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XXV. Observations on the Nebulae. By The Earl of Rosse, Pres. R.S., S^c. 8^c. 
Received June 19, — Read June 20, 1850. 
In laying before the Royal Society an account of the progress which has been made 
up to the present date in the re-examination of Sir John Herschel’s Catalogue of 
Nebulae published in the Philosophical Transactions for 1833, it will be necessary to 
say something of the qualities of the instrument employed. 
The telescope has a clear aperture of 6 feet, and a focal length of 53 feet. It has 
hitherto been used as a Newtonian, but in constructing the galleries provision was 
made for the easy application of a little additional apparatus to change the height of 
the observer, so that the focal length of the speculum remaining the same, the instru- 
ment could be conveniently worked as a Herschelian. 
Although with an aperture so great in proportion to the focal length, the perform- 
ance of a parabolic speculum placed obliquely would no doubt be very unsatisfactory, 
still additional light is so important in bringing out faint details, that it is not im- 
probable in the further examination of the objects of most promise with the full light 
of the speculum, undiminished by a second reflexion, some additional features of in- 
terest will come out. 
The second reflexion is accomplished in the usual way by a surface of speculum 
metal ; some experiments have been made, suggested by Jamin’s paper in the An- 
nales de Chimie for 1848, to procure a surface of silver suited to the purpose, but 
without complete success. Arrangements also have been for some time in contem- 
plation with the view of effecting the second reflexion occasionally by a small glass 
prism ; and about a year ago a prism was procured from Munich for the purpose: in 
both cases there would be a great saving of light ; but I am speaking of the instru- 
ment as it is, not as it may become, if further improved. 
The tube reposes at its lower end upon a very massive universal joint of cast iron, 
resting on a pier of stonework buried in the ground ; and it is counterpoised so that 
it can be moved in polar distance with great facility. A quick motion in polar distance 
is given by a windlass below, and a slow motion is' given by hand above for measure- 
ments. The extreme range of the tube in right ascension at the equator is one hour; 
but greater as the polar distance diminishes. The quick movement in right ascension 
is given below by a wheel turned by a workman, and the slow motion by hand above ; 
the instrument is therefore completely under the dominion of the observer. The tube 
is slung entirely by chains, and is perfectly steady even in a gale of wind. 
As the chain which governs the movement of the telescope passes over a pulley 
capable of being brought by a little subsidiary apparatus into a line drawn from the 
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