their strength, and thus be of more service in distinguishing diseases,
than the knowledge of their frequency. As such a criterion cannot be had,
the frequency of pulsation, the age of the patient being allowed for, will
in some measure assist us to distinguish arterial strength from arterial
debility, since in inflammatory diseases with strength the frequency seldom
exceeds one hundred and eighteen or one hundred and twenty pulsations in a
minute; unless under peculiar circumstance, as the great additional stimuli
of wine or of external heat.

5. After a muscle or organ of sense has been excited into contraction, and
the sensorial power ceases to act, the last situation or configuration of
it continues; unless it be disturbed by the action of some antagonist
fibres, or other extraneous power. Thus in weak or languid people, wherever
they throw their limbs on their bed or sofa, there they lie, till another
exertion changes their attitude; hence one kind of ocular spectra seems to
be produced after looking at bright objects; thus when a fire-stick is
whirled round in the night, there appears in the eye a complete circle of
fire; the action or configuration of one part of the retina not ceasing
before the return of the whirling fire.

Thus if any one looks at the setting sun for a short time, and then covers
his closed eyes with his hand, he will for many seconds of time perceive
the image of the sun on his retina. A similar image of all other bodies
would remain some time in the eye, but is effaced by the eternal change of
the motions of the extremity of this nerve in our attention to other
objects. See Sect. XVIII. 5. on Sleep. Hence the dark spots, and other
ocular spectra, are more frequently attended to, and remain longer in the
eyes of weak people, as after violent exercise, intoxication, or want of
sleep.