164 PROGRESS OF SCIENCE IN THE CENTURY 



period in which it had still to struggle for existence, 

 when it had to justify itself in application to the 

 phenomena of shadows, double refraction, polarisa- 

 tion, colour, interference, diffraction and so on. 



With Young, Fresnel, Arago, and others on the 

 winning side, with Laplace, Biot, and Brewster and 

 others championing the older doctrine, a keen, some- 

 times painfully bitter, struggle of opinions continued 

 till the century had run more than a quarter of its 

 course. 



Joule. — It should not be forgotten that Joule, who 

 contributed so much to the foundation of the dy- 

 namical theory of heat and the kinetic theory of 

 gases, and founded the general doctrine of the con- 

 servation of energy, also made an important experi- 

 ment (1843) bearing on the theory of Light. "He 

 compared the heat evolved in the wire conducting a 

 galvanic current, when the wire was ignited by the 

 passage of the current, with that evolved w^hen (with 

 an equal current, suppose) it was kept cool by immer- 

 sion in water. These experiments showed a small^ 

 but unmistakable, diminution of the heat when light 

 also w^as given out." * 



Foucault. — It was not, however, till 1850 that an- 

 other crucial experiment in favour of the undulatory 

 theory was announced by Foucault (1819-1868). 

 According to the emission-theory the velocity of 

 light should be greater in an optically denser me- 

 dium ; according to the undulatory theory the reverse 

 should be true. By an ingenious and now familiar 

 device, Foucault, the inventor of the gyroscope and 

 the demonstrator of the Earth's rotation by pendulum 

 experiments, gave the death-blow to the Newtonian 



* Tait. Recent Advances, 1876, p. 64. 



