This I can demonstrate, not merely
by the loudness of the sound which can be heard, but by sight; for if a
small mirror be fixed on one of the prongs and a beam of light be cast upon
the mirror, the light being again reflected on to the screen, you will see
the spot of light dance up and down, and the more energetically the
tuning-fork is bowed the greater is the amplitude of the oscillation of the
spot of light. The duration of the time occupied is the same in traversing
a longer as in traversing a shorter space, as is the case of the swinging
pendulum. The vibrating prongs of the tuning-fork throw the air into
vibrations which are conveyed to the ear and produce the sensation of
sound. The duration of time occupied in the vibrations of the tuning-fork
is therefore independent of the space passed over. The greater or less
energy expended does not influence the duration of time occupied by the
vibration; it only influences the amplitude of the vibration.
The second quality of musical sounds is the pitch, and the pitch depends
upon the number of vibrations that a sounding body makes in each second of
time. The most unmusical ear can distinguish a high note from a low one,
even when the interval is not great. Low notes are characterised by a
relatively small number of vibrations, and as the pitch rises so the number
of vibrations increase.
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