42 On the UNEQUAL 



Of the perfect correBion of the aberration arifing from the unequal 

 refrangibility of light. 



This 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. 



This 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 refracting mediums. Though in 

 all the compound object-glaffes which were examined, after be- 

 ing rendered as achromatic as poffible, the fame colours ap- 

 peared, and in the fame order ; yet every trial could only be 

 confidered as fpeaking for itfelf, if the expreflion may be al- 

 lowed. The experiments were indeed numerous, and will, I 

 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 a 

 combination of mediums differing in difperfive power, the 

 green rays will emerge mod refracted, and the red and violet 

 lead refracted, as above explained, could only ferve to prevent 

 farther inveftigation, by reprefenting the perfection of the 

 theory of optical initruments by refraction as a defperate at- 

 tempt. 



The order in which I proceeded farther to explore this fub- 

 ject was the following : 



Having 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 



