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 perpendicular to the
principal axis of the crystal. The cementing medium is linseed oil,
the index of refraction of which is 1.485. This form of prism is
certainly not so well known in this country as it deserves to be; a
very excellent one, supplied to the present writer by Dr. Steeg is of
rectangular form throughout, the terminal surfaces are 19 x 15 mm.,
and the length 41 mm. The lateral shifting of the field is scarcely
perceptible, the prism is perfectly colorless and transparent, and its
performance is far superior to that of the ordinary Nicol. The field
of view afforded by this construction depends upon the cementing
substance used, and also upon the inclination of the sectional cut in
regard to the end of the prism; it may vary from 20 deg. to 41 deg.. If the
utmost extent of field is not required, the prism may be shortened by
lessening the angle of the section, at the expense, however, of
interfering with the symmetrical disposition of the field.
6. _The Glan Prism_ (Carl's "Repertorium," xvi.
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