1 68 



SEVENTEENTH CENTURY. 



Yl. III. 



between these have their own course through the prism. I 

 must warn you, however, not to think that there are exactly 

 seven colours : there are really an infinite number, passing 

 gradually into each other ; Newton only divided them 

 roughly into seven for convenience. 



This spreading out of the different coloured rays is called 

 the dispersion of light. I wish I could give you the many 

 beautiful experiments which Newton made to prove it, but 

 I have only room for one, which you can easily try for 

 yourself, by which the different colours which make up 

 the spectrum can be turned back again into white light. 

 You will see at once that if it is true that white light can 

 be divided up into colours, those same colours when re- 

 united must make white. To show this Newton took a 

 round card and painted upon it the seven colours, as pure as 

 possible, five times over, like a spectrum five times repeated 

 (a, Fig. 31), and then spun it round rapidly, so that the eye 

 received the impression of all the seven colours at once 

 (b, Fig. 31). If you do this you will see the card looks a 



Fig, 31. 



A 



A, Newton's disc, b. Disc routing. 



dirty white, because the colours blend together just as they 

 do in a ray of light. You will not get a pure white, because 



I 



