We now come to the more modern instruments of Gray, Bell, and Edison. So far the receivers of Reis and of Yeates were intended for reproducing any sound; but now for the first time, ten years after the date of these early telephonic receivers, we meet with instruments devised with the express purpose of receiving only certain selected tones. For the purposes of multiple acoustic telegraphy, that is to say for the purpose of signalling the “dots” and “dashes” of the Morse code in a number of different fixed musical notes, each of which is to be signalled out and repeated by a receiver adapted to vibrate in that note alone, it is clear that the instruments of Reis, adapted as they were to transmit and receive _any_ sound that a human ear can hear, would not answer. Accordingly those experimenters, who from about the year 1873 to the year 1870, applied themselves to multiple telegraphy--foremost amongst them being Mr. Elisha Gray and Prof. Graham Bell--dropped the use of the tympanum in the transmitter and devised new transmitters and new receivers, in most of which the ruling idea was that of employing a vibrating tongue or reed, tuned up to one particular note. Now it is obvious that a receiver which, like those of Reis, is adapted to receive _any_ tone, can also receive a musical note. But for the operation of “selective” reception, a receiver must be employed, not only tuned to one note, but tuned to the very note emitted by the particular transmitter with which it is to be in correspondence. Elisha Gray found this out very early in his researches. In the winter of 1873-4[49] he was transmitting musical tones by a sort of tuning-fork interruptor, and received them on an instrument shown in Fig. 46 E, which represents a form of electro-magnet mounted for the purpose. It was “a common electro-magnet, having a bar of iron rigidly fixed at one pole, which extends across the other pole, but does not touch it by about one sixty-fourth part of an inch. In the middle of this armature a short post is fastened, and the whole is mounted on a box made of thin pine, with openings for acoustic effects.” It was, in fact, very similar to Yeates’s receiver just described, and Gray found it capable of receiving not only simple musical tones but composite tones, and even harmonies and discords. In fact, like Reis’s and Yeates’s receivers, it could receive anything that the transmitter sent to it, even including speech. Now this did not suit Gray, who wished to have selective receivers, one to take up note A, another note C, &c. Accordingly in 1870 we find Gray taking out a fresh patent[50] for selective receivers, which he also called harmonic analysers, each of which consisted of “a tuned bar or reed suitably attached to an electro-magnet, and the whole mounted upon a resonant box.” Fig. 46 _F_ is reproduced from Gray’s British patent. “A vibrating tongue reed, or bar” of steel “is united with one pole of the magnet. The free end