particles, separated, it may be, by exceedingly minute distances, but
still separated. To use the scientific phrase, they are not optically
continuous. Now, wherever optical continuity is ruptured we have
reflection of the incident light. It is the multitude of reflections
at the limiting surfaces of the particles that prevents light from
passing through snow, powdered glass, or common salt. The light here
is exhausted in echoes, not extinguished by true absorption. It is the
same kind of reflection that renders the thunder-cloud so impervious
to light. Such a cloud is composed of particles of water, mixed with
particles of air, both separately transparent, but practically opaque
when thus mixed together.

In the case of pigments, then, the light is _reflected_ at the
limiting surfaces of the particles, but it is in part _absorbed_
within the particles. The reflection is necessary to send the light
back to the eye; the absorption is necessary to give the body its
colour. The same remarks apply to flowers. The rose is red, in virtue,
not of the light reflected from its surface, but of light which has
entered its substance, which has been reflected from surfaces within,
and which, in returning _through_ the substance, has had its green
extinguished. A similar process in the case of hard green leaves
extinguishes the red, and sends green light from the body of the
leaves to the eye.

All bodies, even the most transparent, are more or less absorbent of
light. Take the case of water. A glass cell of clear water interposed
in the track of our beam does not perceptibly change any one of the
colours of the spectrum. Still absorption, though insensible, has
here occurred, and to render it sensible we have only to increase the
depth of the water through which the light passes. Instead of a cell