y of manufacture, the end surfaces are cleavage planes, and
the oblique cut, instead of being perpendicular, makes with these an
angle of about 84 deg.. By this alteration the prism becomes shorter, and
is now only 2.83 times its breadth; but if Canada balsam is still used
as the cement, the field will occupy a very unsymmetrical position in
regard to the long axis. If balsam of copaiba is made use of, the
index of refraction of which is 1.50, a symmetrical field of about 24 deg.
will be obtained. A prism of this kind has also been designed by Prof.
B. Hasert of Eisenach (_Pogg. Ann._, cxiii., 189), but its performance
appears to be inferior to the above.

3. _The Nicol Prism with Perpendicular Ends._--The terminal surfaces
in this prism are perpendicular to the long axis, and the sectional
cut makes with them an angle of about 75 deg.. The length of the prism is
3.75 times its breadth, and if the cement has an index of refraction
of 1.525, the field is symmetrically disposed, and includes an angle
of 27 deg.. Prisms of this kind have been manufactured by Dr. Steeg, Mr.
C.D. Ahrens, and others.

4. _The Foucault Prism_ (_Comptes Rendus_, 1857, xlv., 238).--This
construction differs from all those hitherto mentioned, in that a film
of air is employed between the two cut surfaces as the totally
reflecting medium instead of a layer of cement. The two halves of the
prism are kept in position, without touching each other, by means of
the mounting. The length of the prism is in this way much reduced, and
amounts to only 1.528 times its breadth. The end surfaces are cleavage
planes, and the sectional cut makes with them an angle of 59 deg.. The
field, however, includes not more than about 8 deg., so that this prism
can be used only in the case of nearly parallel rays; and in addition
to this the pictures which may be seen through it are to some extent
veiled and indistinct, owing to repeated internal reflection.

5. _The Hartnack Prism_ (_Ann. de Ch. et de Physique_, ser. iv., vii.,
181).--This form of prism was devised in 1866 by MM. Hartnack and
Prazmowiski; the original memoir is a valuable one; a translation of
it, with some additions, has lately been published (_Journ. of the R.
Microscopical Soc._, June, 1883, 428). It is considered by Dr.
Feussner to be the most perfect prism capable of being prepared from
calc-spar. The ends of the prism are perpendicular to its length; the
section carried through it is in a plane pe