963 



in the re-examination of Sir John Herschel's Catalogue of Nebulae 

 published in the Phil. Trans, for 1833. 



Before describing any of the interesting objects the peculiar fea- 

 tures of which the extraordinary powers of the telescope employed 

 for their examination has brought to our knowledge, the author 

 enters upon some details concerning the instrument itself. This 

 telescope, which for aperture and the consequent power it possesses 

 for the examination of faint details must for a considerable time, at 

 least, remain unrivalled, has a clear aperture of 6 feet, with a focal 

 length of 53 feet. It has hitherto been used as a Newtonian, but 

 by the easy application of a little additional apparatus it may be 

 conveniently worked as a Herschelian ; and the author thinks it not 

 improbable that, in the further examination of the objects of most 

 promise with the full light of the speculum undiminished by a second 

 rejiexiony some additional features of interest will come out. 



The tube reposes at its lower end upon a very massive universal 

 joint of cast iron, resting upon a pier of stonework buried in the 

 ground, and it is counterpoised so that it can be moved in polar 

 distance with great facility. The extreme range of the tube in right 

 ascension at the equator is one hour, but greater as the polar distance 

 diminishes. By a little subsidiary apparatus the movement of the 

 telescope can be rendered almost exactly equatorial ; but up to the 

 present time this apparatus has not been used, as, without it, the 

 movement was found to be sufficiently equatorial for such measure- 

 ments as have been required. The whole mounting was planned 

 especially with a view of carrying on a regular system of sweeping ; 

 but as yet the discovery of new nebulsg has formed no part of the 

 systematic work of the observatory, the known objects which re- 

 quire examination being so numerous that hitherto the observers 

 have been fully occupied with them. 



A clock movement was part of the original design, but as yet the 

 telescope is not provided with one, and the want of it has not been 

 very much felt. 



Various micrometers have been tried, but, upon the whole, the 

 common wire micrometer with thick lines has been found to succeed 

 the best ; for the faint details of the nebulse are extinguished by any 

 micrometrical contrivance which either diminishes the light of the 

 telescope or renders the field less dark; and thick lines have been 

 found to be visible without illumination in the darkest night. 



The telescope has two specula, one about three and a half, and 

 the other rather more than four tons weight. Each is provided with 

 a system of levers to afford it an equable support. Upon this 

 system it was placed before it was ground, and has rested upon it 

 ever since. The systems of levers with the mode of applying them 

 in the support of the speculum are described in the paper, and also 

 the precautions taken to guard against strain and consequent flexure 

 of the metal. Notwithstanding these precautions, undoubted evi- 

 dences of flexure in the speculum have occasionally shown them- 

 selves. It has not, however, been found that flexure, even to the 

 extent of materially disfiguring the image of a large star, interferes 



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