degree of light, it will be almost too oppressive for him, and 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 goes out of one room, into another which has an
equal degree of light, he will feel no effect. You may convince
yourselves 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 of what you are doing; then open them, and
the day-light will for a short time appear brighter. If you look
attentively at a window, for about two minutes, and 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 portions of the nerve 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.

Let us next consider what happens with respect to heat; if heat be
for some time abstracted, the excitability accumulates; or in other
words, if the body be for some time exposed to cold, it is more
liable to be affected by heat, afterwards applied; of this also you
may be convinced by an easy experiment--put one of your hands into
cold water, and then put both into water which is considerably warm;