in >, eis tiie, sales ——. rr 
REFRANGIBILITY f LIGHT. as 
the concave and convex lenfes being compofed of mediums 
which do not difperfe the rays proportionally, as before ex- 
plained. But the matter will be beft underftood, by recurring 
to the cafe above mentioned, of fingle achromatic refraction. 
Thus Ijcontinue to denominate it, though, the Society will per- 
ceive that this term achromatic, is here ufed with manifeft im- 
Propriety; ; ; and will alfo; make proper allowance for the fenfe 
in,which,J have employed the term. Aomogeneal light, in. con: 
formity to the common language of optics. 
. Tr was formerly aflerted, that when two mediums differ in 
mean refractive, denfity, and the difperfive power of the rare 
medium exceeds that of the denfe medium in a certain propor- 
tion, light-of. all colours will be equally refracted in the confine 
of the two mediums; and it is true that the red and violet rays 
will be equally.refracted, and the rays of other colours as near- 
ly fo as by any combination of two. mediums of different dif- 
perfive powers. But on account of the two mediums not at- 
tracting, and.confequently feparating the rays of different’ co= 
lours in a given ratio, the fame green and purple-coloured 
fringes appear in an object-glafs of this kind; as in one in which 
oppofite refraGtions are employed; fo that in the refraétion 
which takes place in the confine of two fuch mediums, the rays 
of light are ftill unequally refrangible. But inftead of the de- 
gree of refrangibility being, as in) common refraction, accor- 
ding to the order; of the colours, red, orange, yellow, green, 
blue, indigo, wiolet, the. prifmatic fpeétrum is, as it were, 
doubled, the rays being, at the fame time, compreffed nearer 
to each other ;, and their degrees of refrangibility being now 
according to the following order: Red and violet united, the 
leaft refrangible;, next, to thefe in-refrangibility, the orange and 
indigo united ; then the united yellow and: blue; and, laftly, 
the homogeneal green rays, which are the moft refrangible. 
ei Mila, Bid! sels ctor are h Ff olen Of 
bexsh 7 
