552 Intelligence and Miscellaneous Articles. 
LXIV. Intelligence and Miscellaneous Articles. 
ON A NEW POLARIZER OF ICELAND SPAR.—EXPERIMENT ON 
FLUORESCENCE. BY LEON FOUCAULT. 
HEN the object is to polarize a pencil of white light completely, 
the best known method is to have recourse to the use of the 
Nicol’s prism ; but for operating upon a sheaf of a certain volume, of 
4 to 5 centimetres in diameter, for example, the Nicol’s prism becomes 
expensive and difficult to procure, in consequence of the rarity of 
fine specimens of spar. 
The cutting adopted in the construction of the Nicol’s prism is 
necessarily attended with a great expense of material. ‘The prism 
to be complete, must be taken from a picce of spar the longitu- 
dinal edges of which are at least equal to three times one of the 
equal sides which terminate the bases. The piece is then cut 
from one obtuse angle to the other, through a plane inclined at 88° 
upon the plane of the bases, and perpendicular to the plane of their 
small diagonals. ‘The two faces thus obtained are polished, and 
fastened together by means of Canada balsam. 
When a parallelopiped thus formed is turned towards a uniformly 
illuminated ground, and we look through the piece in the direction 
of its axis, a field of polarization is seen included between two curved 
bands, one red, and the other blue, which correspond with the limit 
directions in which the ordinary and extraordinary rays are trans- 
mitted. These bands enclose an angular space of 32°, which renders 
the Nicol’s prism an analyser applicable in all circumstances when 
the inclination of the rays, which are to be observed simultaneously, 
does not exceed 32°. 
But this angular extent of the field of polarization, which is prized 
in the Nicol’s prism considered as an analyser, no longer possesses 
the same interest when the apparatus is simply to play the part of a 
polarizer; for in that case the action which is to be produced in 
general only affects a pencil of light of nearly parallel rays. So that, 
under such circumstances, there would be an advantage in increasing 
the transverse dimensions of the prism, even when this would cause 
a certain reduction in the extent of the angular field of polarization. 
In reflecting on the data of the question, I have in fact ascertained 
that the cutting of the Nicol’s prism may be modified so as to 
diminish its length considerably without injury to the effects which 
it may produce as a polarizer. 
I take a parallelopiped of spar, of which the longitudinal edges are 
only equal to five-fourths of one of the sides of the bases; I carry 
from one obtuse angle to the other a section inclined at 59° upon 
the plane of the bases, and the new faces being polished, I replace 
the two fragments in their natural position without glueing them, and 
taking care to preserve between the new faces a little space in which 
air remains, and which, under suitable incidence, causes the complete 
reflexion of the ordinary ray. 
On looking through a rhomb cut in this way, and mounted other- 
wise like a Nicol’s prism, we again find that there exists an angular 
field of polarization; but the index of refraction of the air being 
considerably less than those of the two rays propagated by the spar, 
