THE SKY. 133 



of fact, and refines and alters them to suit his needs. 

 The sensation of sound was known to be produced by 

 aerial waves impinging on the auditory nerve. Air being 

 a thing that could be felt, and its vibrations, by suitable 

 treatment, made manifest to the eye, there was here a 

 physical basis for the ' scientific imagination ' to build 

 upon. Both Hooke and Huyghens built upon it with 

 effect. By the illustrious astronomer last named the 

 conception of waves was definitely transplanted from 

 its terrestrial birthplace to a universal medium whose 

 undulations could only be intellectually discerned. 

 Huyghens did not establish the undulatory theory, but 

 he took the first firm step towards establishing it. 

 Laying this theory at the root of the phenomena of 

 light, he went a good way towards showing that these 

 phenomena are the necessary outgrowth of the concep- 

 tion. 



By analysis and synthesis Newton proved the white 

 light of the sun to be a skein of many colours. The 

 cause of colour was a question which immediately oc- 

 cupied his thoughts ; and here, as in other cases, he 

 freely resorted to hypothesis. He saw, with his mind's 

 eye, his luminiferous corpuscles crossing the bodily eye, 

 and imparting successive shocks to the retina behind. 

 To differences of ' bigness ' in the light-awakening 

 molecules Newton ascribed the different colour-sensa- 

 tions. In the undulatory theory we are also confronted 

 with the question of colour ; and here again, to inform 

 and guide us, we have the analogy of sound. Aerial 

 waves of different lengths, or periods, produce notes of 

 different pitch ; and to differences of wave-length in 

 that mysterious medium, the all- pervading ether, dif- 

 ferences of colour are ascribed. Hooke had already 

 discoursed of 6 a very quick motion that causes light, as 

 well as a more robust that causes heat.' Newton had 



