368 Philosophical Transactions, &fc. 



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 Avitli 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 power to one of sixteen or twenty feet. We may therefore, 

 by this means shorten the tube several feet, and yet possess a focal 

 power more considerable than could 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. 

 It has an aperture of 7.8 inches, exceeding by an inch that of tlie 

 largest refracting telescope previously 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 principle just 

 referred to is eighteen 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 likely to re- 

 alize all the anticipations of its ingenious author. 



In the mean time however, Mr. Faraday has been engaged in experi- 

 ments on the manufacture of a glass, intended as a substitute for flint 

 glass in optical instruments. In this he is said by the latest advices 

 to have been completely successful. 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 importance, inasmuch as it now appears probable, that glass of 

 any desirable dimensions can henceforth be obtained without much 

 difficulty. I. R. 



