salt, which is partially insoluble, but which the ammonia helps to remove. If it is not removed there is a sulphur compound left behind, according to J. Spitler, which by time and exposure becomes yellow. The use of potassium cyanide for fixing prints is to be avoided, as this reagent attacks the organic coloured oxide which, if removed, would render the print a ghost. The washing of silver prints should be very complete, since it is said that the least trace of hyposulphite left behind renders the fading of the image a mere matter of time. The stability of a print has been supposed to be increased by immersing it, after washing, in a solution of alum. The alum, like any .acid body, decomposes the hyposulphite into sulphur and sulphurous acid. If this be the case, it seems probable that the destruction of the hyposulphite by time is not the occasion of fading, but that its hygroscopic character is. This, however, is a moot point. It is usual to wash the prints some hours in running water. We have found that half a dozen changes of water, and between successive changes the application of a sponge to the back of each print separately, are equally or more efficacious. On drying the print assumes a darker tone than it has after leaving the fixing bath. Different tones can thus be given to a print by different toning-baths; and the gold itself may be deposited in a ruddy form or in a blue form. The former molecular condition gives the red and sepia tones, and the latter the blue and black tones. The degree of minute subdivision of the gold may be conceived when it is stated that, on a couple of sheets of albuminized paper fully printed, the gold necessary to give a decided tone does not exceed half a grain. Collodio-chloride Silver Printing Process.In the history of the emulsion processes we stated that Gaudin attempted to use silver chloride suspended in collodion, but it was not till the year 1864 that any practical use was made of the suggestion so far as silver printing is concerned. In the autumn of that year George Wharton Simpson worked out a method which has been more or less successfully employed. The formula appended is Simpson's: Silver nitrate 60 parts. I Distilled water 6o Strontium chloride 64 Alcohol woo Citric acid 64 Alcohol t000 To every moo parts of plain collodion 30 parts of No. i, previously mixed with 6o parts of alcohol, are added; 6o parts of No. 2 are next mixed with the collodion, and finally 30 parts of No. 3. This forms an emulsion of silver chloride and also contains citric acid and silver nitrate. The defect of this emulsion is that it contains a large proportion of soluble salts, which are apt to crystallize out on drying, more particularly if it be applied to glass plates. The addition of the citric acid and the excess of silver nitrate is the key to the whole process; for, unless some body were present which on exposure to light was capable of forming a highly-coloured organic oxide of silver, no vigour would be obtained in printing. If pure chloride be used, though an apparently strong image would be obtained, yet on fixing only a feeble trace of it would be left, and the print would be worthless. The collodio-chloride emulsion may be applied to glass, or to paper, and the printing carried on in the usual manner. The toning takes place by means of the chloride of lime or by ammonium sulphocyanide and gold, which is practically a return to the sel d'or bath. The organic salt formed in this procedure does not seem so prone to be decomposed by keeping as does that formed by albumen, and the washing can be more completely carried out. There are in the market several papers which are collodio-chloride. Gelatino-citro-chloride Emulsion.A modified emulsion printing process was introduced by Abney in 1881, which consisted in sus-pending silver chloride and silver citrate in gelatin, there being no excess of silver present. The formula of producing it is as follows: Sodium chloride 40 parts. 1. Potassium citrate 40 Water 500 2 Silver nitrate 150 Water 500 Gelatin 300 Water 1700 Nos. 2 and 3 are mixed together whilst warm, and No. t is then gently added, the gelatin solution being kept in brisk agitation. This produces the emulsion of citrate and chloride of silver. The gelatin containing the suspended salts is heated for five minutes at boiling point, when it is allowed to cool and subsequently slightly washed, as in the gelatino-bromide emulsion. It is then ready for application to paper or glass. The prints are of a beautiful colour, and seem to be fairly permanent. They may be readily toned by the borax or by the chloride of lime toning-bath, and are fixed with the hyposulphite solution of the strength before given. Most, if not all, of the gelatin papers now extant are made somewhat after this manner. Printing with Salts of Uranium.The sensitiveness of the salts of uranium to light seems to have been discovered by Niepce, and was subsequently applied to photography by J. E. Burnett in England. One of the original formulae consisted of 20 parts of uranic nitrate with 60o parts of water. Paper, which is better if slightly sized previously with gelatin, is floated on this solution. When dry it is exposed beneath a negative, and a very faint image is produced; but it can be developed into a strong one by 6 to to % solution of silver nitrate to which a trace of acetic acid has been added, or by a 2 % solution of gold chloride. In both these cases the silver and gold are deposited in the metallic state. Another developer is a 2% solution of potassium ferrocyanide to which a trace of nitric acid has been added, sufficient to give a red coloration. The development takes place most readily by letting the paper float on these solutions. Self-toning Papers.There are several self-toning papers based on the chloride emulsion process. These contain the necessary amount of gold to tone the print. The print is produced in the ordinary way and then immersed in salt and water or in some cases potassium sulphocyanide. The print is finished by immersing in weak hyposulphite of soda. Printing with Chromates: Carbon Prints.The first mention of the use of potassium bichromate for printing purposes seems to have been made by Mungo Ponton in May 1839, when he stated that paper, if saturated with this salt and dried, and then exposed to the sun's rays through a drawing, would produce a yellow picture on an orange ground, nothing more being required to fix it thanwashing it in water, when a white picture on an orange ground was obtained. In 184o Edmond Becquerel announced that paper sized with iodide of starch
Pouncey published his process on the 1st of January 1859; but, as described by him, it was by no means in a perfect state, half-tones being wanting. The cause of this was first pointed out by Abbe Laborde in 1858, whilst describing a kindred process in a note to the French Photographic Society. He says, " In the sensitive film, however thin it may be, two distinct surfaces must be recognizedan outer, and an inner which is in contact with the paper. The action of light commences on the outer surface; in the washing, therefore, the half-tones lose their hold on the paper and are washed away. J. C. Burnett in 1858 was the first to endeavour to get rid of this defect in carbon printing. In a paper to the Photographic Society of London he says, " There are two essential requisites . (2) that in printing the paper should have its unprepared side (and not its prepared side, as in ordinary printing) placed in contact with the negative in the pressure-frame, as it is only by printing in this way that we can expect to be able after-wards to remove by washing the unacted-upon portions of the mixture. In a positive of this sort printed from the front or pre-pared side the attainment of half-tones by washing away more or less depth of the mixture, according to the depth to which it has been hardened, is prevented by the insoluble parts being on the surface and in consequence protecting the soluble part from the action of the water used in washing; so that either nothing is removed, or by steeping very long till the inner soluble part is sufficiently softened the whole depth comes bodily away, leaving the paper white. This method of exposing through the back of the paper was crude and unsatisfactory, and in 186o Fargier patented a process in which, after exposure to light of the gelatin film which contained pigment, the surface was coated with collodion, and the print placed in warm water, where it separated from the paper support and could be transferred to glass. Poitevin success-fully opposed this patent, for he had used this means of detaching the films in his powder-carbon process, in which ferric chloride and tartaric acid were used. Fargier at any rate gave an impetus to carbon-printing, and J. W. Swan took up the matter, and in 1864 secured a patent. One of the great features in Swan's innovations was the production of what is now known as " carbon-tissue," made by coating paper with a mixture of gelatin, sugar and colouring matter, and rendered sensitive to light by means of potassium or ammonium bichromate. After exposure to light Swan placed the printed carbon-tissue on an india-rubber surface, to which it was made to adhere by pressure. The print was immersed in hot water, the paper backing stripped off, and the soluble gelatin containing colouring matter washed away. The picture could then be retransferred to its final support of paper. In 1869 J. R. Johnson of London took out a patent in which he claimed that carbon-tissue which had been soaked in water for a short period, by its tendency to swell further, would adhere to any waterproof surface such as glass, metal, waxed paper, &c., without any adhesive material being applied. This was a most important improvement. Johnson also applied soap to the gelatin to prevent its excessive brittleness on drying, and made its final support of gelatinized paper, rendered insoluble by chrome alum. In 1874 J. R. Sawyer patented a flexible support for developing on; this was a sized paper coated with gelatin and treated with an ammoniacal solution of shellac in borax, on which wax or resin was rubbed. The advantage of this flexible support is that the dark parts of the picture have no tendency to contract from the lighter parts, which they were apt to do when a metal plate was used, as was the case in Johnson's original process. With this patent, and minor improvements made since, carbon-printing has arrived at its present state of perfection. According to P. E. Liesegang, the carbon-tissue when prepared on a large scale consists of from 120 to 150 grains of gelatin (a soft kind), 15 grains of soap, 21 grains of sugar and from 4 to 8 grains of dry colouring matter. The last-named may be of various kinds, from lamp-black pigment to soluble colours such as alizarin. The gelatin, sugar and soap are put in water and allowed to stand for an hour, and then melted, the liquid afterwards receiving the 3. colours, which have been ground on a slab. The mixture is filtered I the whole became coagulated rendered these unmanageable. It through fine muslin. In making the tissue in large quantities the at last occurred to him that if the hardening action of light were two ends of a piece of roll-paper are pasted together and the paper utilized by exposing the surface next the plate to light after or hung on two rollers; one of wood about 5 in. in diameter is fixed before exposingg the front surface to the film and the image, the near the top of the room and the other over a trough containing necessary hardness might be given to the gelatin without adding the gelatin solution, the paper being brought into contact with any chemical hardeners to it. In Tessie de Motay's process the the surface of the gelatin by being made to revolve on the rollers. hardening was almost absent, and the plates were consequently not The thickness of the coating is proportional to the rate at which durable. It is evident that to effect this one of two things had to the paper is drawn over the gelatin: the slower the movement
