430 Prof. A. de la Rive's Memoir of 



interruption of the current which magnetized the electromagnet. 

 The action of magnetism therefore consists simply in causing 

 the plane of polarization to turn by a certain angle, and to give 

 artificially to the glass, while it is under the magnetic influence, 

 a property which certain substances, such as quartz and essence 

 of turpentine, possess naturally. 



Any transparent substance, except gases, may serve, although 

 in different degrees, as the medium for magnetism to act upon 

 the polarized ray. But that by means of which this influence 

 is best manifested is the yellowish heavy glass (borosilicate of 

 lead) which Faraday obtained in his experimental researches 

 upon the fabrication of glass for optical purposes. He happened 

 to have at hand several specimens of this glass ; and it was by 

 using one of these for performing the experiment just described 

 that he discovered the magnetic rotation of the plane of polari- 

 zation, a phenomenon which would probably have escaped him 

 if he had made use of ordinary glass at first starting. Thus the 

 long and painful labours to which he had formerly devoted him- 

 self without any great success, in order to discover a glass fitted 

 for the fabrication of lenses, were not lost to science, since they 

 facilitated his enriching it with one of his finest discoveries. 



Let us now study the new phenomenon a little more closely, 

 so as better to show 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 observer. The discovery of Faraday 

 was that the influence of magnetism or of electric currents 

 develope this same property in nearly all transparent sub- 

 stances, but with this difference, that the direction of rotation 

 of the plane of polarization 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 electromagnet is placed on the same side as the ob- 

 server who receives the ray into his eye, and consequently 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 current, and consequently 

 that of the magnetization, be changed. The action of the mag- 

 net may be replaced by that of a coil in the axis of which the 

 transparent 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 the same as that of the 

 current. 



