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Encyclopedia Britannica - Main :: RON-SAC |
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ROSSE, WILLIAM PARSONS, 3RD EARL of (18001867), Irish astronomer and telescope constructor, was born at York
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The first constructor of reflecting telescopes on a large scale, William Herschel, never published anything about his methods of casting and polishing specula, and he does not appear to have been very successful beyond specula of 18 in. diameter, his 4-foot speculum (" the 4o-foot telescope ") having been little used by him (see discussion between Sir J. Herschel and Robinson in The Athenaeum
Brewster 's Edinburgh Journal of Science for 1828 he described his machine for polishing the speculum, which in all essential points remained unaltered afterwards. It imitates the motions made in polishing a speculum by hand by giving both a rectilinear and a lateral motion to the polisher, while the speculum revolves slowly; by shifting two eccentric pins the course of the polisher can be varied at will from a straight line to an ellipse of very small eccentricity, and a true parabolic figure can thus be obtained. The speculum lies face upwards in a shallow bath of water (to preserve a uniform temperature), and the polisher fits loosely in a ring, so that the rotation of the speculum makes it revolve also, but more slowly. Both the grinding and polishing tools are grooved, to obtain a uniform distribution of the emery used in the grinding process and of the rouge employed in polishing, as also to provide for the lateral expansion of the pitch with which the polisher is coated. In September 1839 a 3-foot speculum was finished and mounted on an altazimuth stand similar to Herschel's; but, though the definition of the images was good (except that the diffraction at the joints of the speculum caused minute rays in the case of a very bright star), and its peculiar skeleton form allowed the speculum to follow atmospheric changes of temperature very quickly, Lord Rosse decided to cast a solid 3-foot speculum. Hitherto it had been felt as a great difficulty in casting specula that the solidification did not begin at one surface and proceed gradually to the other, the common sand mould allowing the edges to cool first, so that the central parts were subject to great straining when their time of cooling came, and in large castings this generally caused cracking. By forming the bottom of the mould of hoop iron placed on edge and closely packed, and the sides of sand, while the top was left open, Lord Rosse overcame this difficulty, and the hoop iron had the further advantage of allowing the gas developed during the cooling to escape, thus preventing the speculum from being full of pores and cavities. This invention secured the success of the casting of a solid 3-foot speculum in 184o, and encouraged Lord Rosse to make a speculum of 6 ft. diameter in 1842. In the beginning of 1845 this great reflector was mounted and ready for work. The instrument has a focal length of 54 ft. and the tube is about 7 ft. in diameter; owing to these large dimensions it cannot be pointed to every part of the heavens, but can only be moved a short distance from the meridian and very little to the north of the zenith; these restrictions have, however, hardly been felt, as there is almost at any moment a sufficient number of objects within its reach.From 1848 to 1878 it was but with few interruptions employed for observations of nebulae (see NEBULA) ; and many previously unknown features in these objects were revealed by it, especially the similarity of " annular " and " planetary " nebulae, and the remarkable " spiral " configuration prevailing in many of the brighter nebulae. A special
Lord Rosse gave a detailed account of the experiments which step by step had led to the construction of the 3-foot speculum in the Philosophical Transactions for 184o. In the same publication for 1844 and 185o he communicated short descriptions and drawings of some of the more interesting nebulae, and in the volume for 1861 he published a paper " On the Construction of Specula of 6-ft. Aperture, and a Selection from the Observations of Nebulae made with them," with numerous engravings. The accounts of the observations given in these papers, however, were fragmentary; but in 18798o a complete account of them was published by the present earl (" Observations of Nebulae and Clusters of Stars made with the 6-foot and 3-foot Reflectors at Birr Castle from 1848 to 1878 ") in the Scient. Trans. R. Dublin Soc. vol. ii. The drawing of the nebula of Orion was published in the Phil. Trans. for 1868See obituary notice in the Proc. Roy. Soc. (1868), 16, 36, and in the Monthly Notices of Roy. Astr. Soc. vol. 29, p. 123. End of Article: ROSSE, WILLIAM PARSONS, 3RD EARL If you wish, you can link directly to this article.
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