The artists repository and drawing magazine: exhibiting the principles of the polite arts in their various branches (1.1787)

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of light pafs to the internal humors, where
they are converged, collected into a focus, and
again diverged, left they fhould be confufed,
and left their action in a focus fhould be too
ftrong for the retina to bear; and indeed they
might be too ftrong for it in fome inftances,
even when diverged, had not the pupil the
power of choofing to admit or reject the rays
of light at pleafure. This curious part, by
a moft admirable contrivance, poflerTes the
faculty of contracting or dilating itfelf, ac-
cording to circumftances. In a ftrong light
which might otherwife be offensive or inju-
rious, its orifice clofes fo as to admit no
greater quantity of light than is convenient,
but the internal parts enjoy that moderation
which is necefiary to the difcharge of their office;
in the {hade, or wherever light is deficient, the
pupil expands, admits all it can collect, and
exerts itfelf to maintain that equilibrium
which is equally deftroyed by want, as by
redundance.

In cats, and other animals that prey
in the dark, the pupil of the eye is fo va-
riable as to admit more than an hundred
times the quantity of light at one time than
at another. The human eye admits more
than ten times the quantity of light at one
time than at another, and perhaps the differ-
ence