felt from spreading the palms of the hands out to
the fire has a "bigness" not felt from heating one solitary finger. The
extensity of a sensation depends on the number of nerve endings
stimulated.

The _duration_ of a sensation refers to the time it lasts. This must not
be confused with the duration of the stimulus, which may be either
longer or shorter than the duration of the sensation. Every sensation
must exist for some space of time, long or short, or it would have no
part in consciousness.

3. SENSORY QUALITIES AND THEIR END-ORGANS

All are familiar with the "five senses" of our elementary physiologies,
sight, hearing, taste, smell, and touch. A more complete study of
sensation reveals nearly three times this number, however. This is to
say that the body is equipped with more than a dozen different kinds of
end-organs, each prepared to receive its own particular type of
stimulus. It must also be understood that some of the end-organs yield
more than one sense. The eye, for example, gives not only visual but
muscular sensations; the ear not only auditory, but tactual; the tongue
not only gustatory, but tactual and cold and warmth sensations.

SIGHT.--Vision is a _distance_ sense; we can see afar off. The stimulus
is _chemical_ in its action; this means that the ether waves, on
striking the retina, cause a chemical change which sets up the nerve
current responsible for the sensation.

The eye, whose general structure is sufficiently described in all
standard physiologies, consists of a visual apparatus designed to bring
the images of objects to a clear focus on the retina at the _fovea_, or
area of clearest vision, near the point of entrance of the optic nerve.

The sensation of sight coming from this retinal image unaided by other
sensations gives us but two qualities, _light and color_. The eye can
distinguish many different grades of light from purest white on through
the various grays to densest black. The range is greater still in color.
We speak of the seven colors of the spectrum, violet, indigo, blue,
green, yellow, orange, and red. But this is not a very serviceable
classification, since the average eye can distinguish about 35,000 color
effects. It is also somewhat bewildering to find that all these colors
seem to be produced from the four fundamental hues, red, green, yellow,
and blue, plus the various tints. These four, combined in varying
proportions and with different degrees of light (i.e.,