2. The material element in sight consists of light,
and colours which are modes of light. If we adopt
the undulation theory, the sensations of light and
colours arise in the nerve substance of the ocular
nerve upon the impact of successive waves, or rays, of
ether atoms, and depend upon the relation of these
to this nerve substance and its modes of activity. A
thorough examination of this whole subject has been
recently given by Prof. Helmholtz in his Handbuch
der Physiologischen Optik, which forms the 9th
volume of the Allg. Encycl. der Physik, edited by
Karsten. And from this I shall attempt to derive
such a brief sketch of the phenomena of sight as may
be necessary for the purposes of the present work.

3. In the first place it must be noticed that the
colours of what are called coloured objects depend
upon their absorbing and reflecting different rays of
ether atoms in different proportions, rays which are
emitted, or the atoms of which are set in motion, by
bodies which are called, from that circumstance, self-
luminous. Black and white are to these coloured
bodies what darkness and extreme light are to self-
luminous bodies or to light itself. The waves of light
and colours which extend from the eye to the object
seen, which is their ultimate or their immediate
source, consist of vibrations of ether atoms in direc-
tions transverse to that of the wave itself; differing
in this respect from the vibrations of air particles in
sound, which have the same direction as that of the
wave of sound.

4. In all the phenomena of sight three features
may be distinguished as modes of the material ele-
ment, corresponding to the three modes of the mate-
rial element of sound, intensity, pitch, and colour.

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