The Wave Theory of Light: Memoirs of Huygens, Young and Fresnel, Vol. 1 (Classic Reprint)

Kirchhoff, Kelvin, Huxley, and others, there is now a widespread opinion that any physical phenomenon is explained only when some one has devised a dynamical model which will duplicate the phenomenon. The completeness of the explanation is to be measured by the completeness with which the model will duplicate the phenomenon. Thus, for instance, a refraction model which, like that of A iry, describes only the path of the refracted ray when the incident ray is given, does not in any true sense explain how the refracted ray comes to take one path rather than another. Such a model illustrates Suell slaw, but does not explain the phenomenon. I f, however, we take a large and shallow tank of water, the floor of the tank being partly covered with a false bottom, so as to give two, and only two, different depths of water, we shall find that the speed of the waves in the deeper portion of the tank bears to the speed in the shallower portion a constant ratio ;hence, in passing from one depth to the other, these waves are refracted according to the sine law. Such a model may be said to be a partial explanation of refraction in so far as it refers the phenomenon to change of speed which accompanies change of medium. It represents, however, only the kinematics of refraction. I f, now, we could go one step further, and make a model in which the wave-producing forces were duplicated in other words, if we could make a model of the medium and of the disturbing forces we should have a fairly complete explanation of refraction; in fact, the dynamics of refraction would be understood. This would imply not only that we knew the substance disturbed, but also that we were acquainted with the laws according to which it is disturbed.
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