, 
42 On the UNEQUAL 
Of the perfe correftion of the aberration arifing from the unequal 
refrangibility of light. 
Tus fact now eftablifhed on the fulleft evidence, that the 
divergency of the heterogeneal rays is not to be removed by a 
combination of crown-glafs with flint-glafs, or with thofe dif- 
perfive fluids employed in the object-glafles, with which the 
experiments above related were made, difcovered a moft im- 
portant problem in optics, namely, the entire removal of the 
aberration from the difference of refrangibility of light, by any 
combination of mediums whatever. 
Tuts problem, it was evident, was only to be attempted by 
again having recourfe to the volume of nature, and fearching’ 
out the hidden ‘qualities of refra@iing mediums. ‘Though in 
all the compound objeét-glafles which were examined, after be- 
ing rendered as achromatic as poflible, the fame colours ap- 
peared, and in the fame order; yet every trial could only be 
confidered as {peaking for itfelf, if the expreflion may be al- 
lowed. The experiments were indeed numerous, and will, ¥ 
hope, be found to have been made with fufficient care and at- 
tention ; yet to have formed from them any general conclufion, 
that in every endeavour to unite the rays of all colours, by 2 
combination of mediums differing in difperfive power, the 
green rays will emerge moft refracted, and the red and violet 
leaft refracted, as above explained, could only ferve to prevent 
farther inveftigation, by reprefenting the perfection of the 
theory of optical inftruments by refraction as a defperate at- 
tempt. 
Tue order in which I proceeded farther to explore this fub- 
ject was the following : 
Havine found fringes of colour, as above defcribed, in 
combinations of crown-glafs with the effential oils, and in com- 
binations of crown-glafs and flint-glafs, when the refraction is 
rendered 
