﻿Mr. W. Spottiswoode on Successive Polarization of Light. 399 



of bringing light into the condition in question is by allowing it to 

 pass through a plate of the crystal called tourmalin, cut parallel to 

 the axis ; and if the light be then examined by causing it to pass 

 through a second similar plate, held parallel to the former and caused 

 to revolve like a wheel in its own plane, it will be found that the in- 

 tensity alternately diminishes and revives, being zero for two posi- 

 tions 180° from one another, and a maximum for two positions at 

 90° from each of the former. This combination of tourmalins con- 

 stitutes in fact a polariscope, which in general consists of two parts, 

 counterparts of each other- — the first for bringing common light into 

 the condition in question (in other words, for polarizing), the second 

 for examining or analyzing the light. 



The explanation of this fundamental phenomenon is as follows : — 

 The vibrations upon which the sensation of light depends, may in 

 ordinary light take place in any direction in a plane perpendicular 

 to the ray. By the process of polarization they are all brought into 

 one direction, still, however, perpendicular to the ray ; so that 

 throughout the entire ray they lie in one plane. On this account 

 the polarization here considered is called plane polarization. There 

 are other kinds of polarization, such as circular and elliptic, whose 

 names are derived from the curves, or orbits, described by the vibra- 

 ting particles. 



There are also other methods for producing plane polarization 

 beside that above described (e. g. reflection at particular angles from 

 the surfaces of transparent media, transmission through parallel plates 

 of glass, &c); but as they all agree in reducing common light to the 

 same condition, it is unnecessary for the present purpose to allude 

 to them more in detail. 



If a ray of polarized light fall upon a plate of doubly-refracting 

 crystal, it is divided into two, whose vibrations lie in planes perpen- 

 dicular to one another. These rays traverse the crystal with different 

 velocities, and therefore emerge with a difference of phase. On 

 entering the analyzer, the vibrations of both rays are resolved into 

 one plane. If the plane of vibration of the analyzer be parallel to 

 one of those of the crystal, one ray will be cut off, the other will be 

 transmitted without change. In any other position of the analyzer 

 the transmitted portions of the two rays will interfere so as to pro- 

 duce colour ; and if the analyzer be then turned through 90°, the 

 portion of the original light cut off in the first position will be trans- 

 mitted, and vice versd. 



Of this theory the following are the experimental results : — If a 

 plate of doubly-refracting crystal, e. g. selenite, be placed between 

 the polarizer and analyzer, and turned round in its own plane, it 

 will be found that in certain positions at right angles to one another 

 no effect is produced. These may be called neutral positions. In 

 all other positions the field is tinted with colour, which is most 

 brilliant when the plate has been turned through 45° from a neutral 

 position. If the analyzer be turned, the crystal remaining still, the 

 colour will fade, and entirely vanish when the angle of turning 

 amounts to 45°. From this position the complementary colour will 



