The latter advantage, 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.
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