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 Nicol
prism which have recently been devised by Prof. S.P. Thompson (_Phil.
Mag._, November, 1881, 349, and _Jour. R. Micros. Soc._, August, 1883,
575), and by Mr. R.T. Glazebrook (_Phil. Mag._, May, 1883, 352), do
not appear to have been known to Dr. Feussner.
The following tabular view of different forms of polarizing prisms is
taken from the conclusion of Dr.
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