From the first and second of these statements it is plain that the vibration in the polarized beam must be transverse to the direction of propagation, for if it were otherwise, there would be some interference of the two rays, even when they are polarized at right angles to each other.
We may here consider the nature of common light. The peculiarity of common light is that it furnishes two images of equal intensity when it passes through a doubly refracting crystal, and that it cannot produce colored fringes when passed through a crystal plate and examined with an analyzer (§ 379). These peculiarities can be explained by supposing that the direction of vibration, in the wave frequently changes. On the other hand, the interference of common light proves that this change of direction does not occur in every wave. In the experiments of Michelson and Morley interference was obtained between two beams of light of which the difference in path was 200,000 wave lengths. Such interference could not have occurred if the direction of the vibration had changed during the time taken by light to traverse that distance. We are accordingly compelled to assume that the vibrations of common light are polarized in one plane for a very short time, which is, however, sufficiently long for the light to execute a large number of vibrations in it, and that at certain intervals the plane of polarization changes its direction.
379. Polariscopes.—In experimenting with polarized light we need a polarizer to produce the polarized beam, and an analyzer to show the effects of the polarization. A piece of plane glass, reflecting light at the polarizing angle, is a simple polarizer. Doubly refracting crystals, if means be employed to suppress one of the beams into which the light is divided, are excellent polarizers. Tourmaline is a doubly refracting crystal which has the property
