240 MICHAEL FARADAY — HIS LIFE AND WORKS. 



biuc, 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 alwaj's be united by the inti- 

 mate relation of their works to the history of the science of electricit}', 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 emi- 

 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 arriving 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 great series of Faraday's works. I have said, and, 

 I 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 or 

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

 times by 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 by theoretical considerations upon the mutual coiTclation of the forces 

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

 nection which exists between light and the magnetic and electnc forces. Instead 

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

 du-ectly 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 length. 

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

 are well polished, and which are those by which the })olarized ray penetrates and 

 issues J'rom the prism, is placed between the poles of an electro-magnet in such a 

 manner that its length and, consequently, the direction of the transmitted ray 

 are parallel 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 Nicol's 

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

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

 this may be effected in any other manner. 



It is well known that by turning the analyzing prism to a certain anigle the 

 polarized raj^ is extinguished in such a manner that the brilliant spot is replaced 

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

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

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

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

 again extinguished; but this extinction ceases as soon as the magnetic action is 

 suppressed by the interruption of the current which magnetized the electro- 

 magnet. The action of magnetism, therefore, consists simpl}' 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 sub- 

 stances, 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 obtaine-d 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 per- 



