Page:Elementary Text-book of Physics (Anthony, 1897).djvu/393

§ 313] absorbed the waves in different degrees according as the grain of the wood was parallel with the electromotive forces of the waves or was transverse to them.

Trouton observed that, when the electromagnetic waves were directed obliquely against a thick stone wall, the waves were, in general, partly reflected and partly transmitted. The ratio between the intensity of the reflected and transmitted waves depended upon the obliquity and upon the angle between the direction of the electromotive forces in the waves and a plane containing the normal to the incident waves and the normal to the reflecting surface. For a certain obliquity, the incident waves were entirely reflected, in case the electromotive forces in the waves were at right angles to the plane of incidence, or were parallel with the reflecting surface. In this case there was no transmitted wave. When, with the same obliquity, the electromotive forces in the waves were in the plane of incidence, there was no reflected wave and the incident wave was entirely transmitted. These properties are exactly analogous to those exhibited by the reflection and refraction of polarized light (§ 377). Trouton found that he could not obtain similar action from sheets of window-glass. These laws of reflection and refraction, and the impossibility of obtaining reflections and refractions consistent with them when the wave-length is long in comparison with the thickness of the reflecting body, are consistent with theory.

Several observers have determined the velocity of the electromagnetic waves in various dielectrics in comparison with their velocity in air. According to Maxwell's theory the ratio of the velocity in air to the velocity in the dielectric, or the index of refraction of the dielectric (§ 334), is equal to the square root of the dielectric constant. This conclusion of theory has been verified in very many cases.

The consideration of these experiments will be resumed in connection with the electromagnetic theory of light.