then pulled taut at right angles to the plane of the diaphragm. When the intermittent beam was focused upon the strip (A), a clear musical tone could he heard by applying the ear to the hearing-tube (C).
This seemed to indicate a rapid expansion and contraction of the substance under trial.
But a vibration of the diaphragm (B) would also have resulted if the thin strip (A) had acquired a to-and-fro motion, due either to the direct impact of the beam or to the sudden expansion of the air in contact with the strip.
2. To test whether this had been the case, an additional strip (D) was attached by its central point only to the strip under trial, and was then submitted to the action of the beam, as shown in Fig. 6.
It was presumed that, if the vibration of the diaphragm (B) had been due to a pushing force acting on the strip (A), the addition of the strip (D) would not interfere with the effect; but, if, on the other hand, it had been due to the longitudinal
expansion and contraction of the strip (A), the sound would cease, or at least be reduced. The beam of light falling upon the strip (D) was now interrupted as before by the rapid rotation of a perforated disk, which was allowed to come gradually to rest.
No sound was heard excepting at a certain speed of rotation, when a feeble musical tone became audible.