352 A SHORT HISTORY OF SCIENCE 



How could wave-propagation be reconciled with sharply defined 

 shadows? Why should not light "go round a corner" as well as 

 sound ? These difficulties were met by Thomas Young who re- 

 vived Huygens' wave theory, which was definitively established 

 by FresnePs researches on refraction, beginning in 1815. 



The determination of the velocity of light by observations of 

 the moons of Jupiter has been mentioned already (page 286). 

 About 1850 this problem was solved by a new method devised by 

 Fizeau. A ray of light passes between the teeth of a wheel to a 

 mirror and back again. During the time required by the ray to 

 pass thus out and back, the gap through which it has passed may 

 have been just replaced by a tooth, in which case the light will be 

 intercepted. By measuring the speed of the wheel when varied in 

 a definite way the speed of the light ray may be determined. The 

 result agreed with that obtained by the astronomical method within 

 about .5 %. At almost the same time Foucault, by an ingenious 

 laboratory device, proved that light travels more slowly in water 

 than in air a result incompatible with the emission theory. 



The sporadic beginnings of a genuine kinetic view of natural 

 phenomena, after having been cultivated ... by Huygens and Euler, 

 and early in the nineteenth century by Rumford and Young, were 

 united into a consistent physical theory by Fresnel, who has been 

 termed the Newton of optics, and who consistently, and all but com- 

 pletely, worked out one great example of this kind of reasoning. He 

 has the glory of having not only established the undulatory theory of 

 light on a firm foundation, but still more of having impressed natural 

 philosophers with the importance of studying the laws of regular 

 vibratory motion and the phenomena of periodicity in the most general 

 manner. 



In astronomy and optics the suggestion of common sense, which 

 regards the earth as stationary and light as an emission travelling in 

 straight lines, had indeed allowed a certain amount of definite know- 

 ledge ... to be accumulated. A real physical theory, however, was 

 impossible until the notions suggested by common sense were com- 

 pletely reversed, and an ideal construction put in the place of a seem- 

 ingly obvious theory. This was done in astronomy at one stroke by 

 Copernicus; in optics only gradually, tentatively, and hesitatingly. 



