Also that the great length of the Nicol--3.28
times its breadth--is not only an inconvenience, but owing to the
large pieces of spar thus required for its construction, prisms of any
but small size become very expensive. To this it may be added that
there is a considerable loss of light by reflection from the first
surface, owing to its inclined position in regard to the long axis of
the prism.
[Illustration: Fig. 4., Fig. 5., and Fig. 6.]
It is with the view of obviating these defects that the modifications
represented in Figs. 2 to 6 have been devised.
2. _The Shortened Nicol Prism_.--This arrangement of the Nicol prism
is constructed by Dr. Steeg and Reuter of Homburg v.d.H. For the sake
of facility 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.
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