slowly dried. As the tissue is coloured, it is not possible to ascertain Woodbury Type.This process was invented by W. Woodbury by inspection whether the printing operation is sufficiently carried about the year 1864, though we believe that' J. W. Swan had been out, and in order to ascertain this it is usual to place a piece of working independently in the same direction about the same time. ordinary silvered paper in an actinometer, or photometer, alongside In October 1864 a description of the invention was given in the the carbon-tissue to ascertain the amount of light that has acted Photographic News. Marc Antoine A. Gaudin claimed the principle on it. There are several devices for ascertaining this amount, the of the process, insisting that it was old, and basing his pretensions simplest being an arrangement of a varying number of thicknesses on the fact that he had printed with translucent ink from intaglio of gold-beater's skin. The value of 1, 2, 3, &c., thicknesses of the blocks engraved by hand; but at the same time he remarked that skin as a screen to the light is ascertained by experiment. Sup- the application of the principle might lead to important results. posing it is judged that a sheet of tissue under some one negative It was just these results which Woodbury obtained, and for which ought to be exposed to light corresponding to a given number of he was entitled to the fullest credit. Woodbury subsequently thicknesses, chloride of silver paper is placed alongside the negative introduced certain modifications, the outcome being what is known beneath the actinometer and allowed to remain there until it takes as the " stannotype process," of which in 188o he read a description a visible tint beneath a number of thicknesses equivalent to the before the French Photographic Society (see PROCESS).strength of the negative. After the tissue is removed from the Photo-lithography.Reference has been made to the effect of printing-frame--supposing a double transfer is to be madeit is light on gelatin impregnated with potassium bichromate, whereby placed in a dish of cold water, face downwards, along with a piece the gelatin becomes insoluble, and also incapable of absorbing water of Sawyer's flexible support. When the edges of the tissue begin where the action of the light has had full play. It is this last to curl up, its surface and that of the flexible support are brought phenomenon which occupies such an important place in photo-together and placed flat. The water is pressed out with an india- lithography. In the spring of 1859 E. J. Asser of Amsterdam rubber squeezer or " squeegee " and the two surfaces adhere. About produced photographs on a paper basis in printer's ink. Being a couple of minutes later they are placed in warm water of about anxious to produce copies of such prints mechanically, he conceived 90 to too F., and the paper of the tissue, loosened by the gelatin the idea of transferring the greasy ink impression to stone, and solution next it becoming soluble, can be stripped off, leaving the multiplying the impressions by mechanical lithography. Following image (reversed as regards right and left) on the flexible support. very closely upon Asser, J. W. Osborne of Melbourne made a An application of warm water removes the rest of the soluble similar application; his process is described by himself in the gelatin and pigment. When dried the image is transferred to its Photographic Journal for April 186o as follows: " A negative is permanent support. This usually consists of white paper coated produced in the usual way, bearing to the original the desired ratio. with gelatin and made insoluble with chrome alum, though it may . A positive is printed from this negative upon a sheet of be mixed with barium sulphate or other similar pigments. This (gelatinized) paper, so prepared that the image can be transferred to transfer-paper is made to receive the image by being soaked in hot stone, it having been previously covered with greasy printer's ink. water till it becomes slimy to the touch; and the surface of the The impression is developed by washing away the soluble matter damped print is brought into contact with the surface of the re- with hot water, which leaves the ink on the lines of print of the transfer-paper, in the same manner as was done with the flexible map or engraving." The process of transferring is accomplished in support and the carbon-tissue. When dry the retransfer-paper the ordinary way. Early in 186o Colonel Sir H. James, R.E., bearing the gelatin image can be stripped off the flexible support, F.R.S., brought forward the Southampton method, of photo-lithowhich may be used again as a temporary support for other pictures. graphy, which had been carefully worked out by Captain de Courcy If a reversed negative be used the image may be transferred at once Scott, R.E. The " papyrotype process " was published by Abney to its final support instead of to the temporary flexible support, in 1870 (see LITHOGRAPHY and PROCESS).. which is a point of practical value, since single-transfer are better than double-transfer prints. I Printing with Salts of Iron.Sir John Herschel and Robert Hunt entered into various methods of printing with salts of iron. At I The first notice on record of coloured light impressing its the present time two or three are practised, being used in draughts- men's offices for copying tracings (see SUN-COPYING), Photo-mechanical Printing Processes.Poitevin claimed to have discovered that a film of gelatin impregnated with potassium bichromate, after being acted upon by light and damning, would receive greasy ink on those parts which had been affected by light. But Paul Oreloth seems to have made the discovery previous to 1854, for in his patent of that year he states that his designs were inked with printing ink before being transferred to stone or zinc. C. M. Tessie de Motay (in 1865) and C. R. Marechal of Metz, however, seem to have been the first to produce half-tones from gelatin films by means of greasy ink. Their general procedure consisted in coating metallic plates with gelatin impregnated with potassium or ammonium bichromate or tri-chromate and mercuric chloride, then treating with silver oleate, exposing to light through a negative, washing, inking with a lithographic roller, and printing from the plates as for an ordinary lithograph. The half-tints by this process were very good, and illustrations executed by it are to be found in several existing works. The method of producing the plates, however, was most laborious, and it was simplified by A. Albert of Munich. He had been experimenting for many years, endeavouring to make the gelatin films more durable than those of Tessie de Motay. He added gum-resins, alum, tannin and other such matters, which had the property of hardening gelatin; but the difficulty of adding sufficient to the mass in its liquid state before Photographs in Natural Colours. own colours on a sensitive surface is in the passage already quoted from the Farbenlehre of Goethe, where T. J. Seebeck of Jena (181o) describes the impression he obtained on paper impregnated with moist silver chloride. In 1839 Sir J. Herschel (Athenaeum, No. 621) gave a somewhat similar description. In 1848 Edmond Becquerel succeeded in reproducing upon a daguerreotype plate not only the colours of the spectrum but also, up to a certain point, the colours of drawings and objects. His method of proceeding was to give the silver plate a thin coating of silver chloride by immersing it in ferric or cupric chlorides. It may also be immersed in chlorine water till it takes a feeble rose tint. Becquerel preferred to chlorinize the plate by immersion in a solution of hydrochloric acid in water, attaching it to the positive pole of a voltaic couple, whilst the other pole he attached to a platinum plate also immersed in the acid solution. After a minute's subjection to the current the plate took successively a grey, a yellow, a violet and a blue tint, which order was again repeated. When the violet tint appeared for the second time the plate was withdrawn and washed and dried over a spirit-lamp. In this state it produced the spectrum colours, but it was found better to heat the plate till it assumed a rose tint. At a later date Niepce de St Victor chlorinized by chloride of lime, and made the surface more sensitive by applying a solution of lead chloride in dextrin. G. W. Simpson also obtained coloured images on silver chloride emulsion in collodion, but they were less vivid and satisfactory than those obtained on daguerreotype plates. Poitevin obtained coloured images on ordinary silver chloride paper by preparing it in the usual manner and washing it and exposing it to light. It was afterwards treated with a solution of potassium bichromate and cupric sulphate, and dried in darkness. Sheets so prepared gave coloured images from coloured pictures, which he stated could be fixed by sulphuric acid (Comptes rend us, 1868, 61, p. 11). In the Bulletin de la Societe FranQaise (1874) Colonel St Florent described experiments which he made with the same object. He immersed ordinary or albuminized paper in silver nitrate and afterwards plunged it into a solution of uranium nitrate and zinc chloride acidulated with hydrochloric acid; it was then exposed to light till it took a violet, blue or lavender tint. Before exposure the paper was floated on a solution of mercuric nitrate, its surface dried, and exposed to a coloured image. It is supposedthough it is very doubtful if it be sothat the nature of the chloride used to obtain the silver chloride has a great effect on the colours impressed; and Niepce in 1857 made some observations on the relationship which seemed to exist between the coloured flames produced by the metal and the colour impressed on a plate prepared with a chloride of such a metal. In s88o Abney showed that the production of colour really resulted from the oxidation of the chloride that was coloured by light. Plates immersed in a solution of hydrogen peroxide took the colours of the spectrum much more rapidly than when not immersed, and the size of the molecules seemed to regulate the colour. He further stated that the whole of the spectrum colours might be derived from a mixture of two or at most three sizes of molecules. In 1841, Robert Hunt published some results of colour-photography by means of silver fluoride. A paper was washed with silver nitrate and with sodium fluoride, and afterwards exposed to the spectrum. The action of the spectrum commenced at the centre of the yellow ray and rapidly proceeded upwards, arriving at its maximum in the blue ray. As far as the indigo the action was uniform, whilst in the violet the paper took a brown tint. When it was previously exposed, however, a yellow space was occupied where the yellow rays had acted, a green band where the green had acted, whilst in the blue and indigo it took an intense blue, and over the violet there was a ruddy brown. In reference to these coloured images on paper it must not be forgotten that pure salts of silver are not being dealt with as a rule. An organic salt of silver is usually mixed with silver chloride paper, the organic salt being due to the sizing of the paper, which towards the red end of the spectrum is usually more sensitive than the chloride. If a piece of ordinary silver chloride paper is exposed to the spectrum till an impression is made, it Rll usually be found that the blue Dolour of the darkened chloride is mixed with that due to the coloration of the darkened organic compound of silver in the violet region, whereas in the blue and green this organic compound is alone affected, and is of a different colour from that of the darkened mixed chloride and organic compound. This naturally gives an impression that the different rays yield different tints, whereas this result is simply owing to the different range of sensitiveness of the bodies. In the case of the silver chlorinized plate and of true collodio-chloride, in which no organic salt has been dissolved, we have a true coloration by the spectrum. At present there is no means of permanently fixing the coloured images which have been obtained, the effect of light being to destroy them. If protected from oxygen they last longer than if they have free access to it, as is the case when the surface is exposed to the air. A method devised by Gabrielle Lippmann, of Paris, by which the natural colours of objects are reproduced by means of interference, may be briefly described as follows: A sensitive plate is placed in contact with a film of mercury, and the exposure to the spectrum, or to the image of coloured objects to be photographed, is made through the back of the plate. On development, the image appears coloured when viewed at one particular angle, the colours being approximately those of the object. The necessary exposure to produce this result was very prolonged in the first experiments in which the spectrum was photographed, and a longer exposure had to be given to the red than was required for the blue. Lippmann at first employed collodion dry plates, prepared, it is believed, with albumen, and it required considerable manipulation to bring out the colours correctly. A. Lumiere used gelatin plates dyed withappropriate dyes (orthochromatic plates); the exposure was much diminished, and very excellent representations were produced of all natural colours. The main point to aim at in the preparation of the plate seems to be to obtain a very sensitive film without any, or, at all events, with the least