detect, with a delicate microphone, the sonorous vibrations that were
so easily observed in our experiments, might be explained upon the
supposition that he had employed the ordinary form of Hughes's
microphone shown in Fig. 1, and that the vibrating area was confined
to the central portion of the disk. Under such circumstances it might
easily happen that both the supports (a b) of the microphone might touch
portions of the diaphragm which were practically at rest. It would of
course be interesting to ascertain whether any such localization of the
vibration as that supposed really occurred, and I have great pleasure in
showing to you tonight the apparatus by means of which this point has
been investigated (see Fig. 2).

[Footnote 1: April 21, 1881.]

[Illustration: Fig. 2. A, Stiff wire. B, Diaphragm. C, Hearing tube. D,
Perforated handle.]

The instrument is a modification of the form of microphone devised in
1872 by the late Sir Charles Wheatstone, and it consists essentially of
a stiff wire, A, one end of which is rigidly attached to the center of
a metallic diaphragm, B. In Wheatstone's original arrangement the
diaphragm was placed directly against the ear, and the free extremity
of the wire was rested against some sounding body--like a watch. In the
present arrangement the diaphragm is clamped at the circumference like
a telephone diaphragm, and the sounds are conveyed to the ear through a
rubber hearing tube, c. The wire passes through the perforated handle,
D, and is exposed only at the extremity. When the point, A, was rested
against the center of a diaphragm upon which was focused an intermittent
beam of sunlight, a clear musical tone was perceived by applying the ear
to the hearing tube, c. The surface of the diaphragm was then explored