e tube. At the
bottom, being suddenly stopped, its re-action will be partly outwards,
and so will drive the strip of air away from the tube. After this will
follow, for a like reason, the other phase of the wave, the rarefaction,
which will swing the strip of air towards the tube. This theory I
verified by filling the bellows with smoke, and watching the motion of
the escaping air and smoke with a stroboscope. This view is now
advocated by an organ-builder in England, Herman Smith; but whether he
discovered it before or after me, I do not know.

When a membrane vibrates, its motion is generally perceptible to the
eye; and it may have a very great amplitude of motion, as in the case of
the drum; and various instruments have been devised for the study of
vibrations, using membranes like rubber, gold-beater's skin, or even
tissue paper, to receive the vibrations. One of the musical instruments
of a former generation of boys was the comb. A strip of paper was placed
in front of it, and placed at the mouth, and sung through, the paper
responding to the pitch with a loose nasal sound. Koeenig fixed a
membrane across a small capsule, one side of which was connected by a
tube to any source of sound, and the other side to a gas-pipe and a
small burner. A sound made in the tube would shake the flame, and a
mirror moving in front of the flame would show a zigzag outline
corresponding to the sound vibrations.

In like manner if a thin rubber be stretched over the end of a tube one
or two inches in diameter and four or five inches long, and a bit of
looking-glass one-fourth of an inch square be made fast to the middle of
the membrane, the motions of the latter can be seen by letting a beam
of sunlight fall upon the mirror so as to be reflected upon a white wall
or screen a few feet away. (Fig. 8.)

[Illustration: FIG. 8.]

When a sound is made in this tube, the spot of light will at once assume
some peculiar form,--either a straight line with some knots of light in
it, or some curve simple or compound, and such as are known as Lissajous
curves. If, while some of these forms are upon the screen, the
instrument be moved sideways, the forms will change to undulating lines
with or without loops, varying with the pitch and intensity, but being
alike for the same pitch and intensity. (Fig. 9)

This instrument I called the opeidoscope.

[Illustration: FIG. 9.]

The vibration of a membrane and that of a solid differ chiefly in the