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 dis- 

 cerned. Huyghens did not establish the undulatory 

 theory, but he took the first firm step towards estab- 

 lishing it. Laying this theory at the root of the phe- 

 nomena of light, he went a good way towards showing 

 that these phenomena are the necessary outgrowth of 

 the conception. 



By analysis atid 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 ' a very quick motion that causes light, as 

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



