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: