Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/43

 culièrement dans l'étrange réfraction du cristal d'Islande, Par C.H.D.Z

The truth of Hooke's hypothesis, that light is essentially a form of motion, seemed to Huygens to be proved by the effects observed with burning-glasses; for in the combustion induced at the focus of the glass, the molecules of bodies are dissociated; which, as he remarked, must be taken as a certain sign of motion, if, in conformity to the Cartesian philosophy, we seek the cause of all natural phenomena in purely mechanical actions.

The question then arises as to whether the motion is that of a medium, as is supposed in Hooke's theory, or whether it may be compared rather to that of a flight of arrows, as in the corpuscular theory. Huygens decided that the former alternative is the only tenable one, since beams of light proceeding in directions inclined to each other do not interfere with each other in any way.

Moreover, it had previously been shown by Torricelli that light is transmitted as readily through a vacuum as through air; and from this Huygens inferred that the medium or aether in which the propagation takes place must penetrate all matter, and be present even in all so-called vacua.

The process of wave-propagation he discussed by aid of a principle which was now introduced for the first time, and has since been generally known by his name. It may be stated thus: Consider a wave-front, or locus of disturbance, as it exists at a definite instant t0: then each surface-element of the wave-front may be regarded as the source of a secondary wave, which in a homogeneous isotropic medium will be propagated outwards from the surface-element in the form of a sphere whose radius at any subsequent instant t is proportional to (t-t0); and the wave-front which represents the whole distur-