possible, " grain " in the sensitive salt. A formula published by Lumiere seems to attain this object. Viewed directly, the developed images appear like ordinary negatives, but when held at an angle to the light the colours are vivid. They are not pure monochromatic colours, but have very much the quality of colours obtained by polarized light. It appears that they are produced by what may be termed " nodes " of different-coloured lights acting within the film. Thus in photographing the spectrum, rays penetrate to the reflecting mercury and are reflected back from it, and these, with the incident waves of light, form nodes where no motion exists, in a somewhat similar way to those obtained in a cord stretched between two points when plucked. In the negative these nodal points are found in the thickness of the silver deposit. When white light is sent through the film after the image has been developed, theoretically only rays of the wave-lengths which formed these nodes are reflected to the eye, and thus we get an impression of colour. Action of Light on Chemical Compounds. Reference has been made above to early investigations on the chemical action of light. In 1777 Karl Wilhelm Scheele (Hunt's Researches in Light) made the following experiments on silver salts: I precipitated a solution of silver by sal-ammoniac; then I edulcorated it and dried the precipitate and exposed it to the beams of the sun for two weeks; after which I stirred the powder, and repeated the same several times. Hereupon I poured some caustic spirit of sal-ammoniac (strong ammonia) on this, in all appearance, black powder, and set it by for digestion. This menstruum dissolved a quantity of luna cornua (horn silver), though some black powder remained undissolved. The powder having been washed was, for the greater part, dissolved by a pure acid of nitre (nitric acid), which, by the operation, acquired volatility. This solution I precipitated again by means of sal-ammoniac into horn silver. Hence it follows that the blackness which the luna cornua acquires from the sun's light, and likewise the solution of silver poured on chalk, is silver by reduction.... I mixed so much of distilled water with well-edulcorated horn silver as would just cover this powder. The half of this mixture I poured into a white crystal phial, exposed it to the beams of the sun, and shook it several times each day; the other half I set in a dark place. After having exposed the one mixture during the space of two weeks, I filtrated the water standing over the horn silver, grown already black; I let some of this water fall by drops in a solution of silver, which was immediately precipitated into horn silver." This, as far as we know, is the first intimation of the reducing action of light. From this it is evident that Scheele had found that the silver chloride was decomposed by the action of light liberating some form of chlorine. Others have repeated these experiments and found that chlorine is really liberated from the chloride; but it is necessary that some body should be present which would absorb the chlorine, or, at all events, that the chlorine should be free to escape. A tube of dried silver chloride, sealed up in vacua, will not discolour in the light, but keeps its ordinary white colour. A pretty experiment is to seal up in vacua, at one end of a bent tube, perfectly dry chloride, and at the other a drop of mercury. The mercury vapour volatilizes to a certain extent and fills the tube. When exposed to light chlorine is liberated from the chloride, and calomel forms on the sides of the tube. In this case the chloride darkens. Again, dried chloride sealed up in dry hydrogen discolours, owing to the combination of the chlorine with the hydrogen. Poitevin and H. W. Vogel first enunciated the law that for the reduction by light of the haloid salts of silver halogen absorbents were necessary, and it was by following out this law that the present rapidity in obtaining camera images. has been rendered possible. To put it briefly, then, the visible action of light is a reducing action, which is aided by or entirely due to the fact that other bodies are present which will absorb the halogens. In the above we have alluded to the visible results on silver salts. It by no means follows that the exposure of a silver salt to light for such a brief period as to leave no visible effect must be due to the same effect, that is, that any of the molecules are absolutely reduced or split up by the light. That this or some other action takes place is shown by the fact that the silver salt is capable of alkaline development, that is, the particles which have suffered a change in their molecules can be reduced to metallic silver, whilst those which have not been acted upon remain unaltered by the same chemical agency. Two theories have been offered to explain the invisible change which takes place in the salts of silver. One is based on the supposition that the molecules of the salt can rearrange their atoms under the vibrations caused by the ether waves placing them in more unstable positions than they were in before the impact of light took place. This, it is presumed, would allow the developer to separate the atoms of such shaken molecules when it came in contact with them. The other theory is that, as in the case of the visible effects of light, some of the molecules are at once reduced and that the developer finishes the disintegration which the light has begun. In the case of the alkaline development the unaltered molecules next those primarily reduced combine with the reduced silver atom and again form an unstable compound and are in their turn reduced. The first theory would require some such action as that just mentioned to take place and cause the invisible image formed by the shaking apart of the light-stricken molecules to become visible. It is hard to see why other unacted upon molecules close to those which were made unstable and which have been shaken apart by the developer should themselves be placed in unstable equilibrium and amenable to reduction. In the second theory, called the " chemical theory," the reduction is perfectly easy to understand. Abney adopts the chemical theory as the balance of unsubstantiated evidence is in its favour. There is another action which seems to occur almost simultaneously when exposure takes place in the absence of an active halogen absorbent, as is the case when the exposure is given in the air, that is, an oxidizing action occurs. The molecules of the altered haloid salts take up oxygen and form oxides. If a sensitive salt be briefly exposed to light and then treated with an oxidizing substance, such as potassium bichromate, potassium permanganate, hydrogen peroxide, ozone, an image is not developed, but remains unaltered, showing that a change has been effected in the compound which under ordinary circumstances is developable. If such an oxidized salt be treated very cautiously with nascent hydrogen, the oxygen is withdrawn and the image is again capable of development.' Spectrum Effects on Silver Compounds.The next inquiry is as to the effect of the spectrum on the different silver compounds. We have already described Seebeck's (181o) experiments on silver chloride with the spectrum whereby he obtained coloured photographs, but Scheele in 1777 allowed a spectrum to fall on the same material, and found that it blackened much more readily in the violet rays than in any other. Senebier's experiments have been already quoted. We merely mention these H A G F E DCBAhave become the foundation of nearly all subsequent researches of the same kind. The effects of the spectrum have been studied by various experimenters since that time, amongst whom we may mention Edmond Becquerel, John William Draper, Alphonse Louis Poitevin, H. W. Vogel, Victor Schumann and W. de W. Abney. Fig. r is compiled from a cut which appeared in the Proc. Roy. Soc. for 1882, and shows the researches made by Abney as regards the action of the spectrum on the three principal haloid salts of silver. No. 7 shows the effect of the spectrum on a peculiar modification of silver bromide made by Abney, which is seen to be sensitive to the infra-red rays. Effect of Dyes on Sensitive Films.In 1874 Dr H. W. Vogel of Berlin found that when films were stained with certain dyes and exposed to the spectrum an increased action on development was shown in those parts of the spectrum which the dye absorbed. The dyes which produced this action he called " optical sensitizers," whilst preservatives which absorbed the halogen liberated by light he called " chemical sensitizers." A dye might, according to him, be an optical and a chemical sensitizer. He further claimed that, if a film were prepared in which the haloid soluble salt was in excess and then dyed, no action took place unless some " chemical sensitizer " were present. The term " optical sensitizer " seems a misnomer, since it is meant to imply that it renders the salts of silver sensitive to those regions of the spectrum to which they were previously insensitive, merely by the addition of the dye. The idea of the action of dyes was at first combated, but it was soon recognized that such an action did really exist. Abney showed in 1875 that certain dyes combined with silver and formed true coloured organic salts of silver which were sensitive to light; and Dr Robert Amory went so far as to take a spectrum on a combination of silver with eosin, which was one of the dyes experimented upon by J. Waterhouse, who had closely followed Dr Vogel, and proved that the spectrum acted simply on those parts which were absorbed by the compound. Abney further demonstrated that, in many cases at all events, the dyes were themselves reduced by light, thus acting as nuclei on which the silver could be deposited. He further showed that even when the haloid soluble salt was in excess the same character of spectrum was produced as when the silver nitrate was in excess, though the exposure had to be prolonged. This action he concluded was due to the dye. Correct Rendering of Colours in Monochrome.In Plate IV., fig. 14 the sensitiveness of a plate stained with homocol is shown, and it is evident that as it is sensitive throughout the visible spectrum there must be some means of cutting off by a transparent screen so much of the spectrum luminosity at different parts that every colour having the same lurnim*ity to the eye shall be shown on a negative of equal density. When this is done the relative luminosities of all colours will be shown by the same relative densities ( 1e . ) blue and an orange glass can be very accurately D. measured; if I-in. squares of these coloured glasses, (I.e.) together with a white glass of the same area, be D. placed in a row and cemented on white glass, we (l.e.) have a colour-screen which we can make available P. for finding the kind of light-filter to be employed. This is readily done by reducing the luminosity of D. the light coming through all the glasses to that of (I.e.) the luminosity of the light coming through the blue glass. If the luminosity of the blue be 5 and that of the white light too, then the luminosity of the former must be re- duced to -210 of its original value, and so with the other glasses. The luminosity of the light coming through each small glass square can be made equal by rotating in front of them A. disk in which apertures are cut corresponding to the reduction required. The AgI+AgNO3 on paper .. P. AgC7-4-AgNO3 on paper . P. Agl+AgNOa in albumen P. AgI prepared in bath, treated with BI, washed, redipped in silver bath, developed with pyrogallic acid. Grey AgBr in gelatin, developed alkaline or ferrous oxalate . . . Orange AgBr in collodion or gelatin, alkaline ferrous oxalate or acid developer. Green AgBr in collodion, developed ferrous oxalate . . . AgCl incollodion, excess of AgNOa or NaCl present, ferrous citrate or acid development. AgI+AgBr, washed from AgNO3 D. or in a print by different depths of greys. Abney (Ie.) devised a sensitometer which should be used to D.e)'ascertain the colour of the screen that should be employed. By proper means the luminosity of D. the light of day coming through a red, a green, a 3AgI+ AgBr + AgNOx collodion, wet plate. acid or alkaline developer [?.=print; D. = developed; I.e. = long exposure]. two for their historical interest
