Analyses of Books. 
151 
VII. An account of a series of experiments made with a view to the const* notion 
of an achromatic telescope^ with a fluid concave tens, instead of the usual lens of flint 
glass. In a letter addressed to Davies Gilbert, Esq. M. P. Pres. R. S. By Peter 
Barlow, Esq. F. R. S. 
The subject of fluid object glasses lias now been long before the public, though as 
yet the practical trials do not appear 10 have been sufficiently encouraging to have 
prompted the construction of any really effective instruments. Mr. Barlow has been 
for some time engaged in a series of experiments on this subject, which originated in 
an attempt to apply to practice the rules given by Mr. Herschel in the Phil. 
Trans. 1821, and a conviction that flint glass of the requisite size and purity could 
not be procured without great difficulty. He has succeeded in constructing two in- 
struments of that description, the one of 3 inches aperture, the other of 6 inches. 
Mr. Barlow’s views have been so far different from Dr. Blair’s, (the first proposer 
of fluid object glasses.) that in the telescopes of the latter it was still necessary to 
retain the flint lens, his only object being to destroy what has been named the se- 
condary spectrum, due to a want of proportionality between the coloured spaces of 
the spectra of flint and plate or crown glass, as compared with their respective re- 
fractive indices, whereas the design of the latter was to dispense altogether with the 
flint glass, by substituting in itsplace a fluid medium of the requisite refractive and 
dispersive power. 
Mr. Barlow was led after many experiments to select sulphuret of carbon, which 
has a refractive index about equal to that of the best flint glass with a dispersive 
power more than double. 44 Perfectly colourless, beautifully transparent, and pos- 
sessing the same optical properties, when hermetically sealed, under all tempera- 
tures, to which it is likely to be exposed for astronomical purposes, (unless indeed 
it should be found that direct observations on the solar disc are inadmissible,) its 
high dispersive power gives it an advantage which no glass ever made or likely to 
he made can possess, though the fixed nature of the latter may probably always give 
it a preference in the construction of telescopes.” 
Having succeeded, after many trials, in confining the fluid, Mr. Barlow attempted 
a telescope of 6 inches aperture ; but after some unsuccessful trials he was induced 
to confine himself to 3 inches aperture. With this instrument, though by no means 
perfect, he separated a great number of Sir W. Herschel *s stars, which have been 
noted as tests of a good3^ iuch refractor. Thus the 44 small star in Polaris is visible, 
as also 70 p Ophiuchi, 39 Bootis, the quadruple star £ Lyra*, C Aquarii, cl Herculis, 
. Encouraged by this success he was led again to attempt the 6 inch aperture ob- 
ject glass, and the result of his endeavours has been highly satisfactory With this 
ni&trumeut the small star in Polaris is so distinct and brilliant with a power of 143, 
that its transit might be taken with the utmost certainty. The small stars in a Lyra, 
Aldebaran,RigeI,£ Bootis, &c are very distinctly exhibited. Amongst the larger close 
ouble stars Castor and y Leonis are well defined with a power of 300, and amongst 
the smaller double stars w Auriga;, 52 Orionis,£ Orionis, &c. The belts and double 
rin g ot Saturn are well exhibited with a power of 150, and the belts and satellites 
Jupiter are very tolerably defined with the same power.” 
> the usual construction of achromatic telescopes, the two or three lenses com- 
posing the object glass are brought into immediate contact, and in the fluid teles* 
Co Pe proposed by Dr. Blair, the construction was the same, the fluid having been 
^closed 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 
the former instance, and between the glass and fluid in the latter, is too close to 
* out ol bringing the concave correcting medium far enough back to be of any sen- 
advantage. The case, however, is very differentwith the sulphuret of carbon. 
n Qr f dispersive ratio here varies (according to the glass employed) betw een the limits 
and 334, which circumstance has enabled me to place the fluid correcting lens 
,? ^stance from the plate lens equal to half its focal length, and I might carry it 
* 11 * Urt her back, ami yet possess sufficient dispersive power to render the object 
^ass achromatic. Moreover, by this means the fluid lens, which is the most difficult 
,° f the construction, is reduced to one half or to less than one half of the size of 
inv i te iens ’ consequently to construct a telescope of ten or twelve inches aperture 
usI.iT U0 S realer difficulty in the manipulation, than in making a telescope of the 
tvW .senption of 5 or ti inches aperture, except in the single plate lens itself; and, 
at will be thought perhaps of greater importance, a telescope of this kind, of ten 
welve feet length, will be equivalent in its focal power to one of 1 6 or 20 feet. 
req ui ? aper Conclud es with an analytical investigation, investigating the condition* 
11 e( t0 correct the secondary spectrum. 
