assing through the lens.

[Fig. 158]

Fig. 158--*Diagram illustrating formation of images.* On the right the
image is formed by a double convex lens; on the left by the lenses of the
eye. The candle flame represents a luminous, or light-giving, body; but
light passes from the large arrow by reflection. (See text.)

In order to form an image, the light waves spreading out from the object
must be brought together, or focused. Focusing means literally the
bringing of light to a point, but it is evident in the formation of an
image that all the waves are not brought to a single point. If they were,
there would be no image. In the example of the candle given above, the
explanation is as follows:

The light from the candle comes from a great number of separate and
distinct points in the candle flame. The lens, by its peculiar shape,
bends the waves coming from any single point so that they are brought to a
corresponding point on the screen. Furthermore, the points of focused
light are made to occupy the same relative positions on the screen as the
points from which they emanate in the candle flame (Fig. 158). This is why
the area of light on the screen has the same form as the candle, or makes
an image of it. The same explanation applies if, instead of the luminous
candle, a body that simply reflects light, as a book, is used.

*The Problem of Seeing.*--What we call _seeing_ is vastly more than the
stimulation of the brain through the action of light upon afferent
neurons. It is the _perceiving _of all the different things that make up
our surroundings. If one looks toward the clear sky, he receives a
_sensation of light_, but sees no object. He may also get a sensation of
light with the eyelids closed, if he turn the eyes toward the window or
some bright light. But how different when the light from various objects
enters the eyes. There is apparently no consciousness of light, but
instead a consciousness of the size, form, color, and position of the
objects. _Seeing is perceiving objects._ Stimulation by the light waves is
only the means toward this end. The chief problem in the study of sight is
that of determining _how light waves enable us to become conscious of
objects._

*Sense Organs of Sight.*--The sense organs of sight consist mainly of the
two eyeballs. Each of these is located in a cavity of the skull bones,
called the _orbit_, where it is held in position by suita