'See Abney, "Destruction of the Photographic Image," Phil. Mae. (1878), vol. v.; also Proc. Roy. Soc. (1878), vol. xxvil. blue glass, for instance, would not be covered by the disk at all, while opposite the white square the disk would have an aperture of an angle of r8. When a plate is exposed behind the row of glass squares, with the light passing through the rotating disk, having the appropriate apertures for each glass, the negative obtained would under ordinary conditions, show square patches of very different opacity. A light-filter of some transparent colour, if placed in the path of the light, will alter the opacities, and eventually one can be found which will only allow such coloured light to be transmitted as will cause all the opacities in the negative to be the same. As the luminosities of the white light passing through the glasses are made equal, and as the photographic deposits are also rendered equal, this light-filter, if used in front of the camera lens, will render all coloured objects in correct monochrome luminosity. Another plan, based on the same principles, is to place segments of annuluses of vermilion, chrome yellow, emerald green, French blue and white on a disk, and to complete the annuluses with black segments, the amount of black depending on the luminosity of the pigments, which can be readily measured. When the disk is rotated, rings of colour, modified in brightness by black, are seen, and each ring will be of the same luminosity. As before, a screen (light-filter) to be used in front of the lens must be found which will cause the developed images of all the rings to appear of equal opacity. It must be remembered that the light in which the object is to be photographed must be the same as that in which the luminosity of the glasses or pigments is measured. Action of the Spectrum on Chromic Salts.The salts most usually employed in photography are the bichromates of the alkalis. The result of spectrum action is confined to its own most refrangible end, commencing in the ultra-violet and reaching as far as in the solar spectrum. Fig. 2 shows the relative action of H h a F E DCBA No. I No.2 v 1 g o oft the various parts of the spectrum on potassium bichromate. If other bichromates are employed, the action will be found to`be tolerably well represented by the figures. No. I is the effect of a long exposure, No. 2 of a shorter one. It should be noticed that the solution of potassium bichromate absorbs those rays alone which are effective in altering the bichromate. This change is only possible in the presence of organic matter of some kind, such as gelatin or albumen. Action of the Spectrum on Asphaltum.This seems to be continued into and below the red, the blue rays, however, are the most effective. The action of light on this body is to render it less soluble in its usual solvents. Action of the Spectrum on Salts of Iron.The commonest ferric salt in use is the oxalate, by which the beautiful platinotype prints are produced. We give this as a representation (fig. 3) of 14 ~ 6 F OCB No.3 - No.t V I B B G YOR the spectra obtained on ferric salts in general. Here, again, we have an example of the law that exists as to the correlation between absorption and chemical action. One of the most remarkable compounds of iron is that experimented upon by Sir j. Herschel and later by Lord Rayleigh, viz. ferrocyanide of s, potassium and ferric chloride. If these two be brushed over paper, and the paper be then exposed to a bright solar spectrum, action is exhibited into the infra-red region. This is one of the few instances in which these light-waves of low refrangibility are capable of producing any effect. The colour of this solution is a muddy green, and analysis shows that it cuts off these rays as well as generally absorbs those of higher refrangibility. Action of Light on Uranium.The salts of uranium are affected by light in the presence of organic matter, and they too are only acted upon by those rays which they absorb. Thus nitrate of uranium, which shows, too, absorption-bands in the green blue, is affected more where these-occur than in any other portion of the spectrum. Some salts of mercury, gold, copper, lead, manganese, molybdenum, platinum, vanadium, are affected by light, but in a less degree than those which we have discussed. In the organic world there are very few substances which do not change by the continuous action of light, and it will be found that as a rule they are affected by the blue end of the spectrum rather than by the red end (see PHOTOCHEMISTRY). The following table gives the names of the observers of the action of light on different substances, with the date of publication of the several observations. It is nearly identical with one given by Dr Eder in his Geschichte der Photo-Chemie. Substance. Observer. Date. Silver. J. H. Schulze . . 1727 Nitrate solution mixed with chalk, gives in sunshine copies 1737 of writing Nitrate solution on paper . Hellot . Nitrate photographically used . Wedgwood and 1802 Nitrate on silk Davy. 1797 Fulhame . . . . Rumford . 1798 Nitrate with white of egg. . B. Fischer . . 1812 Nitrate with lead salts . . Herschel . . 1839 Chloride . . . . J. B. Beccarius 1757 Chloride in the spectrum . Scheele. 1777 Chloride photographically used . Wedgwood 1802 Chloride blackened Lassaigne . . 1839 Iodide Davy . . . . 1814 Iodide by action of iodine (on Daguerre . . 1839 metallic silver). Herschel . . 1840 Iodide photographically used . Iodide with gallic acid . . Talbot . . . 1841 Iodide with ferrous sulphate . . Hunt . . . 1844 Chloride and iodide by chlorine Claudet . . 184o and iodine (on metallic silver). Balard . . 1826 Bromide . Bromide by action of bromine (on Goddard . . 184o metallic silver). Grotthus . . 1818 Sulpho-cyanide . Nitrite Hess . 1828 Oxide with ammonia Mitscherlich . 1827 Sulphate Bergmann. . 1779 Chromate Vauquelin . 1798 Carbonate Buchholz . . 1800 Oxalate Bergmann . . 1779 Benzoate Trommsdorf . 1793 Citrate Vauquelin 1798 Kinate Henry and Plisson 1829 Borate Rose . . . . 183o Pyrophosphate Stromeyer 183o Lactate Pelouze and Gay- 1833 Formiates Lussac. 1844 Hunt . . . Fulminates Hunt . . . 1844 Sulphide by vapour of sulphur Niepce. 1820 (on metallic silver). Niepce. . . 182o Phosphide by vapour of phos- phorus (on metallic silver). Scheele. . . . 1777 Gold. Oxide Chloride on paper Hellot . . . . 1737 Chloride on silk Fulhame . .. 1794 Chloride in ethereal solution Rumford . . 1793 Chloride with ferrocyanide and Hunt . . . 1844 ferricyanide of potassium. Dobereiner . . 1831 Chloride and oxalic acid . . . Chromate . . . Hunt . 1844 Plate of gold and iodine vapour . Goddard . . 1842 Substance. Observer. Date. Platinum. Chloride in ether Gehlen . . . 1804 Chloride with lime Herschel . . . 184o Iodide Herschel . . . 184o Bromide Hunt 1844 Cyanide Dobereiner . . 1828 Double chloride of platinum and potassium. Gay-Lussac and 1811 Mercury. Oxide (mercurous) Oxide Thenard. 1812 Davy . . , . Oxide (mercuric) Davy . . . 1797 Oxide (more accurate observa- , Abildgaard . . 1797 tions) . . . Harup not till . 18o1 Chloride (mercurous) K. Neumann pre- 1739 Chloride (mercuric) viously to 1803 Boullay . . . Chloride with oxalic acid Bergmann . . . 1976 Sulphate Meyer . . . . 1764 Oxalate (mercuric) Bergmann . 1776 Oxalate (mercurous) Harff . . . . 1836 Sulphate and ammonia (mer- Fourcroy . , . 1791 curous). Garot . . . . 1826 Acetate (mercurous) . . . . Bromide (mercuric) . . . . Lowig . . . . 1828 Iodide (mercurous) Torosewicz . . 1836 Artus . 1836 Iodide (mercuric) Field . . . . 1836 Citrate (mercuric) Harff . 1836 Tartrate and potassium (mer- Carbonell and 1831 curous). Bravo 1812 Carbonate (mercuric). . . . Davy . . . . Nitrate Herschel . . . 184o Sulphide (mercuric) Vitruvius . I B.C. Iron. . Chastaing . . . 1877 Sulphate (ferrous) Chloride (ferric) and alcohol . Bestuscheff . . 1725 Chloride and ether . . . . Klaproth . . . 1782 Oxalate (ferric) . . . Dobereiner . . 1831 Ferrocyanide of potassium . . Heinrich . . . 1808 Sulphocyanide . . . . . Grotthus . . . 1818 Prussian blue Scopoli . . . 1783 Ferric citrate with ammonium .. Herschel . . . 184o Ferric tartrate . Herschel . . . 184o Chromate . Hunt . . . . 1844 Copper. Gehlen . , . 1804 Chloride (cupric dissolved in ether). . A. Vogel . . . 1813 Oxalate with sodium Chromate Hunt . . . . 1844 Chromate with ammonium Carbonate Iodide . A. Vogel , . . 1859 Sulphate Chloride- (cuprous) Copper plates (iodized) Kratoch . 1841 Talbot . : : 1841 Manganese. . Brandenburg 1815 Sulphate Oxalate . Suckow . . . 1832 Potassium permanganate . . Frommberg . . 1824 Peroxide and cyanide of potas- Hunt . . . . 1844 slum . Hunt . , . . 1844 Chloride Lead. Davy . . . 1802 Oxide Iodide Schonbein 185o Sulphite . . . 1811 Peroxide . Gay-Lussac Red lead and cyanide of potas- Hunt . . . . 1844 sium . Hunt . . . . 1844 Acetate Nickel. _ 1844 Nitrate Hunt . . . . Nitrate with ferro-prussiates . Iodide . Uncertain . 1844 Tin. Hunt . . . . Purple of cassius . . . . Various Substances. Cobalt salts Arsenic sulphide (realgar) . Sage . . . . 1803 Antimony sulphide Suckow . . . 1832 Substance. Observer. Date. Bismuth salts . Hunt . 1844 Cadmmum salts Roscoe . 1874 Rhodium salts . t Vanadic salts . . Iridium ammonium chloride . Dobereiner 1831 Potassium bichromate . . Mungo Ponton 1838 Potassium with iodide of starch
Metallic chromates . Hunt . . 1843 Chlorine and hydrogen Gay-Lussac and 1809 Chlorine (tithonized) . Thenard. 1842 Draper . Chlorine and ether Cahours 1810 Chlorine in water .. Berthollet . 1785 Chlorine and ethylene Gay-Lussac and 109 Chlorine and carbon-monoxide Thenard 1812 Davy . Chlorine and marsh gas . Henry . 1821 Chlorine and hydrocyanic acid . Serullas 1827 Bromide and hydrogen Balard . 1832 Iodine and ethylene . Faraday 1821 Cyanogen, solution of Pelouze and 1837 Various other methyl compounds Richardson. 1846 Cahours . Hydrocyanic acid . . . Torosewicz 1836 Hypochlorites (calcium and po- Dobereiner 1813 tassium) Gehlen 1804 Uranium chloride and ether . Molybdenate of potassium and Jager . 1800 tin salts. Petit 1722 Crystallization of salts under Chaptal 1788 influence of light. Dize . 1789 Phosphorus (in hydrogen, nitro- Bockmann. 'Soo gen, &c.) A. Vogel 1812 Phosphuretted hydrogen Nitric acid Scheele. 1777 Hog's fat Vogel 1806 Palm oil Fier . 1832 Asphalt Niepce 1814 Resins (mastic, sandarac, gam- Senebier 1782 boge, ammoniacum, &c.). Hagemann 1782 Guaiacum Bitumens all decomposed, all Daguerre . 1839 residues of essential oils. Senebier 1782 Coloured extracts from flowers Similar colouring matters spread Herschel 11842 upon paper. Pliny . 1st cent. A. D Yellow wax bleached Eudoxia macrembolitissa (purple loth cent. dye). Cole 1684 Other purple dyes . . 171I Reaumur Oils generally .. Senebier 1782 Nitric ether . Senebier 1782 Nicotine . . . . Henry & Boutron- 1836 Santonine Charlard. 1883 Merk . . Effect of Hydrogen Peroxide on Sensitive Plates.Dr W. J. Russell made a series of experiments on the effect of exposure of sensitive plates to the action of vapours and gases for long periods. It has long been known that contact of plates with such substances as wood caused a sensitive surface to show " fog " on development. By a somewhat exhaustive series of experiments, Russell showed that the probable cause of this fog is hydrogen peroxide, since substances which favoured its formation produced the same effect. This is somewhat remarkable, as this same substance will completely destroy the effect that light has had on a sensitive plate; indeed, it affords one way of destroying a light image on a sensitive collodion plate. The experiments of Russell give a warning to store exposed plates for brief periods. It appears that negatives wrapped in paraffin paper are secure from this danger. The Application of Photography to Quantitative Measures.In order to employ photography for the measurement of light it was necessary that some means should be devised by which the opacity of the deposit produced on the development of a plate could be determined. It is believed that in 1874 the first attempt was made by Sir W. Abney to do this. In the Phil. gag. he showed how density could be measured by means of an instrument, the diaphanometer, he had devised, in which transparent black wedges were used to make matches between the naked light and the sane light after passing through the photographic opacity that had to be measured. In 1887, owing to the perfecting of the rotating sectors, which could be made to increase or diminish the apertures at pleasure during its rotation, the measurement of opacities became easy. The Rumford method of comparing the light through the deposit with the naked beam, using the sectors to equalize the illumination, was adopted, the deposit being placed between the light and the screen, the comparison light being a beam reflected from the same light on to the screen. Owing to the fact that photographic deposit scatters light more or less, the opacities measured by this plan were slightly greater than was shown when such opacities were to be used for contact printing. The final plan adopted by Abney was to place the part of the plate carrying the deposit to be measured behind a screen constructed as above. C D (fig. 4) is a Cyy/yypO~Jj~~ nl whch card be with an aperture cut j / n it which may be of any desired