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THE LAWS OF THE ABSORPTION OF LIGHT IN CRYSTALS.

By H. BECQUEREL.


1. The absorption spectrum observed through a crystal varies with the
direction of the rectilinear luminous vibration which propagates itself in
this crystal. 2. The bands or rays observed through the same crystal have,
in the spectrum, fixed positions, their intensity alone 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