CH. xx. NEWTON DISPERSION OF LIGHT. 165 



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 

 we 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. 32), and then spun it round rapidly, so that the eye 

 received the impression of all the seven colours at once 



A B 



FIG. 32. 

 A, Newton's disc. B, Disc rotating. 



(n, Fig. 3 2). If you do this you will see the card looks a 

 dirty white, because the colours blend together just as they 

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

 the artificial colours are not pure, and also because it is 

 difficult to paint each colour in the proper proportion. 



But now that we have proved that light is broken up 

 into colours in passing through a denser medium, you may 

 perhaps ask how it is that we do not see coloured rays when- 

 ever we look at the sun through glass or any other trans- 



