1882.J 



in the object-glasses of telescopes. 



233 



of a higher refractive index, and less dispersive than crown-glass, 

 the substitution of the one for the other diminishes the focal 

 length of the compound lens " when the curves of the flint lens are 

 diminished," i.e. when the radii are increased. This seems correct, 

 allowing that it is prudent to diminish the focal length at all. He 

 then proceeds by claiming as original a design to substitute for 

 flint-glass a liquid oil enclosed in glass, which is obviously only a 

 form of Prof. Barlow's invention, which I believe has been found 

 impracticable.^ Thirdly, he proposes to use rock-crystal in eye- 

 pieces ; we may say, an obvious inference from his first invention. 

 It is clear from this short abstract of his Patent, that whatever his 

 own method of working may have been, M. Cauchoix was offering 

 to the public an ingenious invention rather than a scientific 

 novelty. 



For the convenience of my readers I have reprinted Fraun- 

 hofer's and Rudberg's Indices for the three substances we have been 

 considering. The dispersive powers I have given above ; or they 



Vj , where fi, Prof. 



may easily be obtained by the formula 



Stokes informs me, may be safely taken between D and E, about 

 one-third of the distance from D. 



P 



sp. g. /J.B. fiC. fiD. fxE. fiF. fiG. fiH. 



VOL. IV. PT. IV. 



17 



