ROTATION OF A RAY OF LIGHT BY MAGNETISM. 
5 
2161. If a cork or a cylinder of glass, representing the diamagnetic, be marked at 
its ends with the letters N and S, to represent the poles of a magnet, 
the line joining these letters may be considered as a magnetic line of 
force ; and further, if a line be traced round the cylinder with arrow 
heads on it to represent direction, as in the figure, such a simple model, 
held up before the eye, will express the whole of the law, and give 
every position and consequence of direction resulting from it. If a watch be con- 
sidered as the diamagnetic, the north pole of a magnet being imagined 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 proportion 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 inter- 
posed different pieces of the same heavy glass (2151.) between the poles. The greater 
the extent of the diamagnetic 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 rotation 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. 
2164. 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 appears to 
be directly proportionate to the intensity of the magnetic 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 turpentine, sugar, 
tartaric acid, tartrates, &c., the effect of the magnetic 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 magnetism and the light. 
2167- The interposition of copper, lead, tin, silver, and other ordinary non-mag- 
netic bodies in the course of the magnetic curves, either between the pole and the 
diamagnetic, or in other positions, produced no effect either in kind or degree upon 
the phenomena. 
2168. Iron frequently affected the results in a very considerable degree; but it 
always appeared to be, either by altering the direction of the magnetic lines, or dis- 
