[17] Mr. Hughes remarks that the vibrations which affect the microphone, even in speaking at a distance from the instrument, do not proceed from the direct action of the sound waves on the contacts of the microphone, but from the molecular vibrations produced by it on the board which serves to support the instrument; he shows, in fact, that the intensity of sounds produced by the microphone is in proportion to the size of the surface of this board, and when the sending microphone is enclosed in a cylindrical case, its sensitiveness is not much diminished if the surface of the box enclosing the whole is sufficiently large. From this point of view he has sought to increase the sensitiveness of his instruments by fixing the frame on which the moveable parts of the sender and receiver revolve on a spring plate. [18] Helmholtz’s resonator is based upon the principle that a volume of air contained in an open vase emits a certain note when placed in vibration, and that the height of the note depends on the size of the vase and of its opening. Helmholtz makes use of a globe with a large opening on one side and a small one on the other, and the small one is applied to the ear. If a series of musical notes take place in the air, the one which is in harmony with the fundamental note of the globe is intensified, and can be distinguished from the rest. The same effect takes place when, on singing to a piano accompaniment, some strings are heard to vibrate more strongly than others, namely, those which vibrate in unison with the sounds emitted. The resonators are made in various ways; those most generally used are cases of different lengths which also serve as sounding-boxes. [19] I give the text of M. Cros’ sealed paper, opened by his request, at the Académie des Sciences, December 3, 1877:--‘Speaking generally, my process consists in obtaining traces of the movement to and fro of a vibrating membrane, and in using this tracing to reproduce the same movements, with their intrinsic relations of duration and intensity, either on the same membrane or on one adapted to give out the sounds which result from this series of movements. ‘It is therefore necessary that an extremely delicate tracing, such as may be obtained by passing a needle over a surface blackened by fire, should be transformed into a tracing, capable of sufficient resistance to guide an index which will transmit its movements to the membrane of sound. ‘A light index is fastened to the centre of a vibrating membrane; it terminates in a point (a metallic wire or tip of a feather) which rests on a surface which has been blackened by fire. This surface forms part of a disk, to which the double action of rotation and rectilinear progression has been given. If the membrane is at rest, the point will trace a simple spiral; if the membrane vibrates, there will be undulations in the spiral, and these undulations will represent the precise movements of the membrane in their duration and intensity.