of
seeing with an ordinary light.

We find, that we are not always equally capable of performing the
functions of life. When we have been engaged in any exertion, either
mental or corporeal, for some hours only, we find ourselves
fatigued, and unfit to pursue our labours much longer; if in this
state, several of the exciting powers, particularly light and noise,
be withdrawn; and if we are laid in a posture which does not require
much muscular exertion, we soon fall into that state which nature
intended for the accumulation of the excitability, and which we call
Sleep. In this state, many of the exciting powers cannot act upon us,
unless applied with some violence, for we are insensible to their
moderate action. A moderate light, or a moderate noise, does not
affect us, and the power of thinking, which exhausts the
excitability very much, is in a great measure suspended. When the
action of these powers has been suspended for six or eight hours,
the excitability is again capable of being acted on, and we rise
fresh, and vigorous, and fit to engage in our occupations.

Sleep then, is the method which nature has provided to repair the
exhausted constitution, and restore the vital energy; without its
refreshing aid, our worn-out habits would scarcely be able to drag
on a few days, or at most a few weeks, before the vital spring was
quite run down; how properly therefore has the great poet of nature
called sleep the chief nourisher in life's feast.--

'Sleep, that knits up the ravell'd sleeve of care,
'the death of each day's life, sore labour's bath,
'balm of hurt minds, great Nature's second course,
'chief nourisher in life's feast.'

From the internal sensations often excited, it is natural to
conclude that the nerves of sense are not torpid during sleep; but
that they are only precluded from the perception of external
objects, by the external organs being rendered unfit to transmit to
them the impulses of bodies, during the suspension of the power of
volition; thus, the eye-lids are closed in sleep, to prevent the
impulse of the light from acting on the optic nerve; and it is very
probable that the drum of the ear is not stretched; it is likewise
probable that something similar happens to the external apparatus of
all our organs of sense, which may make them unfit for their office
of perception during sleep.

The more violently the exciting powers have acted, the sooner is
sleep brought on; b