circle which does not exceed a
certain magnitude, distinct vision will be produced; the circle
formed by these rays on the retina he calls the circle of
dissipation. The pupil will, by contracting, not only diminish the
circles of dissipation, and thereby help to produce distinct vision,
but will also prevent so great a quantity of light from falling near
the circumferences of those circles; and Dr. Jurin has shown, that,
if the light on the outer side of the circles of dissipation is
diminished, the remainder will scarce affect the sense. In both these
ways, the contraction of the pupil has a tendency to diminish the
circles of dissipation, and, consequently, to produce distinct
vision. This is likewise confirmed by experiment, for when an object
is placed so near, that the pupil cannot be so much contracted as is
necessary for distinct vision, the same end may be obtained by means
of an artificial pupil: for, if a small hole is made in a card, a
very near object may be viewed through it with the greatest ease and
distinctness. Also, if a person have his back turned towards a
window, and hold a book so near his eyes as not to be able to read,
if he turn his face to the light, he will find, that he will be able
to read it very distinctly; which is owing to the contraction of the
pupil by means of the light.

M. Le Roi, a member of the Royal Academy of Montpelier, has attempted
to defend the opinion of M. de la Hire, and, indeed, it seems, of all
others, the best supported by facts; but perhaps it may not account
so well for vision at great distances. It is likewise rendered more
probable by viewing the pictures of external objects, formed in a
dark chamber, by rays coming through a hole in the window shutter;
those pictures will be rendered distinct, by dilating, or contracting
the aperture, without the assistance of a lens, accordingly as the
object is more or less distant; those who have had the crystalline
lens depressed, or extracted, by means of one glass can see objects
pretty distinctly at different distances. These, and several other
arguments that might be brought, tend to prove that the eye
accommodates itself to view objects distinctly at different
distances, chiefly by means of the motion of the pupil; and though
this does not explain the phenomenon so satisfactorily as we could
wish, yet it is certain, that it has a share in it; we are however
certain, that, in whatever manner it may be produced, the eye h