=Vision.= _The Visual Sense Organs._ The details of the anatomy of the eye
can be looked up in a physiological textbook. The essential principles
are very simple. The eye is made on the principle of a photographer's
camera. The retina corresponds to the sensitive plate of the camera. The
light coming from objects toward which the eyes are directed is focused
on the retina, forming there an image of the object. The light thus
focused on the retina sets up a chemical change in the delicate nerve
tissue; this excitation is transmitted through the optic nerve to the
occipital (back) part of the brain, and sets up brain action there. Then
we have visual sensation; we see the object.

The different colors that we see are dependent upon the vibration
frequency of the ether. The higher frequencies give us the colors blue
and green, and the lower frequencies give us the colors yellow and red.
The intermediate frequencies give us the intermediate colors blue-green
and orange. By vibration frequencies is meant the rate at which the
ether vibrates, the number of vibrations a second. If the reader wishes
to know something about these frequencies, such information can be found
in a textbook on physics.

It will be found that the vibration rates of the ether are very great.
It is only within a certain range of vibration frequency that sunlight
affects the retina. Slower rates of vibration than that producing red do
not affect the eye, and faster than that producing violet do not affect
the eye. The lightness and darkness of a color are dependent upon the
intensity of the vibration. Red, for example, is produced by a certain
vibration frequency. The more intense the vibration, the brighter the
red; the less intense, the darker the red.