94 Prof. Barlow's description of a New Fluid Telescope. 



You are aware, that the correcting medium I have em- 

 ployed as a substitute for the flint lens is sulphuret of car- 

 bon. This is enclosed between two plate glasses, ground and 

 polished to the proper curves, and placed at any distance be- 

 hind the plate front lens, from actual contact to two-thirds of 

 the focal length of the plate, every thing else being adjusted 

 accordingly. This being premised, it may be observed, that 

 in the usual construction of achromatic telescopes, the two or 

 three lenses composing the object-glass are brought into im- 

 mediate contact, and in the fluid telescope proposed by Dr 

 Blair the construction is the same, the fluid having been en- 

 closed in the object-glass itself. Nor could any change in this 

 arrangement in either case be introduced with advantage ; be- 

 cause the dispersive ratio between the glasses in the former 

 instance, and between the glass and fluid in the latter, is too 

 close to admit of bringing the concave correcting medium far 

 enough back to be of any sensible advantage. The case, 

 however, is very different with the sulphuret of carbon. The 

 dispersive ratio here varies (according to the glass employed) 

 between the limits .298 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 size of the plate lens ; consequently, to construct 

 a telescope of ten or twelve inches aperture involes no greater 

 difficulty in the manipulation than in making a telescope of 

 the usual description of five or six inches aperture, except in 

 the simple plate lens itself: and what will be thought perhaps 

 of greater importance, a telescope of this kind of ten or twelve 

 feet length will be equivalent in its focal power to one of six- 

 teen or twenty feet. We may therefore, by this means, short- 

 en the tube several feet, and yet possess a focal power more 

 considerable than could be conveniently given to it on the 

 usual principle of construction. This will be better under- 

 stood from Fig. 1. of Plate III. 



