Notices of the proceedings at the meetings of the members of the Royal Institution of Great Britain with abstracts of the discourses . cm., thiscase corresponds to the curve B of our diagram. In connection with the mathematical investigation which led tothe results represented graphically in Fig. 2, there is a point of in-terest which I should like to mention. The investigation was carriedout upon the supposition that the source of sound is at a considerabledistance, so that the waves reaching the sphere are plane ; and thatthe receiver, by which the sound is detected, is situated on thesurfac
Notices of the proceedings at the meetings of the members of the Royal Institution of Great Britain with abstracts of the discourses . cm., thiscase corresponds to the curve B of our diagram. In connection with the mathematical investigation which led tothe results represented graphically in Fig. 2, there is a point of in-terest which I should like to mention. The investigation was carriedout upon the supposition that the source of sound is at a considerabledistance, so that the waves reaching the sphere are plane ; and thatthe receiver, by which the sound is detected, is situated on thesurface of the sphere. At any given position on the surface of thesphere, the receiver will indicate the reception of sound of a certainintensity, which may be read off from Fig. 2. Now the final resultsassume a form which shows that, if the positions of the source andthe receiver are interchanged, the latter will indicate the receptionof sound of the same intensity as in the original arrangement. Thuseach of the curves in Fig. 2 represents the solution of two distinctproblems: the intensity of the sound derived from a distant source. 432 Lord Bayleigh [Jan. 15, and detected at any point on the surface of the sphere; andthe intensity of the sound derived from a source on the surface ofthe sphere, and observed at a distant point. This result forms aninteresting example of a principle of very wide application, which Ihave termed the Principle of Reciprocity. Some special cases weregiven many years ago by Helmholtz. It is a matter of common observation that if one person can seeanother, either directly or by means of any number of reflectionsin mirrors, then the second person can equally well see the same law applies to hearing, apparent exceptions being easilyexplained. For instance, such is the case of a lady sitting in aclosed carriage, listening to a gentleman talking to her through theopen window. If the street is noisy, the lady can hear what thegentleman says very much
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