326 Mr Barlow on the Comtniction of 



give it a preference before any other in the construction of re- 

 fracting telescopes. My object is (as I wish distinctly to be un- 

 derstood) not to supplant the use of flint glass in the construc- 

 tion of this instrument, but to supply its place by a valuable 

 substitute in cases where the former cannot be obtained suffi- 

 ciently large, or where it can only be obtained at an ex pence 

 which must always limit the possession of a good astronomical 

 telescope to persons of fortune and to public institutions. 



Principle of Construction. 



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 was the same, the fluid having been in- 

 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 in- 

 stance, 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 ra- 

 tio 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 re- 

 duced 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 involves 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 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 

 to it on the usual principle of construction. This will be better 

 understood from the annexed diagram. 