shape. A B in aperture was covered with trans- parent paper, as was also a portion B, black cardiitself.A,Lightpthrown from D behind A would be matched with light With this screen accurate measures of printing densities can be made, and it can also be used in the determination of the comparative photographic brightness of the light issuing from different objects. For instance, the relative brightness of the different parts of the corona as seen in a total eclipse can be readily determined if a " time scale " of gradation is impressed on the plate on which it is taken. Both scale and streamer can then be enlarged optically and thrown on the part of the screen A. The measures of the streamer densities can then be directly compared with the densities of the scale and the relative " photographic " brightness of the different parts of the streamer be ascertained by comparison with this scale also. The same method of measurement was adopted in ascertaining quantitatively the sensitiveness of the spectrum of ordinary plates and of plates in which dyes are present. The figures on Pl. IV show reproductions of plates which were exposed to the spectrum. No. I is a continuous spectrum taken with the electric light; no. 7 is an impressed continuous spectrum ; no. 8 shows the bright lines of metals; no. 3 the line spectrum of volatilized lithium and sodium to indicate the position of the spectrum colours. Nos. 4 and 2 are the absorption and fluorescent spectra of eosin. No. 5 is the graduation scale formed by a bromogelatin "Seed" plate stained with homocol, a cyanine derivative sensitive to the red; no. 6 is a similar scale formed by an unstained plate. The small numbers placed below the different bands show an empiric scale which is made to apply to each of them. The first step is to measure soits different parts, and also the curve of sensitiveness of the plate to the different parts of the spectrum. This last is derived from a comparison of the measured densities with those of the gradation scale. Measurement of the Rapidity of a Plate.The first attempt that was made to ascertain the rapidity of a plate was by Abney (Phil. Hag. 1874), who demonstrated that within limits the transparency of deposit varied as the logarithm of the exposure. The last formula has been accepted for general use, though it is believed that it is not absolutely correct, though very approximately true and sufficiently near to be of practical value. This belief is based on the further researches described below.' In I888 Sir W. Abney pointed out that the speed of a plate could be determined by the formula T =E- i(logE+C)2, where T is the transparency, E is the exposure (or time of exposure X intensity of light acting)), and C a constant. If the abscissae (exposures) are plotted as logarithms, the curve takes the same form as that of the law of error, which has a singular point, a tangent through which lies closely along the curve and cuts the axis of Y at a point which has a value of 2/) E. If the total transparency be unity, this ordinate has a value of 1.212, the singular point having a value of o6o6. The ordinate of the zero point of the curve will be where the tangent to the singular point cuts the line drawn at 1.212. The difference between the measurements of this zero point for two kinds of plates (i.e. C in the formula) from the points in the abscissae marking the same exposure, will give the relative sensitiveness of the two plates in terms of log x2. In 1890 Hurter and Driffield (Journ. Soc. Chem. Ind. Jan. 19, 1891) worked out a less empirical formula connecting the exposure E with the density of deposit, which in an approximate shape had the form D =ylog(E/i), where D is the density of deposit (or log 11T), i the " inertia " of the plate, T the transparency of the deposit. In the customary way a small portion of a plate was exposed to a constant light at a fixed distance and for a fixed time, and another small portion to the same light for double the time, and so on. By measuring the densities of the various deposits and constructing a curve, a large part of which was approximately a straight line, it was found possible, by the production of the straight portion to meet the axis of X, to give the relative sensitiveness of different plates by the distance of the intersection from the zero point L. (See also Exposure Meters, below, under 1, APPARATUS.) Effect of Temperature on Sensitiveness.In 1876 Abney showed that heat apparently increased, while cold diminished, the sensitiveness of a plate, but the experiments were rather of the qualitative than the quantitative order. In 1893, from fresh experiments,' he found that the effect of a difference in temperature of some 4o C. invariably caused a diminution in sensitiveness of the sensitive salt at the lower temperature, a plate often requiring more than double the exposure at a temperature of about -18 C. than it did when the temperature was increased to +330 C. The general deduction from the experiments was that increase in temperature involved increase in sensitiveness so long as the constituents of the plate (gelatin, &c.) were unaltered. Sir James Dewar stated at the Royal Institution in 1896 that at a temperature of -18o C. certain sensitive films were reduced in sensitiveness to less than a quarter of that which they possess at ordinary temperatures. It appears also, from his subsequent inquiry, that when the same films were subjected to the temperature of liquid hydrogen (252 C.) the loss in sensitiveness becomes asymptotic as the absolute zero is approached. Presumably, therefore, some degree of sensitiveness would still be preserved even at the absolute zero. Effect of Small Intensities of Light on a Sensitive Salt.3When a plate is exposed for a certain time to a light of given intensity, it is commonly said to have received so much exposure (E). If the time be altered, and the intensity of the light also, so that the exposure (time X intensity) is the same, it was usually accepted that the energy expended in doing chemical work in the film was the same. A series of experiments conducted under differing conditions has shown that such is not the case, and that the more intense the light (within certain limits) the greater is the chemical action, as shown on the development of a plate. Fig. 6 illustrates the results obtained in three cases. The exposure E is the same in all cases. The curves are so drawn that the scale of abscissae ' Those applicable to the correction of star magnitudes as deter-mined by photography have been verified and confirmed by Schwarzchild, Michalke and others. 2 Abney, Proc. Roy. Soc. 1893. 1 Abney, Proc. Roy. Soc. 1893, and Journ. Camera Club, 1893. 8 o to 20 80 40 Empiric Scale of the spectrum the opacity of the gradation scale, next the opacity of the continuous spectrum at the various numbers of the empiric scale, and also the opacity of the other bands at the same scale numbers. The continuous spectrum will give the sensitiveness of the plate to the different parts of the spectrum when the measures of its different opacities are compared with those of the scale of gradation, and a curve of sensitiveness can be plotted from these comparisons. It is evident that the measures of the other two bands. will give us information as to the fluorescence and the absorption of the eosin. Fig. 5 shows the curve of opacity of the image of the spectrum at -20 -10 7 80 v ^^ , "Seed ter o f yJ of the ester of the Sen itiueneas o the Sp Oruro o ^ e 9P et''320 the arc are the ore are om a, itt f ( .8. The bright fides o itf d OAll ot, y to `pQ A o Iv, ress al oo' o ~a N 1u s Li WEe U is the intensity of the light in powers of 2, and the ordinates show the percentages of chemical action produced. If the chemical action remained the same when the intensity of light was reduced, E remaining the same, each of the curves would be shown as a straight line at the height of too, which is the transparency of deposit with the unit of light. As it is, they show diminishing percentages as the light intensity is diminished. o saws set t intensities of Light Thus, when the intensity of the light is reduced to -614- of the original, and the time of exposure is prolonged 64 times, the useful energy expended on a lantern plate is only 50 % of that expended when the light and time of exposure are each unity. In the cases to which the diagram refers, the light used was a standard amyl acetate lamp, and the unit of intensity taken was this light at a distance of 2 ft. from the plate, and the unit of time was lo seconds. The lamp being moved to 16 ft. from the plate, gave an intensity of the unit, and the time of exposure had to be increased to 64o seconds, so that E was the same in both cases. Further, it was found that when the times of exposure on different parts of the plate were successively doubled, light at a fixed distance being used for one series, and altered for a second series, the slopes of the curves of transparency (i.e. the gradation) were parallel to one another. This investigation is of use when camera images are in question, as the picture is formed by different intensities of light, not very different from those of the amyl acetate lamp, the time of exposure being the same for all intensities. The deductions made from the investigation are that with a slow plate the energy expended in chemical action is smaller as the intensity is diminished, while with a quick
/00 90 1 80 70 60 ! 2 .3 4 s 6 Scale of intensities in Powers of 2 intensity of light is, of course, in each case widely different. The slope of the curve due to the spark light is less steep than that due to the arc light, and the latter, again, is much less steep than that due to the amyl acetate lamp. A further investigation was made of the effect of increasing the time of exposure when the intense light was diminished, and it was found that with all plates the useful chemical energy acting on a plate was least with the most intense light, but increased as the intensity diminished, though the time was correspondingly increased. This is the reverse of what we have recorded as taking place when a comparatively feeble light was employed. Further, it was proved that the variation was greatest in those plates which are ordinarily considered to be the most rapid. It follows, therefore, that there is some intensity of light when the useful chemical energy is at a maximum, and that this intensity varies for each kind of plate. Intermittent Exposure of a Sensitive Salt.The same investigator has shown that, if a total exposure is made up of intermittent exposures, the chemical action on a sensitive salt is less than it is when the same exposure is not intermittent. It was also proved that the longer the time of rest between the intermittent exposures (within limits) the less was the chemical action. We may quote one case. Exposures were first made to a naked light, and afterwards to the same light for six times longer, as a rotating disk intervened which had 12 apertures of 5 cut in it at equal intervals apart, and 720 intermittent exposures per second were given. The plate was moved to different distances from the light, so that the intensity was altered. The apparent loss of exposure by the intervention of the disk increases as the intensity diminishes, the ratios of the chemical energy usefully employed of the naked light exposure to that of the intermitting exposures being: For intensity 1 . Ito 815 . 15020 . 1 .423 ,14 1 .370 These results appear to be explicable by the theoretical considerations regarding molecular motion. Effect of Monochromatic Light of Varying Wave-lengths on a Sensitive Salt.It has been a subject of investigation as to whether the gradation on a plate is altered when exposures are made to lights of different colours; that is to say, whether the shades of tone in a negative of a white object illuminated by, say, a red light, would be the same as those in the negative if illuminated by a blue light. Abney I announced that the gradation was different; and, quite independently, Chapman Jones made a general deduction for isochromatic plates that, except with a certain developer, the gradation was steeper (that is, the curve shown graphically would be steeper) the greater the wave-lengths of the light to which the sensitive salt was subjected. For plates made with the ordinary haloid salts of silver Chapman Jones's deduction requires modification. When monochromatic light from the spectrum is employed, it is found that the gradation increases with wave-lengths of light which are less, and also with those which are greater, than the light whose wave-lengths has a maximum effect on the sensitive salt experimented with. Thus with bromo-iodide of silver the maxi-mum effect produced by the spectrum is close to the blue lithium line, and the gradation of the plate illuminated with that light is less steep than when the light is spectrum violet, green, yellow or red. From the red to the yellow the gradation is much the steepest. Whether these results have any practical bearing on ordinary photographic exposures is not settled, but that they must have some decided effect on the accuracy of three-colour work for the production of pictures in approximately natural colours is undoubted, and they may have a direct influence on the determination of star magnitudes by means of photography. Reproduction of Coloured Objects by means of Three Photo-graphic Positives. Ives's Process.A practical plan of producing images in approximately the true colours of nature has been devised by preparing three positives of the same object, one 1 Proc. Roy. Soc., 1900. 