REFRANGlBlLlTr of LIGHT. 4$ 



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 I continue 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 I have employed the term homogeneal light, in con- 

 formity to the common language of optics. 



It was formerly afferted, 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 refracled, 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 refractions are employed ; fo that in the refraction 

 which takes place in the confine of two fuch mediums, the rays 

 of light are {till 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, violet, the prifmatic fpectrum 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 moil: refrangible. 



Vol. III. F Of 



