Royal Society of London. 367 



We shall mention but one of the remaining papers in this half vol- 

 ume. It is that of Barlow on a refracting telescope, the achro- 

 matism of which is produced by a lens of a liquid substance. Mr. 

 Barlow has in the Philosophical Transactions of the Royal Society 

 for 1828, given an account of his prehminary experiments. It ap- 

 pears that in experiments upon the construction of aplanatic object 

 glasses, he had become sensible of the great difficulty of obtaining flint 

 glass of sufficient size and purity for astronomic telescopes. He 

 was therefore led to consider the practicability of substituting a fluid 

 instead of a flint glass lens. Fluid object lenses had been long be- 

 fore constructed by Dr. Blair, but the object was different ; Blair had 

 no other pretension than to lessen a practical difficulty in the con- 

 struction of a compound lens, of which both flint and crown glass 

 still formed essential portions ', while our author sought to dispense 

 with flint glass altogether. After an examination of the refractive 

 and dispersive powers of various substances, he at last fixed upon 

 sulphuret of carbon as best suited to his purpose. This appeared 

 to him, to possess every requisite he could desire ; " having a refrac- 

 tive index about equal to that of the best flint glass, with a dispersive 

 power more than double, perfectly colorless, beautifully transparent, 

 and, although very expansible, possesing the same, or very nearly the 

 same optical properties when hermetically sealed, under all tempera- 

 tures to which it is likely to be exposed for astronomical purposes. 



"Its high dispersive power also gives it an advantage which no glass 

 ever made or likely to be made can possess ; although the fixed na- 

 ture of the latter material may probably always give it a preference 

 in the construction of telescopes ; and I wish clearly to be understood, 

 not as proposing to supplant the use of flint glass in these instruments, 

 but simply to supply its place by a valuable substitute, in cases where 

 it cannot be obtained sufficiently large and pure ; or where it can be 

 obtained only at an expense which must always limit the possession 

 of a powerful astronomical telescope to a small number of individuals, 

 and to public bodies." 



* -s ■«• * -x- * * \ 



" In the usual construction of achromatic telescopes, the two or 

 three lenses composing the object glass are brought into immediate 

 contact ; and in the fluid telescope proposed by Dr. Blair, the con- 

 struction was the same, the fluid having been inclosed in the object 

 glass itself. Nor could any change in this arrangement in either case 

 be introduced with advantage, because the dispersive ratio between 

 the glasses in the former instance, and between the glass and the 



