20 On the UNEQUAL 



refraction in paffing into it from the glafs, conceive the angle 

 BCD to be diminifhed as the refraction diminifhes, fo that the 

 refracted ray G K (hall (till emerge perpendicular to D C. 

 When the angles of the glafs prifm and prifm of difperfive 

 fluid bear to each other a certain proportion, the ray will be 

 found to emerge colourlefs ; and when this happens, it is evi- 

 dent that all the rays are equally refracted at the point G, in 

 paffing out of the glafs into the fluid. For they fufFer no other 

 refraction whatever. 



This is a neceffary confequence of the glafs and fluid differ- 

 ing in their mean refractive denfity, and of the rarer medium 

 poffeffing the requifite degree of difperfive power. 



This cafe of a (ingle refraction taking place, without any 

 difference of refrangibility of the rays, may be illuftrated by 

 Sir Isaac Newton's explication of refraction, by means of 

 attraction, in the following manner. He fuppofes refraction 

 to arife from an attracting force acting on light, in lines per- 

 pendicular to the furface of the medium ; and the cau(e of one 

 kind of rays being more refracted than another, to arife from 

 their being more attracted. 



When the medium is (unrounded by a vacuum, the refrac- 

 tion will be proportioned to the whole attracting force of that 

 medium. But when light paffes from one refracting medium 

 into another, it will only be attracted by the difference between 

 their attracting forces, as they act in oppofitioh to each other. 



Now, if the difference of attraction of the mod and lead 

 refrangible rays were, in all mediums, proportioned to the 

 whole attraction of the mean refrangible ray, it would be im~ 

 poffible to produce refraction without colour *. But fublequent 



experiments 



* This at Ieaft is true as to fenfe in thofe final] refractions which take place in tele- 

 fcopes and microfcopes j and it would be mathematically true in all cafes, if the angles 

 of incidence and refraction were proportional. But as it is not the angles themfelves 

 which are fo, but their fines, it is a miftake to fuppofe that colourlefs refradtion cannot 

 be produced by large contrary refractions of the fame medium, properly difpofed for 

 the purpofe. 



