ROTATION OF A RAY OF LIGHT BY MAGNETISM. 
3 
2151. Sixteen years ago I published certain experiments made upon optical glass*, 
and described the formation and general characters of one variety of heavy glass, 
which, from its materials, was called silicated borate of lead. It was this glass which 
first gave me the discovery of the relation between light and magnetism, and it has 
power to illustrate it in a degree beyond that of any other body ; for the sake of per- 
spicuity I will first describe the phenomena as presented by this substance. 
2152. A piece of this glass, about two inches square and 05 of an inch thick, 
having flat and polished edges, was placed as a diamagnetic (2149.) between the poles 
(not as yet magnetized by the electric current), so that the polarized ray should pass 
through its length ; the glass acted as air, water, or any other indifferent substance 
would do; and if the eye-piece were previously turned into such a position that the 
polarized ray was extinguished, or rather the image produced by it rendered invisible, 
then the introduction of this glass made no alteration in that respect. In this state 
of circumstances the force of the electro-magnet was developed, by sending an electric 
current through its coils, and immediately the image of the lamp-flame became visible, 
and continued so as long as the arrangement continued magnetic. On stopping the 
electric current, and so causing the magnetic force to cease, the light instantly dis- 
appeared ; these phenomena could be renewed at pleasure, at any instant of time, 
and upon any occasion, showing a perfect dependence of cause and effect. 
2153. The voltaic current which I used upon this occasion, was that of five pair 
of Grove’s construction, and the electro-magnets were of such power that the poles 
would singly sustain a weight of from twenty-eight to fifty-six, or more, pounds. A 
person looking for the phenomenon for the first time would not be able to see it with 
a weak magnet. 
2154. The character of the force thus impressed upon the diamagnetic is that of 
rotation ; for when the image of the lamp-flame has thus been rendered visible, revo- 
lution of the eye-piece to the right or left, more or less, will cause its extinction ; and 
the further motion of the eye-piece to the one side or other of this position will pro- 
duce the reappearance of the light, and that with complementary tints, according as 
this further motion is to the right- or left-hand. 
2155. When the pole nearest to the observer was a marked pole, i. e. the same as the 
north end of a magnetic needle, and the further pole was unmarked, the rotation of the 
ray was right-handed ; for the eye-piece had to be turned to the right-hand, or clock 
fashion, to overtake the ray and restore the image to its first condition. When the 
poles were reversed, which was instantly done by changing the direction of the 
electric current, the rotation was changed also and became left-handed, the altera- 
* Philosophical Transactions, 1830, p. 1. I cannot resist the occasion which is thus offered to me of men- 
tioning the name of Mr. Anderson, who came to me as an assistant in the glass experiments, and has re- 
mained ever since in the Laboratory of the Royal Institution. He has assisted me in all the researches into 
which I have entered since that time, and to his care, steadiness, exactitude, and faithfulness in the perform- 
ance of all that has been committed to his charge, I am much indebted. — M. F. 
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