by which waves of one
length produce one kind of sensation and those of another length a
different sensation is not understood, the cones appear to be the portions
of the retina acted on to produce the color. On the other hand, the rods
are sensitive to all wave lengths and give general sensibility to light.

*Visual Perceptions.*--"Seeing" is very largely the mental interpretation
of the primary sensations and the conditions under which they occur. For
example, our ability to see objects in their natural positions when their
images are inverted on the retina is explained by the fact that we are not
conscious of the retinal image, but of the mind's interpretation of it
through experience. Experience has also taught us to locate objects in the
direction toward which it is necessary to turn the eyes in order to see
them. In other words, we see objects in the direction from which the light
enters the eyes. That the object is not always in that direction is shown
by the image in the mirror. The apparent size and form of objects are
inferences, and they are based in part upon the size and form of the area
of the retina stimulated. We judge of distance by the effort required to
converge the eyes upon the objects, by the amount of divergence of the
waves entering the pupil, and also by the apparent size of the object.

*The Lachrymal Apparatus.*--Seeing requires that the light penetrate to the
retina. For this reason all the structures in front of the retina are
transparent. One of these structures, the cornea, on account of its
exposure to the air, is liable to become dry, like the skin, and to lose
its transparency. To preserve the transparency of the cornea, and also to
lubricate the eyelids and aid in the removal of foreign bodies, a
secretion, called _tears_, is constantly supplied.

[Fig. 163]

Fig. 163--*Diagram of irrigating system of the eye.* After wetting the
eyeball the tears may also moisten the air entering the lungs.

The lachrymal, or tear, glands are situated at the upper and outer margins
of the orbits. They have the general structure of the salivary glands and
discharge their liquid by small ducts beneath the upper lids. From here
the tears spread over the surfaces of the eyeballs and find their way in
each eye to two small canals whose openings may be seen on the edges of
the lids near the inner corner (Fig. 163). These canals unite to form the
_nasal duct_, w