6 REPORT—1857. 
a crystal of spar is required, whose longitudinal ridges are at least equal to three 
times one of the equal sides which terminate the bases. The piece is then cut from 
angle to obtuse angle by an inclined plane of 38° on the plane of their bases, and 
perpendicular to that of their smaller diagonals. The two surfaces thus obtained are 
polished and glued together with balsam of Canada, when a parallelopiped thus 
prepared is placed on a bottom uniformly lighted. On looking through the piece, a 
field of polarization is seen contained between two curved bands,—one red, the 
other blue,—which correspond with the direction of the limits according to which 
the ordinary and extraordinary ray are transmitted. These bands comprise an angu- 
lar space of 32°, which makes Nicol’s prism an analyser, applicable in all cases 
where the inclination of the ray, which it is desired to observe simultaneously, 
does not exceed 32°. But this angular extent of the field of polarization, which is 
sought for in the prism of Nicol, considered as an analyser, no longer presents the 
same interest when the apparatus is to fulfil the part of a simple polarizer; for then the 
action desired to be produced acts only, in general, on a pencil of light nearly 
parallel. So that there will be an advantage, in similar circumstances, in increasing 
the extent of the transverse dimension of the prism, even when the consequence 
would be a certain reduction in the extent of the angular field of polarization. Re- 
flecting on the data of the question, I have in effect discovered, that we can modify 
the prism of Nicol in its cut, so as to diminish considerably the length without in- 
jury to its character of polarizer. I take then a parallelopiped of spar, whose longi- 
tudinal ridges equal only five quarters of one of the sides of the base. An inclined 
section of 59° on the plane of the bases, and the new surfaces, being polished, I put 
the two pieces in their natural position without fastening them, taking care to pre- 
serve between the new surfaces a little space, where the air penetrates, and which, 
with the proper incidence, determines the entire reflexion of the ordinary ray. Look- 
ing through a rhomb thus prepared—in other respects mounted like a prism of Nicol 
—there is still discovered an angular field of polarization; but the index of refrac- 
tion of air being considerably below those of the two rays propagated by the spar, 
complete polarization only takes place in an extent of 8°, and the field it presents is 
found comprised between two red bands. The new combination then does not 
fulfil the conditions necessary to the formation of a good analyser; but when it is 
only required to polarize a pencil of solar light, whose extreme rays have an incli- 
nation but of half a degree, the prism, with the thin stratum of air and its eight 
degrees of field, more than suffices to polarize all the elements of such a pencil. 
This kind of polarizer is even in some respects preferable to the prism of Nicol, pro- — 
vided that the reflexion of the ordinary ray takes place under an incidence which 
sends it back almost normally to the intersection of its two lateral faces; this ray 
has no tendency to issue by the base and confound itself, as in Nicol’s prism, with 
the extraordinary ray. Also, when the material of spar is very pure, it accom- 
plishes the extinction of the pencil produced by an analyser in a complete manner 
on the whole extent of the transmitted pencil. It is likely that in cases where the 
prism of Nicol is employed as a polarizer, the new form will be preferred, since it 
produces an effect more complete, at the same time economizing nearly two-thirds 
of the mass of spar. 
On a Telescope Speculum of Silvered Glass. 
By M. Lton Foucautt, Paris. 
The astronomical refractor, compared with the reflecting telescope of the same 
dimensions, has always had the advantage of giving more light; the pencil of rays 
which fall on the object-glass passes through it for the most part, and is employed 
almost entirely in the formation of the image at the focus; while on the metal 
mirror a part only of the light is reflected in a converging pencil, which loses still 
more by a second reflexion being brought back towards the observer. However, as 
the reflecting telescope is essentially free from aberration of refrangibility, as the 
purity of its images depends only on the perfection of a single surface, as with re- 
gard to focal length it possesses a greater diameter than the refracting telescope, and 
thus partly regains the light wasted by reflexions—some observers continue to give 
it the preference, chiefly in England, over the refracting telescope for the examina- 
—.—s)hC 
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