in the Seventeenth Century. 17 



doctrine that light is a material substance. Now Newton had, as 

 a matter of fact, a great dislike of the more imaginative kind of 

 hypotheses ; he altogether renounced the attempt to construct 

 the universe from its foundations after the fashion of Descartes, 

 and aspired to nothing more than a formulation of the laws 

 which directly govern the actual phenomena. His theory of 

 gravitation, for example, is strictly an expression of the results 

 of observation, and involves no hypothesis as to the cause of the 

 attraction which subsists between ponderable bodies ; and his 

 own desire in regard to optics was to present a theory free from 

 speculation as to the hidden mechanism of light. Accordingly, 

 in reply to Hooke's criticism, he protested* that his views on 

 colour were in no way bound up with any particular conception 

 of the ultimate nature of optical processes. 



Xewton was, however, unable to carry out his plan of 

 connecting together the phenomena of light into a coherent 

 and reasoned whole without having recourse to hypotheses. The 

 hypothesis of Hooke, that light consists in vibrations of an 

 aether, he rejected for reasons which at that time were perfectly 

 cogent, and which indeed were not successfully refuted for over 

 a century. One of these was the incompetence of the wave- 

 theory to account for the rectilinear propagation of light, and 

 another was its inability to embrace the facts discovered, as 

 we shall presently see, by Huygens, and first interpreted 

 correctly by Newton himself of polarization. On the whole, 

 he seems to have favoured a scheme of which the following may 

 be taken as a summaryf : 



All space is permeated by an elastic medium or aether, which 

 is capable of propagating vibrations in the same way as the 



*Phil. Trans, vii, 1672, p. 5086. 



t Cf. Newton's memoir in Phil. Trans, vii, 1672 ; his memoir presented to the 

 Royal Society in December, 1675, which is printed in Birch, iii, p. 247; his 

 Opticks, especially Queries 18, 19, 20, 21, 23, 29; the Scholium at the end of 

 the Principia ; and a letter to Boyle, written in February, 1678-9, which is printed 

 in Horsley's Newtoni Opera, p. 385. 



In the Principia, Book I., section xiv, the analogy between rays of light and 

 streams of corpuscles is indicated ; but Newton does not commit himself to any 

 theory of light based on this. 



C 



