A stretched string, as that of a piano-forte, may be made to vibrate
not only from end to end, but in aliquot parts, the portions being
separated by points of rest which interrupted the progress of the
sound. This kind of effect may be shown by shaking a long piece of
cane in the air, when there will be one, two, or three points of rest,
according to the mode of vibrating it.
An elastic surface has, likewise, some parts in motion and others at
rest; and these parts may be made visibly distinct, by strewing pieces
of bristle over them upon the sounding-board of an instrument.
When a bow is drawn across the strings of a violin, the impulses
produced may be rendered evident by fixing a small steel bead upon the
bow; when looked at by light or in sunshine, the bead will seem to form
a series of dots during the passage of the bow.