in focusing arises from the fact that the
degree of divergence of the light waves entering the eye from different
objects, varies according to their distance. Since the waves from any
given point on an object pass out in straight lines in all directions, the
waves that enter the eye from distant objects are at a different angle
from those that enter from near objects. In reality waves from distant
objects are practically parallel, while those from very near objects
diverge to a considerable degree. To adjust the eye to different distances
requires some change in the focusing parts that corresponds to the
differences in the divergence of the light. This change, called
_accommodation_, occurs in the crystalline lens.(124) In the process of
accommodation, changes occur in the shape of the crystalline lens, as
follows:

1. In looking from a distant to a near object, the lens becomes more
convex, _i.e._, rounder and thicker (Fig. 161). This change is necessary
because the greater divergence of the light from the near objects requires
a greater converging power on the part of the lens.(125)

2. In looking from near to distant objects, the lens becomes flatter and
thinner (Fig. 161). This change is necessary because the less divergent
waves from the distant objects require less converging power on the part
of the lens.

The method employed in changing the shape of the lens is difficult to
determine and different theories have been advanced to account for it. The
following, proposed by Helmholtz, is the theory most generally accepted:

The lens is held in place back of the pupil by the suspensory ligament.
This is attached at its inner margin to the membranous capsule, and at its
outer margin to the sides of the eyeball, and entirely surrounds the lens.
It is drawn perfectly tight so that the sides of the eyeball exert a
continuous tension, or pull, on the membranous capsule, which, in its
turn, exerts pressure on the sides of the lens, tending to flatten it.
This arrangement brings the elastic force of the eyeball into opposition
to the elastic force of the lens. The ciliary muscle plays between these
opposing forces in the following manner:

_To thicken the lens_, the ciliary muscle contracts, pulling forward the
suspensory ligament and releasing its tension on the membranous capsule.
This enables the lens to thicken on account of its own elastic force. _To
flatten the lens_, the ciliary muscle relaxes, the elastic force