Not everything that is visible depends upon light for its
visibility. This is only true of the 'proper' colour of things. Some
objects of sight which in light are invisible, in darkness stimulate
the sense; that is, things that appear fiery or shining. This class of
objects has no simple common name, but instances of it are fungi,
flesh, heads, scales, and eyes of fish. In none of these is what is
seen their own proper' colour. Why we see these at all is another
question. At present what is obvious is that what is seen in light
is always colour. That is why without the help of light colour remains
invisible. Its being colour at all means precisely its having in it
the power to set in movement what is already actually transparent,
and, as we have seen, the actuality of what is transparent is just
light.
The following experiment makes the necessity of a medium clear. If
what has colour is placed in immediate contact with the eye, it cannot
be seen. Colour sets in movement not the sense organ but what is
transparent, e.g. the air, and that, extending continuously from the
object to the organ, sets the latter in movement. Democritus
misrepresents the facts when he expresses the opinion that if the
interspace were empty one could distinctly see an ant on the vault
of the sky; that is an impossibility. Seeing is due to an affection or
change of what has the perceptive faculty, and it cannot be affected
by the seen colour itself; it remains that it must be affected by what
comes between.
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