ue as elementary sensations,
since neither of them could be reduced to a blend of the other with
white or black; and there are no other colors present in this form of
color vision to serve as possible elements out of which yellow and
blue might be compounded. That white, black, yellow and blue are
elementary sensations is therefore clear from the study of visual
sensations alone; and there are indications that red and green are
also elements.

Visual Sensations as Related to the Stimulus

Thus far, we have said nothing of the stimulus that arouses visual
sensations. Light, the stimulus, is physically a wave motion, its
vibrations succeeding each other at the rate of 500,000000,000000
vibrations, more or less, per second, and moving through space with a
speed of 186,000 miles per second. The "wave-length", or distance from
the crest of one wave to the crest of the next following, is measured
in millionths of a millimeter.

The most important single step ever taken towards a knowledge of the
physics of light, and incidentally towards a knowledge of visual
sensations, was Newton's analysis of white light into the spectrum. He
found that when white light is passed through a prism, it is broken up
into all the colors of the rainbow or spectrum. Sunlight consists of a
{213} mixture of waves of various lengths. At one end of the spectrum
are the long waves (wave-length 760 millionths of a millimeter), at
the other end are the short waves (wavelength 390), and in between are
waves of every intermediate length, arranged in order from the longest
to the shortest. The longest waves give the sensation of red, and the
shortest that of violet, a slightly reddish blue.

Outside the limits of the visible spectrum, however, there are waves
still longer and shorter, incapable of arousing the retina, though the
very long waves, beyond the red, arouse the sensation of warmth from
the skin, and the very short waves, beyond the violet, though arousing
none of the senses, do effect the photographic plate. Newton
distinguished seven colors in the visible spectrum, red, orange,
yellow, green, blue, indigo and violet; but there is nothing specially
scientific about this list, since physically there are not seven but
an unlimited number of wave-lengths included in the spectrum, varying
continuously from the longest at the red end to the shortest at the
violet; while psychologically the number of distinguishable colors in
the spectrum, thou