26 On the UNEQUAL 
refraCtion in pafling into it from the glafs, conceive the angle 
BCD to be diminifhed as the refraction diminifhes, fo that the 
refracted ray GK fhall ftill) emerge perpendicular to DC. 
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 refraéted at the point G, in 
paffing out of the glais into the fluid. For they fuffer no other: 
refraction whatever. 
Tuis is a neceflary confequence of the glafs and fluid differ- 
ing in their mean refractive denfity, and of the rarer medium 
poffefling the requifite degree of difperfive power. 
Tus cafe of a fingle 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 caufe of one 
kind of rays being more refracted than another, to arife from 
their being more attracted. 
Wuew the medium is furrounded 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 attraéted by the difference between 
their attraGing forces, as they aét in oppofition to each other. 
Now, if the difference of attraGtion of the moft and leaft 
refrangible rays were, in all mediums, proportioned to the 
whole attraction of the mean refrangible ray, it would be im- 
poflible to produce refra€tion without colour *. But fubfequent 
tits got f experiments 
* Tats at leaft is true as to fenfe in thofe {mall refractions which take place in tele~ 
fcopes and microfcopes ; 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 refraction cannot 
be produced by large contrary refrattions of the fame medium, properly difpofed for 
the purpofe. 
