The apparatus is shown in Fig. 43 on the left of which is the transmitter of the sounds to be sent and on the right is the receiver. The transmitter consists of a vibrating diaphragm D, fitted with a mouthpiece

like that of a speaking telephone. A beam of light from any source sufficiently bright is concentrated on the diaphragm D by the lens L, and the diaphragm, which is silvered so as to reflect the light, is placed in such a position relatively to the lens L as to project the light along a line joining the axis of another lens L' with the axis of a parabolic reflector R, placed at the distant station and forming part of the receiving apparatus there. The object of the lens L' is to render the divergent rays proceeding from the diaphragm sufficiently parallel as to strike the interior of the reflector R, and be again reflected to a single focus, wherein is placed a small selenium cell s which is electrically sensitive to light. This selenium cell is connected in the circuit of a voltaic battery в and a pair of speaking telephones T T'.

If a person speaks or makes a sound into the mouthpiece of the transmitting apparatus, the diaphragm D

will vibrate in accordance with it, and the vibrating reflector will undulate a beam of light thrown from it, and these undulations travelling along the parallel beam to the reflector at the receiving station, will be directed on the selenium cell, through which the electric current from the battery is flowing. They will consequently vary the internal resistance of the selenium to the passage of the current, and in this way modify the strength of the current flowing through the telephone. As the resistance of selenium falls under the impact of light and falls the more the stronger the light, it follows that the strength of current will vary in proportion to the intensity of the light shed on the cell. In short, the current passing through the telephone will ebb and flow in sympathy with the flickering beam, and therefore with the vibrations of the diaphragm. The result is that the sounds heard in the receiving telephone are an imitation of those uttered at the transmitter. In this way Professor Bell has sent a whisper along a ray of light for several hundred feet. Different forms of photophone, or, to use a more general term, radiophone, have been devised. In some a conical mirror is used as the receiving reflector with good effect. But the chief differences in these lie in the kind of receiving cell employed.

Fig. 44 represents an excellent form of cell devised by M. Mercadier, a French electrician. It consists of two ribbons of brass a b, separated by two ribbons of parchment paper, and rolled into a spiral, then placed been two plates of brass e d, which are connected to the ends a' b', of the two brass ribbons. The brass plates are held between two boards by ties M N, and terminals B A are fitted to these for connect

ing up the cell. The narrow spaces between the edges of the two brass ribbons is then filled up with selenium, put on by heating the point of a selenium pencil until it softens, and drawing it along the surface of the cell. To get good results, it is best to have a battery of ten or fifteen elements on the cells and to employ telephones having a large coil of very fine wire.

M

FIG. 44.

Instead of selenium, tellurium may be used, as was discovered by Professor W. G. Adams. But a simple streak of lamp-black was also found to respond to the light waves by Mr. Summer Tainter, the associate of Professor Bell in his photophonic researches. With lamp-black it is not absolutely necessary to have a telephone and battery in circuit, as the couch of soft carbon itself gives out sounds under the influence of the light, which are audible to an ear held close to it. The effect is feeble but distinct; and is probably due

to the heat rays of the light expanding the material as they fall upon it. Other substances, such as gases enclosed in glass bulbs, also emit sounds by absorbing the heat, and dilating under it. But in the selenium photophone, where electricity is employed, the effect is proved to be due to the luminous rays, and is strongest with a yellow light.

Musical tones can of course be sent by substituting a sounding instrument for the voice, as in Fig. 43; but they can be elicited from the receiving telephone without being actually transmitted. This is done by simply eclipsing the beam of light directed on the receiver. A rapidly rotating wheel or disc is interposed in the path of the beam, and being drilled with holes round its rim, these holes allow the light to pass as they come in front of it. On the other hand, the body of the disc between the holes cuts off the light. Flashes of light are thus sent to the receiver, and every flash causes a pulse of current to traverse the telephone. These pulses rapidly ensuing, excite a musical note in the telephone, of a pitch determined by the number of eclipses per second, that is to say, by the number of holes in the disc, and the speed of its rotation.

This device, which is in reality a "light-syren," has been turned by M. Mercadier into a tone telegraph, like that of Mr. Elisha Gray. The teleradiophone, as he calls it, will be understood from Fig. 45, where we have a right and left hand station (say R and L), with the telegraph "line" between. For the sake of simplicity let us suppose that R is sending a tone telegram to L. Although each station is provided with both transmitters and receivers, we have in this case only to

consider the transmitters

at R and the receivers at L. There are two transmitters and two receivers, but we need only take note of one at present. Each transmitter consists of a selenium cells, connected to the line on one hand and to "earth" through a battery P on the other. A beam of light from a lamp is directed by a lens on a mirror from which it is reflected through a second lens upon the selenium cell s' after the manner of the photophone. In the path of this beam just at the focus in front of the cell is inserted a rotating disc or wheel I' with holes round its rim, so that when the disc is rotated the wheel will occult the light, and flash it during each turn as often as there are holes in the rim. In this way a continuous musical tone is evoked in the telephone T2 at the receiving station. Now this constant hum