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about another half century led up to Newton's 

 " Principia," from which we trace all the develop- 

 ments of modern astronomical science. 



Another illustration may be drawn from the two 

 theories as to the phenomena of light. Hooke had 

 suggested that the eye should be supposed to receive 

 the stimulus of light from the luminous object 

 through the modifications of an intervening medium ; 

 modifications which might be compared to undu- 

 lations. Newton examined this theory and even 

 proposed certain improvements in its details, but 

 ultimately rejected it in favour of the theory that 

 the stimulus is conveyed to the eye by means 

 of material corpuscles emitted from the luminous 

 body. Either theory provided an intelligible syntax 

 of such phenomena as reflexion, refraction and the 

 like ; but in the seventeenth century no syntax had 

 been found connecting undulations of light with the 

 colours of thin plates ; moreover, Newton was 

 particularly impressed by the fact that waves of 

 light had not been observed to lap round obstacles 

 in the same way as waves of sound. At the time, 

 therefore, it appeared that the observed phenomena 

 could be co-ordinated in a clearer and more direct 

 manner by the corpuscular than by the undulatory 

 theory, and, although the latter was supported by 

 Huygens, it was not generally accepted. At the 

 beginning of the nineteenth century, however, Young 

 and Fresnel turned their attention to the polarisation 



