on has demonstrated. Nature has, however,
contrived a remedy for this also, by making the crystalline humour
more dense and solid near its centre, that the rays of light which
fall near its axis, may have their refraction increased, so as to
meet at the same point with those which fall at a distance from its
axis.

_Of the manner in which the Eye conforms itself in order to see
distinctly at different Distances._

It has been much disputed in what manner the eye conforms itself to
see distinctly at different distances; for it is evident, that,
without some change, the rays which flow from objects at different
distances, could not be collected into a focus at the same point,
and, consequently, though the eye might see distinctly at one
distance, it could not at another.

This subject has given rise to a variety of opinions, but few of them
are satisfactory; and though several of them might explain the
phenomena of vision, at different distances, yet it is by no means
proved that those supposed changes do take place in the eye. I shall
content myself with just mentioning the principal opinions on this
subject, without engaging in a controversy, which has for a long time
employed the ingenuity of philosophers to little purpose.

Some are of opinion, that the whole globe of the eye changes its
figure; becoming more oblong when objects are near, and more flat
when they are removed to a greater distance; and this change in the
figure of the eye is differently explained by different authors; some
maintain that it is rendered oblong by the joint contraction of the
two oblique muscles: others think that the four straight muscles
acting together, compress the sides of the globe, and by this
compression, reduce it to an oblong figure, when objects are near;
and that, by its natural elasticity, it recovers its former figure
when these muscles cease to act. Others again think that when these
four straight muscles act together, they render the eye flat by
pulling it inwards, and pressing the bottom of it against the fat;
and that it is reduced to its former figure, either by the joint
contraction of the two oblique muscles, or by the inherent elasticity
of its parts, which exerts itself when the muscles cease to act.

That, if such a change should take place in the eye, it would produce
distinct vision, will be readily granted; but that such a one does
not take place, at least in any of these ways, is, in my opinion,
very cert