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 preliminary 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 refrae- 
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 j ; 
of a powerful astronomical telescope to a small number of individuals, 
and to public bodies.” ee 
alt siise * % ees ie ae te 
“In the usual construction of achromatic telescopes, the two: 
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 objeet 
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 
