Intelligence and Miscellaneous Articles. 553 
complete polarization only takes place in an extent of about 8°, 
and the field which it occupies is included between two red bands. 
The new combination consequently does not satisfy the conditions 
necessary to form a good analyser; but when we have simply to 
polarize a pencil of solar light, of which the extreme rays are only 
inclined to one another by a half-degree, the prism with a sheet of 
air and its 8 degrees of field is more than sufficient to polarize all the 
elements of suchapencil. This kind of polarizer is in some respects 
even preferable to the Nicol’s prism, inasmuch as the ordinary ray 
is reflected almost normally to the intersection of two of the lateral 
faces, and has therefore no tendency to become again reflected, or 
to escape afterwards by the base and mix, as in the Nicol’s prism, 
with the extraordinary ray. Moreover, when the substance of the 
spar is very pure, and traversed neither by planes of cleavage nor by 
hemitropic plates, complete extinction is produced by an analyser 
upon the whole extent of the transmitted pencil. It is to be ex- 
pected that, in the circumstances in which the Nicol’s prism has 
been employed as a polarizer, the new cutting will be preferred, as 
it produces a more complete effect, at the same time saving nearly 
two-thirds of the mass of spar. 
These trials having called my attention to all the peculiarities 
observed in the Nicol’s prism, I have been struck by finding the 
tints inverted in the band of total reflexion which corresponds with 
the limit direction of transmissibility of the extraordinary ray. This 
inversion js certainly due to the fact, that, notwithstanding the 
slight difference of the average indices of refraction, the dispersive 
power of Canada balsam is greater than that of the spar for the 
limit direction of the extraordinary ray. It follows from this that 
the relative indices of the various simple rays go on increasing from 
the violet to the red, which explains why these different rays are 
totally reflected in the inverse order of their absolute refrangibilities. 
We may avail ourselves of this remark to obtain, by means of the 
Nicol’s prism, a pencil exclusively formed of the most refrangible 
rays contained in the solar light. For this purpose it is sufficient to 
place the prism in the course of the luminous pencils, and to incline 
it gradually in the direction in which complete extinction is pro- 
duced; we then see the transmitted pencil pass to blue, then to 
violet, and lastly become reduced to a radiation which is nearly in- 
visible, but eminently adapted to develope with intensity the phe- 
nomena of fluorescence discovered by Mr. Stokes. Sulphate of 
quinine, uranium-glass, and some diamonds immersed in this sheaf 
immediately acquire a very great brilliancy.—Comptes Rendus, 
August 17, 1857, p. 238. 
ON THE INFLUENCE OF STRUCTURE UPON THE MAGNETIC PRO- 
PERTIES OF IRON. BY F, P. LE ROUX. 
With regard to the experiments of MM. Pliicker and Faraday 
upon magneto-crystalline phenomena, MM. Tyndall and Knoblauch 
have shown, by ingeniously varied experiments, that the position 
taken by crystalline masses in relation to the poles of a magnet 
did not depend, as had been at first supposed, so much upon. the 
