ilarity of voluntary exertion
may be necessary for the perceptions of the other nerves of sense; for it
is observed that the papillae of the tongue can be seen to become erected,
when we attempt to taste any thing extremely grateful. Hewson Exper.
Enquir. V. 2. 186. Albini Annot. Acad. L. i. c. 15. Add to this, that the
immediate organs of sense have no objects to excite them in the darkness
and silence of the night, but their nerves of sense nevertheless continue
to possess their perfect activity subservient to all their numerous
sensitive connections. This vivacity of our nerves of sense during the time
of sleep is evinced by a circumstance, which almost every one must at some
time or other have experienced; that is, if we sleep in the daylight, and
endeavour to see some object in our dream, the light is exceedingly painful
to our eyes; and after repeated struggles we lament in our sleep, that we
cannot see it. In this case I apprehend the eyelid is in some degree opened
by the vehemence of our sensations; and, the iris being dilated, the optic
nerve shews as great or greater sensibility than in our waking hours. See
No. 15. of this Section.

When we are forcibly waked at midnight from profound sleep, our eyes are
much dazzled with the light of the candle for a minute or two, after there
has been sufficient time allowed for the contraction of the iris; which is
owing to the accumulation of sensorial power in the organ of vision during
its state of less activity. But when we have dreamt much of visible
objects, this accumulation of sensorial power in the organ of vision is
lessened or prevented, and we awake in the morning without being dazzled
with the light, after the iris has had time to contract itself. This is a
matter of great curiosity, and may be thus tried by any one in the
day-light. Close your eyes, and cover them with your hat; think for a
minute on a tune, which you are accustomed to, and endeavour to sing it
with as little activity of mind as possible. Suddenly uncover and open your
eyes, and in one second of time the iris will contract itself, but you will
perceive the day more luminous for several seconds, owing to the
accumulation of sensorial power in the optic nerve.

Then again close and cover your eyes, and think intensely on a cube of
ivory two inches diameter, attending first to the north and south sides of
it, and then to the other four sides of it; then get a clear image in your
mind's eye of a