7 opacities produced on Effect of very Intense Light on a Sensitive Salt. Another investigation was made as to the effect of very intense light on sensitive surfaces. In this case a screen of step-by-step graduated opacities was made use of, and plates exposed through it to the action of lights markedly differing in intensity, one being that of the amyl acetate lamp, another that of the arc light, and a third the light emitted from the spark of a Wimshurst machine. The exposures were so made that one of the the plate from exposure to each source of light was approximately the same. The unit of illuminated by a red, the other by a green, and the third by a blue light; the images from these three transparencies, when visually combined, will show the colours of the object. This plan was scientifically and practically worked out by F. E. Ives of Philadelphia, though in France and elsewhere it had been formulated, especially by Hauron Du Cros. The following description may be taken as that of Ives's process: by the trichromatic theory of colour-vision every colour in nature can be accounted for by the mixture of two or three of the three-colour sensations, red, green and blue, to which the eye is supposed to respond. Thus a mixture of a red and green sensation produces the sensation of yellow; of a green and blue, that of a blue-green; of red and blue, that of purple; and of all three, that of white. For the sensations we may substitute those colours which most nearly respond to the theoretical sensations without any material loss of purity in the resulting sensation. We must take the spectrum of white light as the only perfect scale of pure colours. It has been proved that the red sensation in the eye is excited by a large part of the visible spectrum, but with varying intensities. If, then, we can on a photographic plate produce a developed image of the spectrum which exactly corresponds in opacity and position to the amount of red stimulation excited in those regions, we shall, on illuminating a transparent positive taken from such a negative with a pure red light, have a representation of the spectrum such as would be seen by an eye which was only endowed with the sensation of red. Similarly, if negatives could be taken to fulfil the like conditions for the green and for the blue sensations, we should obtain positives from them which, when illuminated by pure green and blue light respectively, would show the spectrum as seen by an eye which was only endowed with a green or a blue sensation. Evidently if by some artifice we can throw the coloured images of these three positives on a screen, superposing them one over the other in their proper relative positions, the spectrum will be reproduced, for the over-lapping colours, by their variation in intensity, will form the colours intermediate between those used for the illumination of the positives. For the purpose of producing the three suitable negatives of the spectrum, three light-filters, through which the image has to pass before reaching the photographic plate, have to be found. With all present plates these are compromises. Roughly speaking, the screens used for taking the three negatives are an orange, a bluish-green and a blue. These transmit those parts of the .spectrum which answer to the three sensations. When these are obtained an image of a coloured object can be reproduced in its true colours. Abney devised sensitometers for determining the colours of the screens to be placed before the lens in order to secure the three-colour negatives which should answer these requirements. Their production depends upon the same principles indicated as necessary for the correct rendering in monochrome of a coloured object. When the sensitometer takes the form of glasses through which light is transmitted to the plate, the luminosities of the coloured lights transmitted are determined, and also their percentage composition in terms of the red, green, and blue lights, and thence are deduced the luminosities in terms of red, green and blue. For ascertaining what screen should be used to produce the red negative the luminosity transmitted through each glass is so adjusted that the luminosity of the red components in each is made equal by rotating a disk with correct apertures cut out close to the row of glasses. This gives a sensitometer of equal red values. A coloured screen has to be found which, when placed in front of the lens, will cause the opacities of the deposit on the plate, corresponding to each square of glass, to be the same throughout. This is done by trial, the colour being altered till the proper result is obtained. In a similar way the " green " and " blue " screens are determined. Coloured pigments rotating on a disk can also be employed, as indicated in the paragraph on the correct rendering of colour in monochrome. As to the camera for the amateur, whose plates are not as a rule large, all of the three negatives should be obtained on one plate, since only in this way can they be developed and the densities increased together. (For commercial work the negatives often cannot be taken on the same plate, as it would make the plate too large to manipulate.) The camera may be of an ordinary type, with a repeating back, bringing successively three different portions of the plate opposite the lens. It is convenient to have a slide, in front of which a holder containing the three screens can be fixed, which will then be close to the plate; such a one has been devised by E. Sanger-Shepherd. The light passes through them one byone as the plate is moved into the three positions. The three exposures are given separately, after which the plate is ready for development. The three separate exposures are, however, a source of trouble at times, particularly in the case of landscapes, for the lighting may vary and the sky may have moving clouds, in which case the pictures would show variations which should not exist. Sanger-Shepherd has a " one-exposure " camera by which the three images are thrown side by side on the plate. Thus any movement
Joly's Process.Professor J. Joly, of Dublin, in 1897 introduced a colour process by which an image in approximately natural colours could be thrown upon a screen by an optical lantern, only one transparency being employed, instead of three, as in the Ives process. A " taking " screen was ruled with alternating orange, blue-green and blue lines z o u to iy in. apart, touching one another and following one another in the above order. When such a screen was placed in front of a sensitive plate in the camera, and exposure made to the image of a coloured object, there were practically three negatives on the same plate, each being confined to the area occupied by lines of the same colour. The shades of colour and the depth of the colours used in ruling depended on the brand of plate. When a perfect triune negative was obtained, a transparency was made from it, and in contact with this was placed a screen ruled with lines the same distance apart, but of the colours corresponding to the three colour sensations, namely red, green and blue. The red lines were made to fall on the image taken through the orange lines, the green on that of the blue-green, and the blue or violet on that of the blue. On the screen there are practically three differently coloured images shown by one transparency. The eye blends the different colours together and a picture is seen in approximately the correct colours of the original. Autochrome.A very remarkable process, founded on J. Joly's process, was introduced in 1907 by A. Lumiere et ses Fils of Lyons. Starch grains of very minute size, some of which were dyed with a red stain, a second portion with a green, and a third portion with a blue, are mixed together in such proportions that a fine layer of them appears grey when viewed by transmitted light. Under a magnifying glass the grains are coloured, but owing to the want of focus in the eye the colours blend one with the other. Such a layer is embedded on the surface of a glass plate in a waterproof vehicle, and a film of sensitive emulsion held in situ in some material, the composition of which has not been published, covers this layer. When such a plate is placed in the camera, with the back of the plate next the lens, the light passes through the coloured granules, and again we have three negatives on one plate, but instead of each negative being represented by lines as in the Joly process they are represented by dots of silver deposit. Owing to the way in which the three-coloured film is prepared, it is evident that a positive taken from such a negative could not be backed with granules of the right colour; as the granules are placed at random in the layer. Lumiere, to overcome this difficulty, converted the negative into a positive in a very ingenious way. The plate was developed with pyrogallic and ammonia in the usual way, but instead of fixing it it was plunged into a solution of potassium permanganate and sulphuric acid. This dissolved all the silver that had been deposited during development and left a film of unaltered silver salt. On looking through the plate the colours of the coloured layer coming through the different dots where the silver was at first deposited appeared in view, and the image was the image in colour of the object photographed. The plate after being washed was taken into the light and redeveloped with an alkaline developer, which converted the sensitive salt of silver to the metallic state. The image now consisted of black particles of silver and the coloured image. The plate was next fixed in hyposulphite of soda to remove any unreduced silver salt that might be left, and the picture after washing was complete. The coloured image so obtained is a very close representation of the true colours, but as the " taking " screen is the same as the " viewing " screen some little variation must result. Positives in Three Colours.Ives was the first to show that a transparency displaying approximately all the colours in nature could be produced on the same principles that underlie the three-colour printing. This he effected by printing each of the three negatives, produced for his triple projection process as already described, on gelatine films sensitized by bichromate of potash. Each of the three transparent films was dyed with a colour complementary to the colour of the light which he transmitted through the positives when used for projection. Thus the " red " positive he dyed with a blue-green dye, the " green " positive with a purple dye, and the " blue " positive with a yellow dye. These three films, when superposed, gave the colours of the original object. Sanger-Shepherd has made the process a commercial success (see PROCESS) and produces lantern slides of great beauty, in which all colours are correctly rendered. Instead of using a dye for the " red " transparency, he converts the silver image of a positive image into an iron salt resembling Prussian blue in colour. (W. DE W. A.) II.PHOTOGRAPHIC APPARATUS Photographic apparatus consists essentially of the camera with lens and stand, lens shutters, exposure meters, prepared plates for the production of negatives or transparencies, sensitive papers and apparatus for producing positive prints, direct or by enlargement. Besides these there are many subsidiary accessories. Since the introduction of highly sensitive dry plates and their extended use in hand cameras, the art and practice of photography have been revolutionized. Numerous special forms of apparatus have been created suitable for the requirements of the new photography, and their manufacture and sale have become important industries. The value of the exports of photographic materials from the United Kingdom in 1906 was 22,716. The most important improvement has been in the construction of anastigmatic lenses, which, having great covering power, flatness of field, and freedom from astigmatism, can be worked with very much larger apertures than was possible with the earlier forms