THE PROGRESS OF PHYSICS. 151 



While the corpuscular theory served to interpret a 

 number of the phenomena of light, it failed more or 

 less markedly in regard to others — for instance, the 

 reflection which accompanies refraction, the unequal 

 refrangibility of the different colours of the spec- 

 trum, double refraction, and so on. The result was 

 that subsidiary hypotheses had to be invented to cover 

 the defects of the main assumption. Eventually 

 it became necessary to discard the main assumption 

 altogether. 



Newton s Position, — The central idea of the un- 

 dulatory theory was suggested by Hooke and others, 

 and was formulated as early as 1678 by Huygens, 

 who interpreted double refraction, but its establish- 

 ment was due to the work of Thomas Young and 

 Fresnel. Although Descartes had suggested that 

 light is produced by waves excited in the subtle mat- 

 ter which pervades the universe (analogous to but 

 different from the non-atomic ether of to-day), and 

 had also ventured the suggestion that the mechanism 

 of light and that of gravitation are inseparable, and 

 although Hooke had made the important suggestion 

 of substituting for the progressive wave of Descartes 

 a vibrating one, we find Newton weighing the merits 

 of the wave-theory and the emission-theory, finding 

 both unsatisfactory and deliberately refraining from 

 accepting either. Apart from his " theory of fits,^' — 

 in which he states that the phenomena of thin plates 

 prove that the luminous ray is put alternately in a 

 certain state or fit of easy reflection and of easy 

 transmission — he abstains from taking up a definite 

 position, though " he shall sometimes, to avoid cir- 

 cumlocution and to represent it conveniently, speak 

 of it (the emission) as if he assumed it and pro- 

 pounded it to be believed." It does not seem to be 



