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 sometimes 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, he 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 