of rectilinear or aplanatic lenses. The increased rapidity of working thus gained has rendered it easy to photograph objects in very rapid motion with great perfection. This has encouraged the construction of the very light and compact hand cameras now so universally in use, while, again, their use has been greatly simplified by improvements in the manufacture of sensitive plates and films and the introduction of light, flexible, sensitive films which can be changed freely in daylight. The introduction in 1907 of Messrs Lumiere's " Autochrome " process of colour photography has also been a great advance, tending to popularize photographic work by the facility it offers for reproducing objects in the colours of nature. The Camera. Historical.The camera obscura (q.v.) was first applied to photographic use by Thomas Wedgwood between 1792 and 1802. No description of his camera is available, but it was probably one of the sketching cameras then in use. In 1812 W. H. Wollaston found that by using a meniscus lens with a concave surface towards the object and the convex towards the screen, a diaphragm being placed in front, the projected image of the camera obscura was greatly improved in sharpness over a larger field. The first photographic lenses made by V. and Ch. L. Chevalier in Paris (183o-1840) were on this principle. The photographic camera in its simplest form is a rectangular box,one end of which is fitted to carry a lens and the opposite one with a recess for holding the focusing screen and plate holders, these ends being connected by a rigid or expanding base-board and body, constructed to keep out all light from the sensitive plate except that passing through the lens. In 1816 Joseph Nicephore Niepce, of Chalon-sur-Saone, for his photographic experiments made a little camera, or artificial eye, with a box six inches square fitted with an elongated tube carrying a lenticular glass. There are now in the Chalon Museum cameras of his with an iris diaphragm for admitting more or less light to the lens; some with an accordion bellows, others with a double expanding rigid body for adjusting the focus. The iris diaphragm was adopted later by Chevalier for his photographic lenses. In 1835 W. H. Fox Talbot constructed simple box cameras for taking views of his house on sensitive paper, and claimed them as the first photographs of a building (Phil. Mag. 1839, 14, p. 206). Fr. von Kobell and C. A. Steinheil, early in 1839, made a camera with an opera glass lens for taking landscapes on paper. Later in 1839 J. W. Draper successfully used a camera for his daguerreotype experiments made of a spectacle lens, 14 in. focus, fitted into a cigar box. He also used a camera fitted with a concave mirror instead of a ,lens. Similar cameras were constructed by A. T. Wolcott (184o) and R. Beard (1841) for reversing the image in daguerreotype portraits. They have also been recommended by V. Zenger (1875) and D. Mach (189o) for scientific work. L. J. M. Daguerre's camera, as made by Chevalier in 1839 for daguerreotype, was of Niepce's rigid double body type, fitted with an achromatic meniscus lens with diaphragm in front on Wollaston's principle, the back part with the plate moving away from the lens for focusing, and fixed in its place with a thumb-screw. This expanding arrangement enabled lenses of different focal lengths to be used. With modifications cameras of this type were in use for many years afterwards for portrait and studio purposes. For work in the field they were found inconvenient, and many more portable forms were brought out, among them G. Knight's and T. Ottewill's single and double folding cameras (1853), made collapsible with hinges, so as to fold on to the base-board. Cameras with light bodies made of waterproof cloth, &c., also came into use, but these were superseded by cameras with collapsible bellows-body of leather, which, invented by Niepce, were used in France, in 1839, by Baron A. P. de Seguier and others for daguerreotype. The first record of them in England is, apparently, J. Atkinson's portable stereoscopic camera of parallel-side bellows form (Ph. Journ. 1857, 3, p. 261), which was soon followed by C. T. H. Kinnear's lighter conical form, made by Bell of Edinburgh (Ph. Journ. 1858, 4, p. 166). They have since been made in various patterns, conical, oblong and square, by P. Meagher, G. Hare and others, and are still, in modified forms, in general use as studio, field or hand cameras. When wet collodion plates were used many cameras were fitted with arrangements for developing in the field. Information on these and other early cameras will be found in the photographic journals, in C. Fabre's Traite encyclopedique de Photographie, vol. i., and in J. M. Eder's Ausfiihrliches Handbuch der Photographie, 2nd ed., vol. i., pt. ii. The distinctive feature of present day photography is the world-wide use of the hand camera. Its convenience, the ease with which it can be carried and worked, and the remarkably low prices at which good, useful cameras of the kind can be supplied, concurrently with improvements in rapid sensitive plates and lenses, have conduced to this result. It has also had a valuable educational Influence in quickening artistic perception and scientific inquiry, besides its use in depicting scenes and passing events for historical record. Small portable cameras had been made by B. G. Edwards (1855), T. Scaife (Pistolgraph, 1858), A. Bertsch (186o), T. Ottewill (1861), and others, but it was not until rapid gelatin dry plates were available in 1881 that T. Bolas brought out his " detective " camera (Ph. Journ. 1881, p. 59). It consisted of a double camera (one as finder, the other for taking the picture) enclosed in another box, suitably covered, which also contained the double-plate carriers and had apertures in front of the viewing and taking lenses. In another form the bright, well-defined object on the screen and then on a ground-finder was omitted. A month later A. Loisseau and J. B. 1 Germeuil-Bonnaud patented an opera glass camera. Various forms of portable magazine cameras followed, among them A. Pumphrey's " Repeating Camera " (1881), W. Rouch's " Eureka" (1887), R. Krugener's camera (book form, 1888), and others in collapsible or box forms disguised as books, watches, &c., but they did not come into general use before 1888, when the East-man Company of Rochester, U.S.A., brought out their very portable roll-film cameras, now known under the trade name of " Kodak." The manufacture of these and other light hand cameras has since become a very important and flourishing industry in Great Britain, Germany, France and the United States. It is noteworthy that the most modern form of hand camera, the reflex, goes back to an early type of portable camera abscura, figured by Johann Zahn in 1686, in which a mirror was used for reflecting the image on to a horizontal focusing screen, at the same time reversing it. The first photographic camera on this principle was T. Sutton's (186o), which has served as a basis for many subsequent developments. A. D. Loman's (1889) and R. Krugener's (1891) were early examples of the hand camera type, but great improvements have since been made. Modern cameras differ so much in details of improved construction that only a few of the more important requirements can be noticed. A camera should be well and strongly made of seasoned wood or of metal, perfectly rigid when set up, to avoid any shifting of the axis of the lens in respect to the sensitive plate. The front and back of the camera should normally be vertical and parallel, and the axis of the lens perpendicular to the centre of the plate, but arrangements are usually made by vertical and lateral adjustments on the camera front for raising the lens to take in less foreground or vice versa, or for moving it right or left, the latter becoming a vertical movement when the camera has to be turned on its side. In the Adams " Idento'" camera the lens and finder can be rotated together on the rising front according as the camera is used horizon-tally or vertically, the finder showing in either case the identical view projected on the plate. The best modern field cameras are fitted with a swing-back or swing-front and sometimes with both. A swing-back is necessary for bringing back the plate to the vertical position, so as to prevent convergence of vertical lines, when the camera has to be tilted. A rising swing-front, in which the lens is tilted, answers the same purpose, provided the camera is kept level. If further tilting is necessary, when taking high buildings &c., the swing-back and front may both be required, but must be kept vertical and parallel and the effect is that of an abnormal rising front. Many modern cameras are fitted with a double rising front. The vertical and side swings are also useful for equalizing the definition of objects at different distances from the camera, but they alter the perspective. These swing-movements should preferably be round the central horizontal or vertical axis of the back or front, but are frequently effected by simple inclination of the back or lens front on a hinge. When the rising front is used a lens of extended covering power is desirable, and it may be necessary to stop it down to ,obtain good definition over the extended area of the picture. A slight inclination of the lens may also be useful in readjusting the focus. The camera and plate carriers must be perfectly light-tight and all inner bright surfaces made dead black to prevent reflections from bright spots being thrown on the plate. The black varnish used, preferably of shellac and lampblack in spirit, must have no deleterious effect on the plates. Although the weight and bulk are increased it is convenient to have the camera square and fitted with a reversible back, so that the greatest length of the plate may be horizontal or vertical, as desired. Many cameras are fitted with revolving backs to be used in either position. In some French cameras the back part of the camera with the bellows is reversible, to be used upright or horizontal. Focusing.The earlier cameras were focused by drawing out the back and clamping it with a thumb-screw working in a slot in the base-board. When bellows cameras were introduced they were focused by an endless screw, and these are still used for large copying cameras. Most modern cameras are fitted with rack and pinion movements working either in front or at the back of 'the camera or both. Many hand cameras, requiring to be brought to focus at once, are fitted with studs (infinity catches) which fix the front in focus for distant objects, nearer distances being noted on an engraved scale attached to the base-board. Such scales should be verified by measurement. In hand cameras with fixed infinity focus, the necessary adjustments for distance of near objects are made on the lens mount. The focusing screen may be ruled with parallel cross lines for purposes of measurement, and as a check on the verticality of the camera when photographing buildings or other objects with vertical lines. The distance of the lens from the focusing screen and from the sensitive plate in the dark slide must coincide exactly. This can be tested by measurement or by focusing aglass plate placed in each of the slides to be examined. A evel or other means of showing that the camera is level and the plate vertical should be attached to the camera, also a view meter or finder, showing the exact extent of the picture on the focusing glass. In the view meter the picture is viewed directly through a pin-hole mounted at the back of the caplera as it appears in a frame with cross wires on the rising front, adjusted to the size of the plate and the 'ncus of the lens. Finders are practically small reflex cameras, and a reduced image is seen reflected from a mirror or prism. A rectangular concave glass mounted on the camera is also a convenient form, it can be combined with a mirror for vertical observation, and in Watson's new form is also arranged as a level and telemeter (B. J. A. p. 724, 1908). The image seen in the finders should correspond exactly with that on the plate. When the rising front is used special arrangements have to be made to ensure the correspondence of the images in the finder and on the ground-glass. This is done in the " Adams Identoscope " (19o8), which is fitted to the swing front and adjusted by a lever to follow the movement of the lens. Plate-holders or Dark-slides.The dark-slides or backs, holding sensitive plates, are made either single or double, the former usually for wet plates, the latter for dry plates. The ordinary book-form double dark-slide has been in use since the early days of calotype paper negatives, and contains two plates separated by a blackened metal plate; three of them usually form a set, the shutters being