By means of certain doubly refracting crystals, of which tourmalin is the best ex ample, it is possible to obtain a single plane polarized ray of light without recourse to any artificial device for the separation of the two rays. Such crystals possess the property of absorbing one of the polarized com ponents much more strongly than the other, so that when light is seen through a layer of moderate thickness a considerable percentage of one of the components is transmitted while the other component is completely absorbed.
The use of tourmalin has the disadvantage that the intensity of the transmitted ray is greatly reduced and that owing to the selective charac ter of the absorption within the crystal the composition of the light is greatly modified. Thus white light transmitted by tourmalin is changed by the almost complete absorption of the blue and violet rays to a yellowish green color.
Polariscopes.—An instrument by means of which the investigation of the properties of polarized light and of the numerous effects ob tained with the same is carried on is called a polanscope. In its simplest form this instru ment consists of two parts. The function of the first, which is called the polarizer, is to pro duce plane polarized light ; that of the second to enable one to determine the direction of the plane of polarization of the ray thus produced. This part is called the analyzer. The most con venient form of polariscope for many purposes consists of two Nicol prisms P and A, Fig. 5, mounted in a common axis, each of which is capable of rotation about this axis. The posi tion of the polarizing plane of each of these prisms is indicated by means of a pointer V attached to the prism and moved as the prism is turned around a graduated circle. The polar ized ray produced by the passage of light through the first Nicol prism enters the second where it is separated into two components, one of which, the extraordinary, is polarized in the polarizing plane of this prism, while the other is polarized at right angles to that plane. The intensity of these components depends upon the angle which its polarizing plane makes with thepolarizing plane of the first Nicol prism. When these two polarizing planes are parallel the ordinary component in the second prism becomes equal to zero and the whole of the polarized ray is transmitted, subject only to losses by reflection. As the angle be tween the polarizing planes of the two prisms increases, the intensity of the extraordinary or transmitted rays falls off and that of the ordi nary ray increases until, When this angle reaches 90 degrees, the intensity of the ex traordinary ray is reduced to zero and no light is transmitted. Whatever be the angle between the polarizing planes of the two prisms the intensity of the ray transmitted by the analyzer is proportional to I cost a where I is the In tensity of the incident ray and a is the angle between the polarizing planes. When the angle is 90 degrees, so that no light is transmitted, the prisms are said to be crossed.
When between the polarizer (P) and analy zer (A) of a polariscope (Fig. 5), with prisms crossed, a layer (C) of any transparent doubly refracting crystal is placed there is restoration of light. Although as already explained the polarized beam from the first prism was un able to pass through the second, the insertion of the crystal between the two prisms has en abled it to do so. The explanation of this beau tiful phenomenon is as follows: The .beam of plane polarized light from the first Nicol, which we may suppose to be vibrat ing in a vertical plane, enters a medium when it reaches the crystal, in which in general light vibrating in two oblique positions at right angles to each other, and in those positions mly, is transmitted. The beam then is resolved into two components havi these oblique direc tions of vibration. The light which reaches the analyzer after passing rough the layer of crystal is vibrating in these two oblique planes each of which has a horizontal component which is capable of passing through the prism. Not the whole of the light from the polarizer is transmitted by the analyzer, the vertical com ponent of each of the oblique rays being sup pressed.
A great variety of crystalline substances, such as quartz, mica, selenite, calcite and in deed all doubly refracting substances, are cap able of producing this effect. If the crystalline layer be of proper thickness not only do we have.transmission of light but likewise a change of white light into light of other colors pro duced by the interference of the two compo nents after passage through the second prism. This production of colors by means of a polar iscope is one of the most striking phenomena known in optics. The two polarized rays in the crystal, the ordinary and the extraordinary rays, move with different velocities. We have, therefore, emerging from the crystal two simi lar rays, one of which is somewhat behind the other on account of its greater retardation and which consequently differs from it in phase. So long as these rays have planes of vibration at right angles to one another they cannot interfere, but in the second prism those por tions of each which are transmitted vibrat ing in the same plane and interference becomes possible. The result of this interference is the destruction of certain of the wave-lengths of which the light is made up. If the incident beam consist of white light that emerging from the analyzer will be white minus the particular constituent or color which has been destroyed by the interference and the beam will have a color other than white. The character of the color of the ray will depend upon the thickness of the crystal layer since it is this which de termines the degree of relative retardation of the two components passing through it. If the thickness of the crystal be everywhere the same, the color produced in different parts of it will be uniform, but difference in thickness gives change of color so that the field of view is in many instances most varied and striking.