crystals
of calc-spar, especially in the beautiful transparent variety from
Iceland, in which it was first observed and described by Erasmus
Bartholin in a work published at Copenhagen in 1669.
The
subjoined figure will illustrate this singular property. It represents
a rhomb of Iceland spar, on the surface of which a ray of light lì r falls. As seen in the figure, this ray divides into two, one of which r ο 6 follows the ordinary law of refraction, or the sines of
the
angles of incidence and refraction maintain a constant ratio. This is
named the ordinary ray O. The other, hence named the extraordinary ray
E, docs not obey the usual law of the sines, and has no general index
of refraction. In the plane perpendicular to the axis it is most
widely separated from the ordinary ray, but in others oblique to it
approaches nearer to O, and in one at light angles coincides, or there
is no double refraction. This plane, or rather direction, in which
there is no double refraction, is named the optical axis of the
crystal, or the axis of double refraction. Now, in certain minerals, it
is found that there is only one plane with this property, whereas in
others there are two such planes, and they have in consequence been
divided into monoaxial and binaxial. To the former (monoaxial) belong
all crystals of the tetragonal and hexagonal systems ; to the latter
(binaxial) all those of the three other systems. In the former the
optic axis coincides with, or is parallel to, the crystallographic
