Dr. Faraday on the Magnetic Affection qf Light. 155 



would be rotated to a certain jTjg g. 



amount. But if, instead of pro- 

 ceeding onwards at A, it were 

 reflected by the surface A F to 

 E, and were there observed, it 

 would be found to have received 



no rotation, for the effect produced in going from D to A would 

 be exactly compensated by its return from A to E. Or if the 

 reflexions were made more numerous and recurred at E F 

 and C, so that the ray should traverse the body five times, still 

 an amount of rotation equal only to that which its passage 

 once along the substance could effect would be finally pro- 

 duced. 



Such would not be the case if ABCD were a diamagnetic, 

 rotating the ray by means of magnetic force ; for then, which- 

 ever way the ray was passing, it would still be rotated in the 

 same direction in relation to the lines of force. So if observed 

 issuing forth at A, it would have an amount of rotation (which 

 we may call right-handed) equal to what one transit across 

 the diamagnetic could produce; if observed at E, it would 

 have an amount of left-handed rotation double the amount of 

 the first or unit quantity ; if observed at F, it would have three 

 times the first amount of right-handed rotation ; if observed 

 at C, four times the amount of left-handed rotation ; and at B 

 would possess five times the original amount of right-handed 

 rotation. 



This was confirmed by the result of an experiment. The 

 great magnet described in the Philosophical Transactions* 

 was employed, and the parallelopiped of glass (fig. 2) sub- 

 mitted to its powers; the direct ray, or that producing the first 

 image of the luminous object, acquired a right-handed rotation 

 equal to 12°. Moving the glass a little the second image was 

 brought into view, or that produced by the ray which had tra- 

 versed the glass three times, and its rotation was 36°. The 

 third image was then observed, and the rotation of the ray 

 producing it was 60°, as nearly as my rough apparatus could 

 measure angular quantities. The same general results were 

 obtained with the second piece of glass described. 



The experiment proves in a very striking manner, that 

 whichever way the ray of light is passing through the dia- 

 magnetic, the direction of its rotation depends essentially and 

 alone upon the direction of the lines of magnetic force. 



It also proves and manifests in a manner not to be mistaken, 

 the difference in this respect between the magnetic rotation of 



* 184S, p. 22, par. 2247. Phil. Mag. vol. xxviii. p. 398. 

 M 2 