numbered 1 to 6, the odd numbers on the opening side. Inner frames can be used for smaller plates if desired. The slides should fit easily into the camera and the shutters run smoothly out and in. They must be perfectly light-tight, the corner joints, the hinges in the shutters, and the openings in the sides and top of the book-form slides are all weak points requiring occasional careful examination or protection by metal plates. The shutters of dark-slides are either jointed or solid and removable; the former is perhaps the more convenient, but both forms may become liable to let in light. Various forms of solid slides, single and double, are now made in wood or metal, or of wood for the frame and metal for the shutters; they are lighter, more compact and less liable to admit light to the plates. In some cases one slide can suffice for the exposure of several plates or stiff films, enclosed in separate envelopes, as in the " Wishart-Mackenzie " slide, the " Victrix " and other similar ones, or contained in a single packet, as in the " Premo Filmpack," and similar arrangements which enable twelve thin celluloid films to be placed in the camera, exposed one after the other, and removed again safely in daylight, the pack being replaced, if necessary, by another. The packets of films are made of light cardboard, and effect a great saving of bulk and weight (fig. 1). Roll-holders are also a convenient way of carrying sensitive celluloid films in lengths of six or twelve exposures, rolled on spools, which can be changed in daylight. Changing boxes for holding a. reserve of plates or celluloid films in sheaths, are used with some magazine and other cameras. They are arranged to fit on the camera in place of the dark-slide and the plates are changed automatically so that exposed plates are placed in order successively at the back, a fresh plate going forward for exposure and the number of the exposure being recorded at the same time. Studio cameras, for portraiture, are usually of the square bellows type, of solid construction, to take large and heavy lenses; adjustable from front and back with rack and pinion movements, to enable long or short focus lenses to be used, with extra extension for copying or enlarging. They are generally fitted with repeating backs, allowing two or more exposures to be made on one plate. The backs are square or reversible, so that the plates can be used up-right or lengthways, and are fitted with double swing movements at the back. When single dark slides are used they are best fitted with a flexible shutter to avoid jerking and movement of the camera. For portraiture they are mounted on solid pillar stands, being raised or lowered with an endless screw or rack-work, and the table-top usually has vertical and horizontal angular movements. Large cameras with long extension for copying purposes are made in many forms with special arrangements for the various photo-mechanical processes, and are mounted on substantial table-stands with screw adjustments for obtaining the various motions above noted, and also a rectilinear traversing motion right or left. All these stands should be absolutely rigid and free from tremor. Process cameras are, however, sometimes mounted, together with the copying board, on swinging stands, to avoid the effects of vibration. Portable and field cameras include cameras of the Hare and Meagher types for outdoor work and general purposes on plates 15 in. X 12 in. to 81 in. X 61 in., and in lighter forms from 61 in. X 41 in. to 44 in. X 31 in. For general purposes they are usually made with square bellows and folding tail-board, rather more substantially than those with conical bellows intended for outdoor work. There are many patterns, the principal modern improvements in field cameras being swinging fronts, tripod head and turn-table in the base-board, double and sometimes triple extension movements from the back and front for long or short focus lenses, and the use of aluminium for some of the metal-work. They are fitted with a focusing screen and are intended for use on a tripod stand, though some of the smaller sizes of the modern light hand or stand cameras can be used as hand cameras with finders. The plates are carried in the usual dark-slides, but the smaller sizes, from half-plate downwards, can be fitted with roll-holders for flexible films, or with film packs or other daylight changing arrangements. Folding and Hand Cameras.Folding cameras form a class of modern portable cameras which have many conveniences for hand or stand work from quarter-plate to 7 in. X 5 in. They may have all the fittings of a stand camera and be made to take glass plates, flat or roll films, but have the advantage of forming when closed a convenient package enclosing cam- era, lens and shutter, all in position for immediate use when opened out (fig. 2). Most of them are fitted with focusing glass and finders, and may focus by scale in the same way as hand cameras. With an ap- 2.Sinclair Folding Camera. paratus of this kind on a light stand any class of ordinary indoor or outdoor.work can be undertaken within the size of the plate, and the extension of the bellows, which should be quite double the focus of the lens. The multiplicity of forms and arrangements of hand cameras makes it difficult to classify them into distinct types; but they may be mainly divided into box and folding cameras, and further into (a) cameras with enclosed changing magazines for plates or flat films; (b) with enclosed roll film on spools; (c) with separate changing magazines, changing boxes or roll-holders; (d) .with single, double or multiple plate carriers or film-packs. Most cameras that will take glass plates in the ordinary plate-holders will take cut films in suitable sheaths or can be fitted with envelope slides, film-packs or roll-holders. The normal size for hand cameras is the quarter-plate (4; in. X 31 in.), or the continental size 9 X 12 cm. ; 5 in. X 4 in. is also a popular size, and cameras for the post-card size, 51 in. X 31 in. or 15 X io cm. have been largely adopted. Smaller sizes are also made for lantern plates and for the lighter pocket cameras, some in the form of stereoscopes, field-glasses or watches, as in the Ticka," but the pictures are small and require enlarging. Hand cameras are constructed on the same principles as stand cameras, but, being specially intended for instantaneous work, they are simplified and adapted for rapid focusing and exposing. The focusing screen is superseded or supplemented by finders arranged to show the limits of the subject on the plate, the focus being adjusted by the infinity catches and focusing scales above noticed. Swing-backs and fronts are.often dispensed with, but are desirable adjuncts, and a rising and falling front particularly so. Lenses of fairly large aperture, f/6 to f/8, and good covering power, preferably of the anastigmatic type, or a rapid aplanat, should be used, but for very rapid work anastigmats working from f/4 to f/6 will be more useful. Hand cameras can also be fitted with telephoto objectives of large aperture. Some cheap hand cameras are fitted with single landscape lenses or aplanats working about fill or lower, but the want of intensity limits their use to well-illuminated subjects. Shutters of the between-lens type are now generally used in hand cameras, and for ordinary purposes should give fairly accurate exposures from i to h of a second or less and also time exposures. Some central shutters are speeded for shorter exposures to spa of a second, but for these focal plane shutters are preferable, and for the more rapid exposures to I~uo of a second and less are necessary. The shutter should be efficient, regular in action, and readily released by gentle pressure, pneumatic or otherwise. Mechanism for automatically changing plates or films in hand cameras of the box magazine type must be certain in action, simple and not readily put out of order, special care being taken to avoid rubbing or abrasion of the plates in changing or transport. In changing plates or films the number of plates exposed should be recorded automatically, and duplicate exposures prevented asfar as practicable. A circular level placed near the finder is useful. The choice of a hand camera depends upon the circumstances in which it is to be used, and the purpose for which it is principally required. For general work and with the modern facilities for carrying and changing plates and films in daylight, the numerous folding hand or stand cameras for plates, flat or roll films, with full adjustments, will be found most useful. Box or magazine cameras in which a supply of cut films or plates can be carried, changed mechanically, and exposed rapidly in succession, are convenient, but their use is limited and they are liable to get out of order. A third class are the reflex and other hand cameras with focal plane shutters for specially rapid instantaneous work as noticed below. There are two types of light folding hand or stand cam-eras, specially adapted for hand camera workthose made for taking glass plates and cut films, and the folding pocket Kodak or other roll - film cameras. The former are now made of very light construction with mahogany or metal bodies, wooden or aluminium base-boards, thin metal dark-slides (fig. 3). The cameras of the pocket Kodak type are of similar construction, but made to take roll films Fin. 3.Ernemann's Pocket Camera. on spools, or with an attach- ment for focusing glass and dark-slides for taking plates and cut films. Attached to a sling-strap the quarter-plate size can be quite conveniently carried in a side-pocket. Watson's " Deft " folding camera is fitted with a focal plane shutter (fig. 4). The " Selfix carbine " camera has a self-erecting front bringing the lens at once into position for use on opening out. Those fitted with lenses of fairly large aperture, double extension, and rising and falling fronts are to be preferred. Of box or magazine cameras there is an immense variety. In some the lens is fixed in focus for all objects within a certain distance, in others it is adjusted by a focusing scale on the lens or by an FIG. 4.The " Deft " Folding extending front. Some have Focal-plane Camera. a single magazine, others two or more. Some take only glass plates, others plates or cut films. All of them are, however, self-contained and ready for immediate exposure. One of the earliest forms of single magazine cameras, still in use, as in the " Eureka " and " Yale," is the " bag," in Fin. 6.The Verascope, Richard. which a supply of plates or films in sheaths, is kept in a magazine behind the camera, ready for exposure, the plates as exposed being lifted with the fingers into a bag or expanding chamber above the magazine and placed behind the rest of the plates at the back, a fresh plate taking its place in front. In some forms the magazines are removable and replaceable by others. The arrangement is simple and effective, but the bag, usually made of soft leather or cloth, is liable to wear and puncture, and may make dust. The cameras with double magazines in which unexposed plates are kept in one recess and transferred successively after exposure to a second recess are more complicated, and many Camera. ingenious devices have been invented for effecting the change (fig. 5). Some forms are effective and popular on account of their compactness and readiness for immediate exposure, but there is always a risk
Dai-Cornex " is a great improvement in this form of camera, being a daylight-loading box magazine camera for plates, the plates being packed in a bundle of ridged can he put into or taken out of the camera in full daylight. In other respects it resembles other magazine cameras (fig. 7). Another useful magazine camera is the " Zambex," carrying either plates or films, held in skeleton frames in envelopes which can be loaded or unloaded in daylight, and are kept ready for use in the back of the camera and exposed consecutively. For work in which speed is of primary importance hand cameras fitted with very rapid lenses and focal plane shutters are necessary, and several forms of portable collapsible cameras of this kind are now available, such as the Goerz-Anschiitz, Zeiss's Palmos," Watson's " Vril " (fig. 8), Adams, " Idento," &c., and are lighter and more portable than the reflex cameras. Hand cameras are generally fitted with screw-bushes for mounting on a tripod stand when time exposures are wanted. The light folding e wooden or aluminium stands noted below are specially suitable. Twin-lens and Reflex Cameras.For photographing animals, objects in motion, with Twin Lenses, section have the means of watching the movement to show working. till the critical moment of exposure A, Hood of finder. arrives. For this it is convenient to B, Ground glass screen, have a camera f |
