The Magic Lantern And Its Applications
By L. H. Laudy PH. D.
Attachments For Polarized Light.
Published in Anthony's Photographic Bulletin
Vol. 17. No. 9.
May 8, 1886
LIGHT which has been refracted from certain surfaces, or transmitted through certain substances under certain special conditions, assumes new properties, and is no longer reflected, refracted or transmitted, as is ordinary light. This change in the action of light is called polarization, and rays thus modified are said to be polarized. This phenomenon was discovered in 1808 by Malus, a young engineer officer of Paris.
According to the undulatory theory, a ray of common light contains vibrations in all planes passing through the ray. In polarized light the vibrations are reduced to one plane only. The term polarized, as applied to light, is an unfortunate one, but it is too firmly engrafted upon science to be changed for a better name, and we are still obliged to use it. It originated from the analogy of two ends of a magnet, which consist of poles; and as the beam of light was said to have sides and ends, it was called polarized. Strictly and scientifically speaking, it is light vibrating in one plane.
There are four principal methods by means of which a beam of light may be polarized: the are, reflection, ordinary refraction, double refraction, and absorption. To of these methods only can be used practically for purposes of projection with the lantern: they are double refraction and ordinary reflection. Any instrument used to polarize light is called a polarizer, and the instrument to examine the light polarized is called an analyzer, and either may be used to polarize or analyze. A large number of crystals possess the property of double refraction -- that is, a ray of light in passing through a double refracting crystal is divided into two rays -- hence an object seen through one of these crystals appears double (Fig. 36). Such a crystal possessing this property to a high degree is Iceland spar, and if we look at a dot or letter or line, a double image will be perceived, as two dots, two letters two lines, etc. Now the light in passing through is divided into two rays, one of which is called the ordinary, the other the extraordinary ray, and both are polarized. The ordinary image will seem slightly nearer to the eye than the extraordinary, and if the crystal to be turned round, the ordinary image will continue fixed, and the extraordinary will describe a circle around it.
The objection to the use of a natural rhombohedron of Iceland spar, is that it gives two rays, one of which must be suppressed. This was most perfectly accomplished by the invention of the Nicol prism, which is made by cutting a rhomb of Iceland spar along a diagonal plane, and then cementing the two pieces together in their natural position by Canada balsam, whose refractive index is intermediate between the ordinary and extraordinary indices of the crystal. A ray of common light suffers double refraction on entering the prism, but the ordinary ray is totally reflected on meeting the first surface of the balsam, and is thrown to one side, while the extraordinary ray passes through the balsam, and emerges at the end of the prism parallel to the original ray (Fig. 37). The lateral faces of this compound prism are all painted black, leaving only the terminal faces for the transmission of the light. The ordinary ray suffers total reflection , and is absorbed by the blackened side of the prism.
The Nicol prism is the most valuable means of polarizing light; yet, while it more perfectly polarized the light than the bundle of glass plates, it produces a small beam or circle, which is not governed by the distance of the lantern from the screen, but by the size of the prism.
