72 On the UNEQUAL 



and the quantity of light tranfmitted by equal apertures of the 

 telefcopes much greater. 



Such are the advantages which the theory prefents. In re- 

 ducing this theory to practice, difficulties muft be expected in 

 the firft attempts. Many of thefe it was neceffary to furmount 

 before the experiments could be completed. For the delicacy of 

 the obfervations is fiich, as to require a confiderable degree of 

 perfection in the execution of the object-glafTes, in order to ad- 

 mit of the phenomena being rendered more apparent by means 

 of high magnifying powers. Great pains feem to have been 

 taken by mathematicians to little purpofe in calculating the ra- 

 dii of the fpheres requifite for achromatic telefcopes, from 

 their not considering that the objecl-glafs itfelf is a much nicer 

 tell of the optical properties of refracting mediums than the 

 grofs experiments made by prifms, and that the refults of their 

 demonftrations cannot exceed the accuracy of the data, however 

 much they may fall fhort of it. 



I shall conclude this paper, which has now greatly exceed- 

 ed its intended bounds, by enumerating the feveral cafes of 

 unequal refrangibility of light, that their varieties may at once 

 be clearly apprehended. 



In the refraction which takes place in the confine of every 

 known medium and a vacuum, rays of different colours are 

 unequally refrangible, and the red-making rays are leaft re- 

 frangible, and the violet-making rays are mofl refrangible. 



This difference of refrangibility of the red and violet rays 

 is not the fame in all mediums. Thofe mediums in which the 

 difference is greateft, and which, by confequence, feparate or 

 difperfe the rays of different colours moft, have been diftin- 

 guifhed by the term difperfive, and thofe mediums which fepa- 

 rate the rays leaft have been called indifperfive. Difperfive me- 

 diums differ from indifperfive, and flill more from each other, 

 in another very eftential circumftance. 



It 



