Precisely the same result takes place when, instead of being reflected, the polarised ray is transmitted.
Some substances have peculiar polarizing powers: the tourmaline is a familiar example. If a slice of tourmaline is taken, and we look at a common pencil of light through it, we see it in whatever position we may place the transparent medium. If, however, we look at a pencil of polarised light, and turn the crystal round, it will be found that in two positions the light is stopped, and that in two other positions it passes freely through it to the eye.
By way of endeavouring to conceive something of what may be the conditions which determine this very mysterious state, let us suppose each ray of light to vibrate in two planes at right angles to each other: one wave being vertical and the other horizontal. We have many examples of this compound motion. The mast of a ship, by the force with which she is urged through the water, describes a vertical wave, while by the roll of the billows across which she sails, a lateral undulation is produced at the same time. We may sometimes observe the same thing when a field of corn is agitated by a shifting wind on a gusty day.
The hypothesis therefore is, that every ray of ordinary light consists of two rays vibrating in different planes; and that these rays, separated one from the other, have the physical conditions which we call polarized.
The most transparent bodies may be regarded as being made up of atoms arranged in certain planes. Suppose the plane of lamination of any substance to be vertical in position, it would appear that the ray which has a vertical motion passes it freely, whereas if we turn the body round so that the planes of lamination are at right angles to the plane of vibration of the ray, it cannot pass.
That some action similar to that which it is here endeavoured to express in popular language does take place, is proved by the correctness of the results deduced by rigid mathematical analyses founded on this hypothesis.
There are two other conditions of the polarization of light—called circular and elliptical polarization. The first is produced by light when it is twice reflected from the second surface of bodies at their angle of maximum polarization, and the second by reflexions from the surfaces of metals at angles varying from 70° 45' to 78° 30'. The motion of the wave in the first is supposed to be circular, or to be that which is represented by looking along the centre of a corkscrew as it is turned round. At every turn of the medium effecting circular polarization the colour of the ray of light is changed after a uniform order. If turned in one direction, they change through red, orange, yellow, green, and violet; and if in the other direction, the colours appear in the contrary order.
The variety of striking effects produced by the polarization of light; the unexpected results which have sprung from the investigation of the laws by which it is regulated; and the singular beauty of many of its phenomena, have made it one of the most attractive subjects of modern science.
Ordinary light passes through transparent bodies without producing any very striking effects in its passage; but this extraordinary beam of light has the power of insinuating itself between the molecules of bodies, and by illuminating them, and giving them every variety of prismatic hue, of enabling the eye to detect something of the structure of the mass. The chromatic phenomena of polarized light are so striking, that no description can convey an adequate idea of their character.