Michael Faraday, his Life and Works. 431 



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 experiment the direc- 

 tion 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 pro- 

 duce in all transparent bodies exactly the same property that 

 certain substances naturally possess. Faraday well shows this 

 difference by an experiment which consists in producing by an 

 ingenious artifice the internal reflexion 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 quadruples the angle of rotation of the plane of polar- 

 ization, according as the ray is reflected once, twice, or three 

 times. But when, instead of the magnetic, we have to do with 

 the natural rotatory 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 direc- 

 tion. In this case the angle of rotation of the plane of polari- 

 zation 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 hitherto remained unexplained, notwithstanding 

 many investigations, and especially the persevering and remark- 

 able 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 mag- 

 netic rotatory power. Faraday, however, drew from it a general 

 consequence which led him to another discovery, namely, that 

 magnetism acts upon all bodies, since all transparent bodies 

 may be modified under its influence sufficiently to acquire, in 

 different degrees indeed, a power which they do not possess of 

 themselves. The discovery to which I have just alluded is that 

 as the magnet acts by attraction upon magnetic bodies, it acts 

 also by repulsion upon all other bodies in nature. From this 

 it results that whilst a rod of iron, or of some other magnetic 

 substance, suspended between the poles of an electromagnet, 

 places itself axially (that is to say, parallel to the line which joins 

 the poles), a prism of heavy glass (the same, for example, which 

 served for the experiments on light) places itself equatorially 

 (that is to say, transversely to this line). A rod of bismuth is 

 in the same case; and this metal and heavy glass are the sub- 



