e that
again is the _retina_, or curved focussing screen of the eye, which may
best be described as a network of fibres ramifying from the optic nerve,
which carries sight sensations to the brain. The hollow of the ball is
full of a jelly-like substance called the _vitreous humour_; and the
cavity between the lens and the cornea is full of water.

We have already seen that, in focussing, the distance between lens and
image depends on the distance between object and lens. Now, the retina
cannot be pushed nearer to or pulled further away from its lens, like
the focussing screen of a camera. How, then, is the eye able to focus
sharply objects at distances varying from a foot to many miles?

[Illustration: FIG. 116.--Section of the human eye.]

As a preliminary to the answer we must observe that the more convex a
lens is, the shorter is its focus. We will suppose that we have a box
camera with a lens of six-inch focus fixed rigidly in the position
necessary for obtaining a sharp image of distant objects. It so happens
that we want to take with it a portrait of a person only a few feet from
the lens. If it were a bellows camera, we should rack out the back or
front. But we cannot do this here. So we place in front of our lens a
second convex lens which shortens its principal focus; so that _in
effect_ the box has been racked out sufficiently.

Nature, however, employs a much more perfect method than this. The eye
lens is plastic, like a piece of india-rubber. Its edges are attached to
ligaments (L L), which pull outwards and tend to flatten the curve of
its surfaces. The normal focus is for distant objects. When we read a
book the eye adapts itself to the work. The ligaments relax and the lens
decreases in diameter while thickening at the centre, until its
curvature is such as to focus all rays from the book sharply on the
retina. If we suddenly look through the window at something outside, the
ligaments pull on the lens envelope and flatten the curves.

This wonderful lens is achromatic, and free from spherical aberration
and distortion of image. Nor must we forget that it is aided by an
automatic "stop," the _iris_, the central hole of which is named the
_pupil_. We say that a person has black, blue, or gray eyes according to
the colour of the iris. Like the lens, the iris adapts itself to all
conditions, contracting when the light is strong, and opening when the
light is weak, so that as uniform an amount of light as