100 FRAGMENTS OF SCIENCE. 



repelled from the surfaces of bodies and sometimes per- 

 mitted to pass through. Newton may have scorned the 

 levity with which hypotheses are some times framed; but he 

 lived in an atmosphere of theory, which he, like all pro- 

 found scientific thinkers, found to be the very breath of 

 his intellectual life. 



The theorist takes his conceptions from the world 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 con- 

 ception of waves was definitely transplanted from its ter- 

 restrial 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 toward establishing it. Laying this theory at the root 

 of the phenomena of light, he went a good way toward 

 showing that these phenomena are the necessary out- 

 growth of the conception. 



By analysis and synthesis Newton proved the white light 

 of the sun to be a skein of many colors. The cause of 

 color was a question which immediately occupied his 

 thoughts; and here, as in other cases, lie freely resorted to 

 hypothesis. He saw, with his mind's eye, hisluminiferous 

 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 color-sensations. In the undulatory theory we 

 are also confronted with the question of color; 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, differences of color 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 

 ascribed the sensation of red to the shock of his grossest, 

 and that of violet to the shock of his finest luminiferous 

 projectiles. Defining the one, and displacing the other of 



