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. Spectra more beautiful and intense than the prismatic image,--systems of rings far excelling those of thin plates,--and forms of the most symmetric order, are constantly presenting themselves, as the polarized ray is passed through various transparent substances; the path of the ray indicating whether the crystal has been formed round a single nucleus or axis, or whether it has been produced by aggregation around two axes. The coloured rings, and the dark or luminous crosses which distinguish the path of the polarized ray, are respectively due to different states of tension amongst the particles, although those differences are so slight, that no other means is of sufficient delicacy to detect the variation.