Composition of While light. 59 



fracted light is suffered to fall perpendicularly on a screen, 

 the figure of the image, if all the rays were equally refracted, 

 ought to be round. Finding, then, by experiment, that 

 this was not the case, and that the length of the image 

 greatly exceeded its breadth, he concluded, that the rays of 

 light are unequally refracted at the same angle of incidence. 



He says, that, " By the mathematical proposition above 

 mentioned, it is certain that the rays which are equally re- 

 frangible do fall upon a circle answering to the sun's appa- 

 rent disque. Now, let A G, (Fig. [.) represent the circle 

 which all the most refrangible rays, propagated from the 

 whole disque of the sun, would illuminate and paint upon the 

 opposite wall if they were alone ; E L the circle which all 

 the least refrangible rays would in like manner illuminate if 

 they were alone ; B H, C J, D K, the circles which so many 

 intermediate sorts would paint upon the wall, if they were 

 singly propagated from the sun in successive order, the rest 

 being intercepted ; and conceive that there are other circles 

 without number, which innumerable other intermediate 

 sorts of rays would successively paint upon the wall, if the 

 sun should successively emit every sort apart. And seeing 

 that the sun emits all these sorts at once, they must alto- 

 gether illuminate and paint innumerable equal circles ; of 

 all which, being according to their degrees of refrangibility 

 placed in order in continual series, that oblong spectrum is 

 composed which was described in the first experiment." 

 (Newton's Optic's, p. 31.) See Smith's Optics, Article 174. 



Again he says, " The solar image P T formed by the 

 separated rays in the fifth experiment, did in the progress 

 from its end P, on which the most refrangible rays fell, 

 into which its end Ton which the least refrangible rays fell, 

 appear tinged with this series of colours ; violet, indigo, 

 blue, green, yellow, orange, red, together with all tbeir in- 

 termediate degrees in a continual succession perpetually 

 varying : So that there appeared as many degrees of colours 

 as there were sorts of rays differing in refrangibility." — 

 (Newton's Optics, p. 106.) See Smith's Optics, Art. I7i>. 



It i- evident from these quotations, that Newton not only 

 considered the different colours differently refrangible, 

 which he very satisfactorily proved ; but, also, that each of 

 the colours, into which lie divided the spectrum, had innume- 

 rable different degrees of refrangibility, and shadesof colour ; 



