Page:Collected Physical Papers.djvu/396

376 2.04, which is much higher than the index 1.53 for the sodium light. The index for sulphur is 1.73 (pp. 21, 31).

On account of extreme shortness of wave-length of the visible rays, the thinnest air-film produces total reflection at the critical angle. The case is, however, different with comparatively greater wave-length of electric radiation. The critical thickness of air-space is modified by the angle of incidence and by the wave-length. When a cube of glass is interposed between the radiator and receiver placed opposite to each other, the radiation striking one face perpendicularly is transmitted across the opposite without deviation and causes response of the receiver. On cutting the cube across a diagonal, two right-angled isosceles prisms are obtained. When the two prisms are separated slightly, keeping the two hypoteneuses parallel, thus securing parallel air-space, the incident radiation is found divided into two complementary portions, of which one is transmitted and the other reflected by the air-film at right angles to the incident ray. When the thickness of air-space is reduced to about 0.3 mm., there is a complete transmission and no reflection. The two prisms in spite of the breach due to the air-space, are electro-optically continuous. A greater separation of the two prisms increases the reflected, at the expense of the transmitted portion; and at a certain critical thickness, there is no transmission but total reflection. If a thin piece of cardboard or any other refracting substance be next interposed, a portion of the radiation becomes transmitted, necessitating further separation of the prisms to reduce the transmitted portion to zero. This offers