rpendicular 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., 570, and xvii.,
195).--This is a modification of the Foucault, and in a similar manner
includes a film of air between the sectional surfaces. The end
surfaces and also the cut carried through the prism are parallel to
the principal axis of the calc-spar. The ends are normal to the
length, and the field includes about 8 deg.. This prism is very short, and
may indeed be even shorter than it is broad. It is subject to the same
defect as that mentioned in the case of the Foucault, although perhaps
not quite to the same extent.

II.--THE NEW POLARIZING PRISM.

This prism differs very considerably from the preceding forms, and
consists of a thin plate of a doubly refracting crystal cemented
between two wedge-shaped pieces of glass, the terminal faces of which
are normal to the length. The external form of the prism may thus be
similar to the Hartnack, the calc-spar being replaced by glass. The
indices of refraction of the glass and of the cementing medium should
correspond with the greater index of refraction of the crystal, and
the directions of greatest and least elasticity in the latter must
stand in a plane perpendicular to the direction of the section. One of
the advantages claimed for the new prism is that, it dispenses with
the large and valuable pieces of spar hitherto found necessary; a
further advantage being that other crystalline substances may be used
in this prism instead of calc-spar. The latter advantage,