166 Prof. A. de la Rive’s Memoir of 
larized ray penetrates and issues from the prism, is placed be- 
tween the poles of an electromagnet in such a manner that its 
length, and, consequently, the direction of the transmitted ray 
are parallel to the line joining the magnetic poles. Lastly, the 
polarized ray on issuing from the glass prism only reaches the 
eye after passing through a Nicol’s prism which serves as an 
analyzer. - It is also by traversing a Nicol’s prism before pene- 
trating into the glass prism that the ray of light is polarized; 
but this may be effected in any other manner. 
It is well known that by turning the analyzing prism toa 
certain angle the polarized ray is extinguished, in such a man- 
ner that the brilliant spot is replaced by a black spot. If, 
after this operation has been‘effected, a strong electric current 
is passed through the wire surrounding the electromagnet, the 
black spot disappears and the bright one again makes its ap- 
pearance, Then by turning the analyzing prism a little fur- 
ther in the same direction, the luminous ray is again extin- 
guished, but this extinction ceases as soon as the magnetic ac- 
tion is suppressed by the interruption of the current which mag- 
netized the electromagnet. Theaction of magnetism therefore 
consists simply in causing the plane of polarization to turn by 
a certain angle, and to give artificially to the glass, while itis , 
under the magnetic influence, a property which certain sub- 
stances, such as quartz and essence of turpentine, possess 
naturall : 
Any transparent substance, except gases, may serve, al- 
though in different degrees, as the medium for magnetism to 
act upon the polarized ray. But that by means of which this 
influence is best manifested is the yellowish heavy glass (boro- 
silicate of lead) which Faraday obtained in his experimental 
researches upon the fabrication of glass for optical purposes. 
He happened to have at hand several specimens of this glass; 
and it was by using one of these for performing the experiment 
just described that he discovered the magnetic rotation of the 
—~ of polarization, a phenomenon which would probably 
lave escaped him if he had made use of ordinary g! -at first 
starting. Thus the long and painful labors to which he had 
orm i i ess, in order 
- Let us now study the new phenomenon a little more closely, 
impo . Some substances we 
ing the plane 
a 
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