lly notice the after-images that
occur many times in the course of the day.

The Sense of Hearing

Sound, like light, is physically a wave motion, though the sound
vibrations are very different from those of light. They travel 1,100
feet a second, instead of 186,000 miles a second. Their wave-length is
measured in feet instead of in millionths of a millimeter, and their
vibration frequencies are counted in tens, hundreds and thousands per
second, instead of in millions of millions. But sound waves vary among
themselves in the same three ways that we {229} noticed in light
waves: in amplitude, in wave-length (or vibration rate), and in degree
of mixture of different wave-lengths.

Difference of amplitude (or energy) of sound waves produces difference
of loudness in auditory sensation, which thus corresponds to
brightness in visual sensation. Sounds can be arranged in order of
loudness, as visual sensations can be arranged in order of brightness,
both being examples of intensity series such as can be arranged in any
kind of sensation.

Difference of wave-length of sound waves produces difference in the
_pitch_ of auditory sensation, which thus corresponds to color in
visual sensation. Pitch ranges from the lowest notes, produced by the
longest audible waves, to the highest, produced by the shortest
audible waves. It is customary, in the case of sound waves, to speak
of vibration rate instead of wave-length, the two quantities being
inversely proportional to each other (in the same conducting medium).
The lowest audible sound is one of about sixteen vibrations per
second, and the highest one of about 30,000 per second, while the
waves to which the ear is most sensitive have a vibration rate of
about 1,000 to 4,000 per second. The ear begins to lose sensitiveness
as early as the age of thirty, and this loss is most noticeable at the
upper limit, which declines slowly from this age on.

Middle C of the piano (or any instrument) has a vibration rate of
about 260. Go up an octave from this and you double the number of
vibrations per second; go down an octave and you halve the number of
vibrations. Of any two notes that are an octave apart, the upper has
twice the vibration rate of the lower. The whole range of audible
notes, from 16 to 30,000 vibrations, thus amounts to about eleven
octaves, of which music employs about eight octaves, finding little
use for the upper and lower extremes of the {230} pitch series. The
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