f it, many of them discover a
partiality to it in the direction of their flowers; and all of them
perspire oxygen gas only when exposed to it; nay it would seem that
organization, sensation, spontaneous motion, and life, exist only at
the surface of the earth, and in places exposed to light. Without
light nature is lifeless, inanimate, and torpid.

Let us now examine if the action of light upon the body is subject to
the law that has been mentioned. If a person be kept in darkness for
some time, and then be brought into a room in which there is only an
ordinary degree of light, it will be almost too oppressive for him,
and will appear excessively bright; and if he have been kept for a
considerable time in a very dark place, the sensation will be very
painful. In this case, while the retina or optic nerve was deprived
of light, its excitability accumulated, or became more easily
affected by light: for if a person go out of one room into another,
which has an equal degree of light, he will perceive no effect.

You may convince yourselves of the truth of this law, by a very
simple experiment; shut your eyes, and cover them for a minute or two
with your hand, and endeavour not to think of the light, or what you
are doing; then open them, and the daylight will for a short time
appear brighter.

If you look attentively at a window for about two minutes, then cast
your eyes upon a sheet of white paper, the shape of the window frames
will be perfectly visible upon the paper; those parts which express
the wood work appearing brighter than the other parts. The parts of
the optic nerve on which the image of the frame falls, are covered by
the wood work from the action of the light; the excitability of these
parts will therefore accumulate; and the parts of the paper which
fall upon them must of course appear brighter.

If a person be brought out of a dark room where he has been confined,
into a field covered with snow, when the sun shines, it has been
known to affect him so much as to deprive him of sight altogether.

This law is well exemplified when we come into a dark room in the day
time. At first we can see nothing; but with the absence of light the
excitability accumulates, and we begin to have an imperfect glimpse
of the objects around us; after a while the excitability of the
retina is so far accumulated, and we become so sensible of the feeble
light reflected from the surfaces of bodies, that we can discern
their sh