368 Philosophical. Transactions, Sc. 
fluid in the latter, is too close to admit of bringing the concave cor- 
recting medium far enough back to be of any sensible advantage. 
The case however is widely different with sulphuret of carbon. The 
dispersive ratio here varies (according to the glass employed,) be- 
tween the limits 299 and 334; which circumstance has enabled 
me to place the fluid correcting lens at a distance from the plate lens 
equal to half its focal length; and I might carry it still farther back, 
and yet possess sufficient dispersive power to render the object glass 
achromatic. Moreover, by this means the fluid lens, which is the 
most difficult part of the construction, is reduced to one half, or to 
less than one half of the plate lens ; consequently, to construct a tel- 
escope of.ten or twelve inches aperture, involves no greater difficulty 
in the manipulation than in making a telescope of the usual descrip- 
tion of five or six inches aperture, except in the simple plate lens it- 
self... And. what will be thought of greater importance, a telescope 
of this kind of ten or twelve feet in length will be equivalent. in. its 
focal poner to one of sixteen or twenty feet. We may therefore, 
means shorten the tube several feet, and yet possess a focal 
power more considerable than mae be conveniently given it on the 
usual principle of construction.’ 
Having established the correctness of his views by the application 
of these principles to the construction of smaller instruments, he ob- 
tained the aid of the Board of Longitude to enable him to construct 
one of great dimensions. This is described in the paper before us. 
Igdihe, an aperture of 7.8 inches, exceeding by an inch that of the 
evious! ly existing in England. Its tube is 
eleven feet in , length, which, with the eye piece makes a total length 
of twelve feet ; but its effective focal distance upon the pineinle just 
referred to is ightenn feet; it has a power of 700. 
Of the. performance of this instrument,. the account is most satis- 
factory, .and there is little doubt that the construction is _ to re- 
— anticipations of its te ingenious author. 
hee 
ments on the manufacture of a Dane: intended. as a substitute for flint: 
glass in optical instruments. In this he is said by the latest advices 
to,have been one successfule’, Many of the difficulties attend- 
ing-the construction of solid. object lenses for refracting. telescopes 
will be obviated by this discovery, and the plan of Barlow rendered 
of less impo erinasmauch ait now Ce AE i 
can henceforth be obtained without much 
I. R. 
