204 IJNDULATOliY THEORY OF LIGHT. 



decrements of the other, unci each becoming periodically zero. It accordingly 

 follows that if two circularly polarized rays, whose molecular gyrations arc 

 performed in opposite directions, be thrown together in a nearly comnwu direc- 

 tion, and observed by means of an analyzer, fringes of interference may be de- 

 tected in the two principal azimuths, those in one of these azimuths being com- 

 plementary to those in the other. 



Mr. Babinet made this observation, employing Arago's prism to produce in- 

 terfering pencils of plane polarized light ; which pencils he polarized circularly 

 by means of " quarter-wave lamince " of mica placed in their paths. The gyra- 

 tions were made of opposite kmds in the two pencils, by placing the two laminae 

 so that their principal planes should be at right angles to each other. As an 

 analyzer, he employed a doubly refracting prism. The fringes immediately 

 appeared ; thus furnishing a very interesting experimental corroboration of a 

 theoretic anticipation. 



In the year 1845 Mr. Faraday communicated to the Royal Society of London 

 a very remarkable discovery which he had made, of the apparent influence of 

 7nagnctism upon light. If any homogeneous transparent body be placed under 

 the influence of a powerful electro-magnet, it will possess the property, whiln 

 the magnetism is maintained, of turning the plane of a ray of polarized light 

 traversing it in the direction of a line joining the magnetic poles, in the same 

 manner as such a ray is turned by quartz, or by liquids possessing the property 

 of rotatory polarization. Mr. Faraday was at first disDOsed to attribute this effect 

 to a direct action of magneti;im on light, but that idea is now abandoned ; and 

 the received opinion on the subject supposes that the molecules of the medium 

 undergo some modification during the continuance of the magnetic influence, 

 which assimilates their action upon the ether to that of substances which pos- 

 sess permanently the power of rotatory polarization. The direction in which the 

 plane of polarization was turned in these experiments depended on the direction 

 of the electric currents. When the cun-ents were reversed, the rotation was 

 reversed also. It is impossible in this place to do more than to allude to this 

 interesting discovery. 



^ X. CHROMATICS OF POLARIZED LIGHT. 



We will now proceed to apply the principles we have been considering to the 

 '•xplanation of the colors produced in doubly refracting substances by polarized 

 lio'ht. We have seen that double refraction consists in the generation of two 

 waves of unequal velocity and of dissimilar form in the doubly refracting body. 

 We have also seen that the molecular movements in the two waves are at right 

 ano-les to each other. In consequence of the inequality of velocity the two rays 

 into which a doubly refracting body divides a single incident ray may emerge 

 from a surface opposite and parallel to the surface of incidence in different 

 phases. If not entirely separated by the deviation of their paths, they may 

 thus, so far as 'plia&c is concerned, be in condition to interfere. But we hav*- 

 seen that interference is impossible between waves whose molecular movements 

 are perpendicular to each other. If, then, by any contrivance, we can turn the 

 planes of polarization of two rays which, by double refraction, have been made 

 to differ in llieir length of path by half an undulation, or by any odd number 

 (if half undulations, so that these planes shall coincide, interference will be pro- 

 duced. It is this which is done in the arrangements wliich have been described, 

 by which the gorgeous colors first observed by Arago in plates of doubly re- 

 fi'acting crystals, are made to appear. In the first place, the lamina must be 

 doubly refracting, in order that there may be two rays. In the second place, 

 it must be thin, that the difterence of length of path may be small. In the third 

 place, the original light must be polarized, otherwise there will be two systems 

 of interferences compensating each other, and obliterating each other's effects. 



