M. Pouillet on the Recent Researches of Prof. Faraday. 325 



it is that which relates to the action of an electro-magnet on a 

 ray of polarized light in turning its plane of polarization either 

 to the right or left, according to the relative directions of the 

 luminous ray and of the resultant of the magnetic actions. 



This fact is justly considered by Faraday as a fundamental 

 one, for hitherto there is nothing analogous in science ; and 

 it constitutes of itself a discovery of the very highest import- 

 ance. Undoubtedly many persons have hastened to repeat 

 and investigate it in order to ascertain its perfect accuracy, 

 and to find out the most marked characters and the most 

 essential conditions. Immediately after having read the Philo- 

 sophical Magazine I set to work, as I stated at the last meeting 

 of the Academy, but my first trials having been without re- 

 sult, and other persons not having been more fortunate than 

 myself in the attempt, it appeared to me necessary to resume 

 them with greater attention, varying the mode of experiment 

 and making up in the best manner I was able for the want 

 of precision in the directions which had come to my know- 

 ledge. 



I hasten to lay before the Academy today the result of 

 these researches, and with a twofold motive ; in the first place, 

 to render homage to the author of the discovery, and in the 

 next place, to furnish those physicists who may desire to 

 follow this new path of science with some indications that 

 may be of service to them, if, as I believe, they add anything 

 to what has hitherto been published on the subject. 



The apparatus employed by me* is composed, — 1st, of a Bun- 

 sen's battery; 2nd, of one or more electro-magnets ; 3rd, of M. 

 Soleil's instrument for exhibiting the least angular displace- 

 ments of the planes of polarization ; 4th, of the various sub- 

 stances to be submitted to examination. The elements of the 

 Bunsen's battery are of the ordinary size ; in the majority of 

 cases ten suffice to render the phenomenon perceptible ; but 

 to measure it and to compare the intensities with a certain 

 approximation, forty, fifty, and even 100 elements must be 

 employed. The electro-magnets are capable of supporting 

 1600 pounds when excited by a battery of twenty pairs. 

 They are soft iron cylinders, seven to eight centimetres in 

 diameter and about fifty centimetres in length, which are 

 curved in a horse-shoe form, the distance of the axes of the 

 two arms or poles being not more than fifteen to twelve centi- 

 metres. From 500 to 600 metres of copper wire, coated twice 

 with silk, are wound round each arm. The instrument of 

 M. Soleil is composed of two parts, one objective, the other 

 ocular. The objective part, or that which is turned towards 

 the light, is nothing more than a Nicol's prism, behind which 



