REFRANGIBILITY of LIGHT. 39 
lefs refrangible rays, and fome other ray, Rw, which is one of 
the more refrangible rays in the medium AB, is the mean re- 
frangible ray in this medium C.D. — 
Tuer 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 moft refrangible rays; and that thefe colours appear more 
diftin@tly 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 refraGtion through any lens, whether 
fimple orc 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 refra€tion, 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 object-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 
