CH. xx. THE DISPERSION OF LIGHT. 161 



CHAPTER XX. 



SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). 



Newton on Dispersion of Light Chester More Hall Dispersive 

 Power in Flint and Crown Glass Newton on the Transmission of 

 Sound Last years of Newton's life. 



Newton publishes his Discovery of the Dispersion of 

 Light, 1671. We must now return to Newton, and con- 

 sider his third great discovery, which was about light. You 

 will remember that he had to wait sixteen years between his 

 first attempt to investigate the law of gravitation, and that 

 new measurement of the earth which enabled him to prove 

 the truth of his theory. During this time he had by no 

 means been idle. He once said that the reason he had 

 succeeded in making discoveries was that he gave all his 

 attention to one subject at a time; from 1666 to 1671, 

 when his papers on gravitation were quite laid aside, the 

 subject to which he devoted himself was Light. 



In the early part of the seventeenth century several 

 people had tried to find out what it was that gave rise to 

 different colours. An Italian Archbishop named Antonio 

 de Dominis (died 1625) had given a better explanation of 

 the rainbow than Roger Bacon had given before him ; and 

 Descartes had gone farther, and had pointed out that a ray 

 of light seen through a clear, polished piece of glass, cut into 

 the shape of a prism (see Fig. 29), is spread out into colours 



