mph-like liquid which contains an
occasional white corpuscle. It has a feeble motion and is slowly added to
and withdrawn from the eye. It is supplied mainly by the blood vessels in
the ciliary processes and finds a place of exit through a small lymph duct
at the edge of the cornea (Fig. 159).

The back portion of the eyeball is filled with a soft, transparent,
jelly-like substance, called the _vitreous_ humor. It is in contact with
the surface of the retina at the back and with the attachments of the lens
in front, being surrounded by a thin covering of its own, called the
_hyaloid membrane_. The aqueous and vitreous humors aid in keeping the
eyeball in shape and also in focusing.

*How we see Objects.*--To see an object at least four things must happen:

1. Light must pass from the object into the eye. Objects cannot be seen
where there is no light or where, for some reason, it is kept from
entering the eye.

2. The light from the object must be focused (made to form an image) on
the retina. In forming the image, an area of the retina is stimulated
which corresponds to _the form of the object_.

3. Impulses must pass from the retina to the brain, stimulating it to
produce the sensations.

4. The sensations must be so interpreted by the mind as to give an
impression of the object.

*Focusing Power of the Eyeball.*--The eyeball is essentially a device for
focusing light. All of its transparent portions are directly concerned in
this work, and the portions that are not transparent serve to protect and
operate these parts and hold them in place. Of chief importance are the
crystalline lens and the cornea. Both of these are lenses. The cornea with
its inclosed liquid is a plano-convex lens, while the crystalline lens is
double convex.(123) Because of the great difference in density between the
air on the outside and the aqueous humor within, the cornea is the more
powerful of the two. The crystalline lens, however, performs a special
work in focusing which is of great importance. The iris also aids in
focusing since it, through the pupil, regulates the amount of light
entering the back chamber of the eyeball and causes it to fall in the
center of the crystalline lens, the part which focuses most accurately.

[Fig. 161]

Fig. 161--*Diagram showing changes in shape of crystalline lens* to adapt
it to near and distant vision.

*Accommodation.*--A difficulty