i.] NEWTON'S WORK. 7 



from one colour to another is not abrupt, but is made in an 

 imperceptible manner, so that it can scarcely be said, for 

 instance, where the yellow ends or the green begins. 



Next came the further experiments to which so much interest 

 attaches. Newton, by making a hole in the screen, allowed 

 one of the colours thus produced say red to pass on and 

 through a second prism, receiving the image on a second screen : 

 the colour remained unaltered. 



This experiment proved that the rays producing the red 

 colour of the spectrum so experimented on, were simple. The 

 same result was soon found for all the others. 



As Newton in his experiment operated with sunlight, the 

 band of colours was in this case called the solar spectrum ; 

 and, indeed, the rainbow is nothing more nor less than a 

 solar spectrum, caused by rain-drops playing the part of so 

 many prisms. This being so, Newton could deal with the 

 components of the light instead of with the light as a whole. 



He then took two beams of differently coloured light red 

 and blue and passed these two beams of different colour 

 through the same prism. The action of the prism on these 

 two differently coloured beams was found to be unequal ; in 

 other words, he got the red beam deflected to a certain distance 

 from a straight line, and the blue deflected to a certain other 

 distance. This experiment showed that there is a distinct differ- 

 ence in the amount of refrangibility, or, in other words, in the 

 degree to which the original beam is deviated out of its course 

 by its passage through the prism that the red light is not 

 diverted so far out of its original direction by the prism as 

 the blue light is. 



This leads us to Newton's first proposition, which is this : 

 " Lights which differ in colour differ in refrangibility." This may 

 be translated as follows : " Lights which differ in colour are 

 differently acted upon by a prism." 



We now reach that proposition of Newton's which makes 



