m top to
bottom; and it should be lighted at least ten minutes before using the
light for testing. A cylindrical glass chimney surrounds it to ward
off air currents. About 2 per cent. of the light is absorbed by the
glass. The power of the flame is that of a standard English candle;
and experiments have shown that amyl acetate, which besides is not
expensive, is the best fuel for steadiness and brilliance. Neither the
substitution of commercial amyl-acetate for pure nor the use of a wick
of cotton thread for loose cotton fiber alters the illuminating power;
but the wick should be trimmed square across the mouth of the tube,
for if it project and droop the illuminating power is increased.

* * * * *

[NATURE.]

DR. FEUSSNER'S NEW POLARIZING PRISM.

In a recent number of the _Zeitschrift fur Instrumentenkunde_ (iv.,
42-50, February, 1884), Dr. K. Feussner of Karlsruhe has given a
detailed description of a polarizing prism lately devised by him,
which presents several points of novelty, and for which certain
advantages are claimed. The paper also contains an account, although
not an exhaustive one, of the various polarizing prisms which have
from time to time been constructed by means of different combinations
of Iceland spar. The literature of this subject is scattered and
somewhat difficult of access, and moreover only a small part of it has
hitherto been translated into English; and it would appear therefore
that a brief abstract of the paper may not be without service to those
among the readers of _Nature_ who may be unacquainted with the
original memoirs, or who may not have the necessary references at
hand.

Following the order adopted by Dr. Feussner, the subject may be
divided into two parts:

I.--OLDER FORMS OF POLARIZING PRISMS.

In comparing the various forms of polarizing prisms, the main points
which need attention are--the angular extent of the field of view, the
direction of the emergent polarized ray, whether it is shifted to one
side of, or remains symmetrical to the long axis of the prism; the
proportion which the length of the prism bears to its breadth; and
lastly, the position of the terminal faces, whether perpendicular or
inclined to the long axis. These requirements are fulfilled in
different degrees by the following methods of construction:

[Illustration: Fig. 1., Fig. 2., and Fig. 3.]

1. _The Nicol Prism_ (_Edin. New Phil. Journal_, 1828, vi.,