240 MICHAEL FARADAY — HIS LIFE AND WORKS. 



bine, in an equally ingenious and satisfactory manner, the mechanical phenomena 

 of electro-dynamics, discovered by Ampere, with the electrical phenomena due 

 to mechanical movement, discovered by Faraday. 



Ampere and Faraday : two names which will always be united by the inti- 

 mate relation of their works to the history of the science of electricity, in which 

 they have opened such new and vast horizons; and yet minds as dissimilar in 

 their mode of proceeding as similar in the power of their genius. Both eirii- 

 nently endowed with that faculty of divination which generates great discoveries, 

 but one of them, Faraday, arriving at them by impression, by a kind of instinct 

 which never deceived him, the other, Ampere, advancing with a more certain 

 step, having as his instrument those calculations which he handled with such 

 remarkable ability, and thus aniving at results which he hardly required experi- 

 ment to confirm, so certain was he that this would not contradict him. 



IV. I now pass to the last gi'eat series of Faraday's works. I have said, and, 

 1 think, proved, that induction was the most important of his discoveries ; I 

 must now say that the action of magnetism and electricity upon light was the 

 most brilliant. Often the attempt had been made to see whether magnetism and 

 electricity exerted any direct influence upon light ; but these attempts had always 

 failed. Investigators had operated upon luminous rays travelling in the air of 

 in liquids, and endeavored to act upon them, sometimes by strong magnets, somor 

 times 1)y electric currents or by statical electricity ; but these attempts had led 

 to nothing, absolutely nothing. All these negative investigations have never 

 been published, but they have nevertheless been made. 



Guided bj" theoretical considerations upon the mutual correlation of the forc-es 

 of nature, Faraday, after many fruitless attempts, succeeded in finding the con- 

 iiection wdiich exists between light and the magnetic and electric forces. Instead 

 of taking an ordinary ray, he operated with a polarized ray; instead of acting 

 directly upon this ray by means of a magnet, he submits it to the influence of 

 magnetism while it is traversing a glass prism in the direction of its lengtlv 

 This prism, terminated V)y two square and parallel bases, the surfaces of which 

 are well polished, and which are those by which the ].x>larized ray penetrates and 

 issues from tlie prism, is placed between the poles of an electro-magnet in such a 

 manner that its length and, consequently, the direction of the transmitted rav^ 

 are pai'allel to the line joining the magnetic poles. Lastly, the polarized ray on 

 issuing from the glass prism only reaches the eye after passing through a Nioors 

 prism, which serves as an analyzer. It is also by traversing a jS^icol's prism 

 before penetrating into the glass prism that the ray of light is polarized ; but 

 this may be eflected in any other manner. 



It is well known that by tm'uing the analyzing prism to a certain angle the 

 polarized ray is extinguished in such a manner that the brilliant s})ot is replaced 

 by a black spot. If, after this operation has been effected, a strong electric cur- 

 rent is passed through the wire suiTounding the electro-magnet, the black spot 

 disappears and the bright one again makes its appearance. Then by turning 

 tlie analyzing prism a little further in the same direction, the luminous ray Is 

 again extiiiguislied; but this extinction ceases as soon as the magnetic action ia 

 suppressed by the inten'uption of the current which magnetized the electrjo- 

 magnet. The action of magnetism, therefore, consists simply in causing the 

 plane of polarization to tiu'n by a certain angle, and to give artificially to tlic 

 glass, while it is under the magnetic influence, a property which certain §ii\y- 

 stances, such as quartz and essence of turpentine, possess naturally. 



Any transparent substance, except gases, may serve, although in difFcren:! 

 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 ghiss 

 (borosilicate of lead) which Faraday obtained in his experimental researches 

 upon the fabrication of glass for optical purposes. He happened to have a^ 

 hand several specimens of this glass ; and it was by using one of these for per- 



