Polarization of Heat by Magnetism. 345 . 
the plane of polarization equal to 4, the deviation will be, accord- 
ing to the direction of the @urrent, cos” (45° - 9) or cos?(45°+ 9), 
and we shall then have, for the difference of the effects observed 
when the current is madé to pass in a contrary direction, 
cos? (45° — 5) —cos? (459+) =sin 26. 
On placing the principal sections at 90°, the difference of the 
deviations would be on = 
cos? (90° — 5) —cos? 90° =sin? 4, 
or 
cos? (90°-+-0)—cos? 90°=sin? 6. 
Now sin? 4 is considerably less than sin 20. If, for example, we 
Suppose d=8°, sin? 6 is equal to more than fourteen times sin 20. 
The eye, it is true, appreciates readily the transition from dark- 
hess to light, but not so the difference in brightness of two lumi- 
nous images. This is not the case with the thermoscopic appa- 
t There is therefore, when heat is concerned, a great ad- 
vantage in proceeding as above directed. 
‘The following are the details of the experiment: the solar ray, - 
reflected by a heliostat, traverses at first a doubly-refracting achro- 
matic prism. ‘The extraordinary bundle was intercepted ; the or- 
dinary bundle traverses the electro-magnet of M. Ruhmkorfl’s ap- 
paratus, and a flint-glass of 38 millimetres in thickness between 
the poles of the electro-magnet. It afterwards encounters, at 
about 3™-50, the second prism of spar, bifurcates again, aud gives 
two images, one of which may be received on the thermo-electric 
pile placed at four metres from the electro-magnet. ve galvan- 
ometer was still a little further removed from this disturbing force. 
It was ascertained, by direct and repeated experiments, that on es- 
tablishing the current there were no phenomena of induction, and 
that the electro-magnets had no appreciable action on the mag- 
hetic needle which, under their infiuence, remained at zero in a 
State of perfect rest. In order to understand this, it must he borne 
in mind that the two opposite poles are very close together, and 
that they act simultaneously upon a system already very distant 
and almost completely astatic. It might be feared that the elec- 
tro-magnet, without action on the needle at zero, acted on the nee- 
dle already displaced by the action of the calorific radiation. This 
would be possible in fact, if, in its first position, the needle had 
the same direction as the line which joins its centre to the elec- 
tro-magnet, and if, when it deviates, it made a notable angle in 
that direction. In our experiments, precisely the inverse condi- 
tion was realized ; so that the component of the magnetic action 
diminished more and more during the movement of the needle, 
and became perfectly null when it attained its greatest deviation. 
If therefore it had no action in the first case, such ought for a 
Stronger reason to be the case in the second. 
Szconp Serms, Vol. IX, No. 27.—May, 1850. 44 
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