20 The Theory of the Aether 



received the name of Bologna stone or Bologna phosphorus, has- 

 the property of shining in the dark after it has been exposed 

 for some time to sunlight ; and the storage of light which 

 seemed to be here involved was more easily explicable on the 

 corpuscular theory than on any other. The evidence in 

 this quarter, however, pointed the other way when it was 

 found that phosphorescent substances do not necessarily emit 

 the same kind of light as that which was used to stimulate 

 them. 



In accordance with his earliest discovery, Newton considered 

 colour to be an inherent characteristic of light, and inferred 

 that it must be associated with some definite quality of the 

 corpuscles or aether-vibrations. The corpuscles corresponding 

 to different colours would, he remarked, like sonorous bodies of 

 different pitch, excite vibrations of different types in the 

 aether ; and " if by any means those [aether- vibrations] of 

 unequal bignesses be separated from one another, the largest 

 beget a Sensation of a Red colour, the least or shortest of a 

 deep Violet, and the intermediate ones, of intermediate colours ; 

 much after the manner that bodies, according to their several 

 sizes, shapes, and motions, excite vibrations in the Air of various 

 bignesses, which, according to those bignesses, make several 

 Tones in Sound."* 



This sentence is the first enunciation of the great principle 

 that homogeneous light is essentially periodic in its nature, and 

 that differences of period correspond to differences of colour. 

 The analogy with Sound is obvious ; and it may be remarked 

 in passing that Newton's theory of periodic vibrations in an 

 elastic medium, which he developed! in connexion with the 

 explanation of Sound, would alone entitle him to a place among 

 those who have exercised the greatest influence on the theory 

 of light, even if he had made no direct contribution to the 

 latter subject. 



* Phil. Trans, vii (1672), p. 5088. 



t Newton's Prmcipia, Book ii., Props, xliii.-l. 



