with the point of the microphone, and sounds were obtained in all parts
of the illuminated area and in the corresponding area on the other side
of the diaphragm. Outside of this area on both sides of the diaphragm
the sounds became weaker and weaker, until, at a certain distance from
the center, they could no longer be perceived.

At the point where we would naturally place the supports of a Hughes
microphone (see Fig. 1) no sound was observed. We were also unable to
detect any audible effects when thepoint of the microphone was rested
against the support to which the diaphragm was attached. The negative
results obtained in Europe by Mr. Preece may, therefore, be reconciled
with the positive results obtained in America by Mr. Tainter and myself.
A still more curious demonstration of localization of vibration occurred
in the case of a large metallic mass. An intermittent beam of sunlight
was focused upon a brass weight (1 kilogramme), and the surface of the
weight was then explored with the microphone shown in Fig. 2. A feeble
but distinct sound was heard upon touching the surface within the
illuminated area and for a short distance outside, but not in other
parts.

In this experiment, as in the case of the thin diaphragm, absolute
contact between the point of the microphone and the surface explored was
necessary in order to obtain audible effects. Now I do not mean to
deny that sound waves may be originated in the manner suggested by Mr.
Preece, but I think that our experiments have demonstrated that the kind
of action described by Lord Raleigh actually occurs, and that it is
sufficient to account for the audible effects observed.

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