The lens is a compact body of transparent cells, concave in form, and
closely similar to the glass lens of a camera. The lens of the eye,
however, differs from the camera’s glass lens because it changes its
shape in focusing for objects at different distances. This focusing,
which takes place automatically, is known as “accommodation.”
The object of the change in the shape of the lens is that no matter at
what angle the rays of light reflected from the object looked at fall on
the outer surface of the lens (through the opening in the iris), they
may be accurately focused on the surface of the retina, or lining
membrane at the back of the eye. When looking at a distant object the
lens is fairly flat, because when in this position the rays of light
will be accurately focused on the retina. If the eye is now turned to an
object near at hand the rays of light from the object are more divergent
than in the previous case, and if the lens retained its previous shape
they would fail to be focused accurately on the surface of the retina.
Hence Nature has arranged that the lens of the eye is elastic,
automatically becoming flatter by the action of the ciliary muscle when
distant objects are looked at and rounder or deeper when nearer objects
are looked at.