Nov. 1845.] Rotation of a Hay of Light by Magnetism. 299 



against the face, and a south pole against the back, then the 

 motion of the hands will indicate the direction of rotation 

 which a ray of light undergoes by magnetization. 



2162. I will now proceed to the different circumstances 

 which affect, limit, and define the extent and nature of this 

 new power of action on light. 



2163. In the first place, the rotation appears to be in pro- 

 portion to the extent of the diamagnetic through which the 

 ray and the magnetic lines pass. I preserved the strength of 

 the magnet and the interval between its poles constant, and 

 then interposed different pieces of the same heavy glass (2151.) 

 between the poles. The greater the extent of the diamag- 

 netic in the line of the ray, whether in one, two, or three 

 pieces, the greater was the rotation of the ray ; and, as far as 

 I could judge by these first experiments, the amount of rota- 

 tion was exactly proportionate to the extent of diamagnetic 

 through which the ray passed. No addition or diminution of 

 the heavy glass on the side of the course of the ray made any 

 difference in the effect of that part through which the ray 

 passed. 



21 64. The power of rotating the ray of light increased with 

 the intensity of the magnetic lines of force. This general 

 effect is very easily ascertained by the use of electro-magnets; 

 and within such range of power as I have employed, it ap- 

 pears to be directly proportionate to the intensity of the mag- 

 netic force. 



2165. Other bodies, besides the heavy glass, possess the 

 same power of becoming, under the influence of magnetic 

 force, active on light (2173.). When these bodies possess a 

 rotative power of their own, as is the case with oil of turpen- 

 tine, sugar, tartaric acid, tartrates, &c, the effect of the mag- 

 netic force is to add to, or subtract from, their specific force, 

 according as the natural rotation and that induced by the 

 magnetism is right- or left-handed (2231.). 



2166. I could not, perceive that this power was affected by 

 any degree of motion which I was able to communicate to the 

 diamagnetic, whilst jointly subject to the action of the mag- 

 netism and the light. 



2167. The interposition of copper, lead, tin, silver, and 

 other ordinary non-magnetic bodies in the course of the mag- 

 netic curves, either between the pole and the diamagnetic, or 

 in other positions, produced no effect either in kind or degree 

 upon the phaenomena. 



2168. Iron frequently affected the results in a very consi- 

 derable degree ; but it always appeared to be, either by alter- 

 ing the direction of the magnetic lines, or disposing within 



