however,
occurs only when the difference between the indices of refraction for
the ordinary and extraordinary rays in the particular crystal made use
of is greater than in calc-spar. When this is the case, the field
becomes enlarged, and the length of the prism is reduced.

[Illustration: Fig. 7.]

The substance which Dr. Feussner has employed as being most suitable
for the separating crystal plate is nitrate of soda (_natronsalpeter_),
in which the above-mentioned values are [omega] = 1.587 and [eta] =
1.336. It crystallizes in similar form to calcite, and in both cases
thin plates obtained by cleavage may be used.

As the cementing substance for the nitrate of soda, a mixture of gum
dammar with monobromonaphthalene was used, which afforded an index of
refraction of 1.58. In the case of thin plates of calcite, a solid
cementing substance of sufficiently high refractive power was not
available, and a fluid medium was therefore employed. For this purpose
the whole prism was inclosed in a short glass tube with airtight ends,
which was filled with monobromonaphthalene. In an experimental prism a
mixture of balsam of tolu was made use of, giving a cement with an
index of refraction of 1.62, but the low refractive power resulted in
a very considerable reduction of the field. The extent and disposition
of the field may be varied by altering the inclination at which the
crystal lamina is inserted (Fig. 7), and thereby reducing the length
of the prism, as in the case of the Hartnack.

In order to obviate the effects of reflection from the internal side
surfaces if the prism, the wedge-shaped blocks of glass of which it is
built up may be made much broader than would otherwise be necessary;
the edges of this extra width are cut obliquely and suitably
blackened.

The accompanying diagram (Fig. 8) represents a prism of cylindrical
external form constructed in this manner, the lower surface being that
of the incident light. In this the field amounts to 30 deg., and the
breadth is about double the length.

[Illustration: Fig. 8.]

Dr. Feussner remarks that a prism similar in some respects to his new
arrangement was devised in 1869 by M. Jamin (_Comptes Rendus_,
lxviii., 221), who used a thin plate of calc-spar inclosed in a cell
filled with bisulphide of carbon; and also by Dr. Zenker, who replaced
the liquid in M. Jamin's construction by wedges of flint glass.

Among others, the carefully considered modifications of the