REFRANGIBILirr of LIGHT. 39 



lefs refrangible rays, and fbme other ray, R&», which is one of 

 the more refrangible rays in the medium AB, is the mean re- 

 frangible ray in this medium CD. 



The moft obvious way of examining the optical properties 

 of different mediums, is by means of prifms. But I have not 

 found this method either fo eafy or fo accurate as that by means 

 of lenfes, which has been already explained. It has been fhewn, 

 that the image of a lucid point (fee the thirteenth figure) is 

 every where, between the lens and that point where the rays 

 crofs, furrounded with a fringe of the colour of the leaft re- 

 frangible rays ; and that every where beyond the point of crof- 

 fing, the image is furrounded with a fringe of the colour of 

 the mod refrangible rays ; and that thefe colours appear more 

 diftinctly at the focus itfelf, when one half of the lens is co- 

 vered. Hence, in order to determine which rays are the moft 

 or leaft refrangible, after refraction through any lens, whether 

 fimple ore om pound, it is only neceflary to examine the colours 

 of thefe fringes, which is the more eafily done, as they are 

 greatly magnified by the eye-glafs. 



In fingle lenfes, the fringe within the focus, which is com- 

 pofed of the leaft refrangible rays, will always be found to be 

 of a red colour, with a mixture of orange ; and the fringe be- 

 yond the focus, compofed of the moft refrangible rays, will be 

 found to be of a blue colour. Thefe are the colours which, it 

 is well known, are produced by fimple refraction, made in the 

 confine of every known medium and a vacuum. 



From what hath been already related, it appears, that colour 

 is likewife produced in what has been termed achromatic refrac- 

 tion, though it be lefs in quantity, in proportion to the whole 

 refraction ; and the rays which are found moft and leaft refran- 

 gible, in thefe two cafes, differ very widely. 



In a compound objecl-glafs, formed of a concave, which dif- 

 perfes the rays in a greater degree, and a convex, which dif- 

 perfes the rays in a lefs degree, there was always found, when 



the 



