568 



or differ but little from the course of the ray in its passage through 

 the glass. It was then found that if the eye-piece had been so turned 

 as to render the ray invisible to the observer looking through the 

 eye-piece before the electric current had been established, it be- 

 comes visible whenever, by the completion of the circuit, the mag- 

 netic force is in operation ; but instantly becomes again invisible on 

 the cessation of that force by the interruption of the circuit. Fur- 

 ther investigation showed that the magnetic actioti causes the plane 

 of polarization of the polarized ray to rotate, for the ray is again 

 rendered visible by turning the eye-piece to a certain extent; and 

 that the direction of the rotation impressed upon the ray, when the 

 magnetic influence is issuing from the south pole, and proceeding 

 in the same direction as the polarized ray, is right-handed, or si- 

 milar to that of the motion of the hands of a watch, as estimated by 

 an observer at the eye-piece. The direction in which the rotation 

 takes place will, of course, be reversed by reversing either the 

 course of the ray or the poles of the magnet. Hence it follows that 

 the polarized ray is made to rotate in the same direction as the cur- 

 rents of positive electricity are circulating, both in the helices com- 

 posing the electro-magnet, and also in the same direction as the 

 hypothetical currents, which, according to Ampere's theory, circu- 

 late in the substance of a steel magnet. The rotatory action was 

 found to be always directly proportional to the intensity of the 

 magnetic force, but not to that of the electric current ; and also to 

 be proportional to the length of that portion of the ray which re- 

 ceives the influence. The interposition of substances which occa- 

 sion no disturbance of the magnetic forces, produces no change in 

 these effects. Magnets consisting only of electric helices act with 

 less power than when armed with iron, and in which magnetic ac- 

 tion is consequently more strongly developed. 



The author pursues the inquiry by varying in a great number of 

 ways the circumstances in which this newly-discovered influence is 

 exerted ; and finds that the modifications thus introduced in the re- 

 sults are all explicable by reference to the general law above stated. 

 Thus the effect is produced, though in a less degree, when the po- 

 larized ray is subjected to the action of an ordinary magnet, instead 

 of one that derives its power from a voltaic current ; and it is also 

 weaker when a single pole only is employed. It is, on the other 

 hand, increased by the addition of a hollow cylinder of iron, placed 

 within the helix, the polarized ray traversing its axis being then 

 acted upon v/ith great energy. Helices act with equal power in 

 any part of the cylindric space which they enclose. The heavy glass 

 used in these experiments was found to possess in itself, no specific 

 magneto-inductive action. 



Different media differ extremely in the degree in which they are 

 capable of exerting the rotatory power over a polarized ray of light. 

 It is a power which has no apparent relation to the other physical 

 properties, whether chemical or mechanical, of these bodies. Yet, 

 however it may differ in its degree, it is always the same in kind ; 

 the rotation it effects is invariably in one direction, dependent, how- 



