ne varying. 3. For a
given band or ray there exist in the crystal three rectangular directions
of symmetry, according to one of which the band generally disappears, so
that for a suitable direction of the luminous vibrations the crystal no
longer absorbs the radiations corresponding to the region of the spectrum
where the band question appeared. These three directions may be called the
principal directions of absorption, relative to this band. 4. In the
orthorhombic crystals, by a necessary consequence of crystalline symmetry,
the principal directions of absorption of all the bands coincide with the
three axes of symmetry. We may thus observe three principal absorption
spectra. In uniaxial crystals the number of absorption spectra is reduced
to two. 5. In clinorhombic crystals one of the principal directions of
absorption of each crystal coincides with the only axis of symmetry; the
two other principal rectangular directions of each band may be found
variously disposed in the plane normal to this axis. Most commonly these
principal directions are very near to the principal corresponding
directions of optical elasticity. 6. In various crystals the characters of
the absorption phenomena differ strikingly from those which we might expect
to find after an examination of the optical properties of the crystal. We
have just seen that in clinorhombic crystals the principal absorption
directions of certain bands were completely different from the axis of
optical elasticity of the crystal for the corresponding radiations. If we
examine this anomaly, we perceive that the crystals manifesting these
effects are complex bodies, formed of various matters, one, or sometimes
several, of which absorb light and give each different absorption bands.
Now, M. De Senarmont has shown that the geometric isomorphism of certain
substances does not necessarily involve identity of optical properties, and
in particular in the directions of the axes of optical elasticity in
relation to the geometric directions of the crystal. In a crystal
containing a mixture of isomorphous substances, each substance brings its
own influence, which may be made to predominate in turn according to the
proportions of the mixture. We may, therefore, admit that the molecules of
each substance enter into the crystal retaining all the optical properties
which they would have if each crystallized separately. The principal
directions of optical elasticity are given by the result