88 A. G. Bell—Applicability of a modification of 
He was thus foreed—by the supposed insufficiency of the 
explanation—to seek in some other direction the cause of the 
phenomenon observed, and, as a consequence, he adopted the 
ingenious hypothesis alluded to above. But the experiments 
which had proved unsuccessful in the hands of Mr, Preece were 
perfectly successful when repeated in America under better 
conditions of experiment, and the supposed necessity for 
another hypothesis at once vanished. I have shown, in a recent 
paper read before the National Academy of Science,* that audi- 
le sounds result from the expansion and contraction of tlie 
material exposed to the beam; and that a real to-and-fro vibra- 
tion of the diaphragm occurs capable of producing sonorous 
effects. It has occurred to me that Mr. Preece’s failure to 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 
A 
Sy SIE it might easily happen that both the 
\ 
5 
supports (A, B,) of the microphone might 
touch portions of the diaphragm which 
Z were practically at rest. It would of 
course be interesting to ascertain wheth- 
er any such localization of the vibration 
as that supposed really occurred, and I 
have great pleasure in showing to you 
t u e 
B, carbon supports ; . ‘ . ” ; : 
Bg baa PP vba: A point has been investigated 
The instrument is a modification of the form of microphone 
devised in 1827 by the late Sir Charles Wheatstone, and it con- 
sists essentially of a stiff wire (A), one end of which is rigidly 
attached to the center of a metallic diaphragm (B). In Wheat- 
stone’s original arrangement the diaphragm was placed directly 
against the ear, and the free extremity of the wire was rest 
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). e 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 focussed an intermittent beam 0 
sunlight, a clear musical tone was perceived by applying the 
ear to the hearing tube (C). The surface of the diaphragm was 
* April 21, 1881. 
