242 MICHAEL FARADAY — HIS LIFE AND WORKS. 



forming the experiment just described that he discovered the magnetic rotation 

 of the ])lane of poLirization, a phenomenon which would prohably have escaped 

 him if he had made use of ordinary ghiss at first starting. Thus the long and 

 painful labors to which he had formerly devoted himself without any great suc- 

 cess, in order to discover a glass fitted for the fabrication of lenses, were not 

 lost to science, since they facilitated his em-iching it with one of his finest dis- 

 coveries. 



Let us now study the new phenomenon a little more closely, so as better to 

 sl^ow all its importance. Some substances, we have said, naturally possess the 

 property of causing the plane of polarization of a polarized ray traversing them 

 to rotate through a larger or smaller angle ; some cause it to turn to the right, 

 and others to the left, of the obseiwer. The discoveiy of Faraday was that the 

 influence of magnetism or of electric currents develops this same property in 

 nearly all transparent substances, but with this difference, that the direction of 

 rotation of the plane of polaiization depends only upon the position of the 

 magnetic poles, or the direction of the currents with relation to the transparent 

 substance. The law is, that if the north pole of the electro-magnet is placed 

 on the vsarae side as the observer who receives the ray into his eye, and conse- 

 quently the south pole on the side by which the polarized ray enters into the 

 substance, the rotation of the plane of polarization takes place, to the observer, 

 from left to right. It takes place from right to left if the direction of the cur- 

 rent, and consequently that of the magnetization, be changed. The action of 

 the magnet may be replaced l)y that of a coil in the axis of which the trans- 

 parent substance is placed. In this case, again, the rotation of the plane of 

 polarization is very well observed when a rather strong current is transmitted 

 through the wire of the coil ; and the direction of the rotation is always tlie 

 same as that of the current. 



Thus, whilst in substances naturally endowed with circular polarization the 

 rotation of the plane of polarization always takes place, according to the nature 

 of the substance, either to the right or left of the observer, in Faraday's experi- 

 ment the direction of this rotation only depends upon the direction of electric 

 currents or the relative position of the magnetic poles, since it is completely 

 independent of the position of the observer. These two kinds of action are 

 therefore not identical, and we cannot say that by the influence of the magnet < 

 or of electricity we produce in all transparent bodies exactly the same property 

 that certain substances naturally possess. Faraday well shows this ditlerence 

 by an experiment which consists in producing, by an ingenious artifice, the inter- 

 nal reflection of the polarized ray upon the extreme surfaces of the prism ; this 

 may be done once or several times before the ray is allowed to escape, and 

 doubles, triples, or (j^uadruples tiie angle of rotation of the plane of polariza- 

 tion, according as the ray is reflected once, twice, or three times. But when, 

 instead of the magnetic, we have to do with the natural rotary polarization, the 

 result is quite different, the return of the reflected ray neutralizing the effect 

 which the direct ray had undergone while travelling in an opposite direction. 

 In this case the angle of rotation of the plane of polarization reflected twice, 

 and which consequently has. three times traversed the transparent substance, is 

 no greater than that of a ray which has only traversed it once. 



The general phenomenon so unexpectedly discovered by Faraday has hith- 

 erto remained unexplained, notwithstanding many investigations, and especially 

 the persevering and remarkable researches of M. Verdet. 



It has not even been possible to connect it with some other property of bodies, 

 although each substance has its specific magnetic rotatory power. Faraday, 

 however, drew from it a general consequence which led him to another dis- 

 covery, namely : that magnetism acts upon all bodies, since all transparent 

 bodies may be modified under its influence sufficiently to acquire, in difierent 

 degrees indeed, a power which they do not possess of themselves. The dis- 



