distinct perception of the outline of a distant hill, and also of a book
lying before us. The rays of light we receive from these objects cannot
have the same focus. How, then, can we account for the evident
accommodation of the eye to the varying distances? Various theories have
been advanced to explain this adjustment; such as changes in the
curvature of the cornea and lens; a movement of the lens, or a general
change in the form of the eyeball, by which the axis may be lengthened
or shortened.

[Illustration: Fig. 63.]

Two facts comprise all the positive knowledge which we possess on this
subject. Every person is conscious of a muscular effort in directing the
eye to a near object" as a book, and of fatigue, if the attention is
prolonged. If, now, the eyes be directed to a distant object, there will
result a sense of rest, or passiveness. By various experiments it has
been proved that the accommodation or adjustment of the eye for near
objects requires a muscular effort, but for distant objects the muscles
are in an essentially passive condition. An increase in the convexity of
the crystalline lens is now admitted to be necessary for a distinct
perception of near objects. We may give two simple illustrations, cited
by Dr. Dalton in his recent edition of Human Physiology. If a candle be
held near the front of an eye which is directed to a distant object,
three reflected images of the flame will be seen in the eye, one on each
of the anterior surfaces of the cornea and lens, and a third on the
posterior surface of the latter. If the eye is directed to a near
object, the reflection on the cornea remains unchanged, while that on
the anterior surface of the lens gradually diminishes and approximates
in size the reflection on the cornea, thus giving conclusive evidence
that, in viewing a near object, the anterior surface of the crystalline