Until within recent years it was never supposed that a sunbeam would ever laugh except in poetry. But the modern scientist has taken it out of the realm of poetry and put it into the prosy play of every-day life. The Radiophone, invented by A. G. Bell, is an instrument by which articulate or other sounds are transmitted through the medium of a ray of light. It has as yet no practical application and has never gone beyond the experimental stage, but as a bit of scientific information it is very interesting.

If we introduce into an electric circuit a piece of selenium, prepared in a certain way, its resistance as an electric conductor undergoes a radical change when a beam of sunlight is thrown upon it. For instance, a selenium cell, so called, that in the dark would measure 300 ohms resistance, would have only about 150 ohms when exposed to sunlight. This amount of variation in a short circuit of low resistance would produce a considerable change in the strength of a current passing through it from a battery of a given voltage.

If now we connect a selenium cell to one pole of a battery, and thence through a telephone and back to the other pole, we have completed an electric circuit, of which the selenium cell is a part, and any variation of resistance in this cell, if made suddenly, will be heard in the telephone. Let the diaphragm of a telephone transmitter have a very light, thin mirror on one side of it, and a beam of sunlight be thrown upon it and reflected from that on to the selenium cell, which may be some distance away. Then, if the diaphragm is thrown into vibration by an articulate word or other sound, the light-ray is also thrown into vibration, which causes a vibratory change of resistance in the selenium cell in sympathy with the light-vibrations; and this in turn throws the electric current into a sympathetic vibratory state which is heard in the telephone. So that if a person laughs or talks or sings to the diaphragm, the sunbeam laughs, talks and sings and tells its story to the electric current, which impresses itself upon the telephone as audible sounds—articulate or otherwise. Instead of the telephone, battery and selenium cell, a block of vulcanite or certain other substances may be used as a receiver; as a light-ray thrown into vibration has the power to produce sound or sympathetic vibration in certain substances.

Another curious application of the selenium cell has been attempted, but has scarcely gone beyond the domain of theory. This apparatus, if perfected, might be called a Telephote. It is an apparatus by which an illuminated picture at one end of a line of many wires is reproduced upon a screen at the other end. The light is not actually transmitted, but only its effects. Suppose a picture is laid off into small squares and there is a selenium cell corresponding to each square and for each selenium cell there is a wire that runs to a distant station in which circuit there is a battery. At the distant station there are little shutters, one for each wire, that are controlled by the electric current and so adjusted that when the cell at the transmitting-end is in the dark the shutter will be closed. Now if a strong light be thrown upon the picture at the transmitting-end, and each square of the picture reflects the light upon its corresponding selenium cell, the high lights of the picture will reflect stronger light than the shadows, and therefore the wires corresponding to the high-light squares will have a stronger current of electricity flowing through them, because the resistance of the circuit is less than the ones connected with the darker shadows. So that the degree of current-strength in the various wires will correspond to the intensity of light reflected by the different sections of the picture. The shutters are so adjusted that the amount of opening depends upon the strength of current. The shutters corresponding to the high lights of the picture will open the widest and throw the strongest light upon the screen, from a source of light that is placed behind the shutters. The shutters that open the least will be those that are operated upon by the shadows of the picture. Inasmuch as a picture thrown on a screen from a source of light is wholly made up of lights and shadows, the theory is that this apparatus perfectly constructed would transmit any picture to a distance, through telegraph-wires. It must not be understood that the rays of light are transmitted through the wires as sound-vibrations are. Light, per se, can be transmitted only through the luminiferous ether, as we have seen in the chapter on light in Volume II.

While we are talking about these curious methods of telegraphic transmission, I wish to refer to an apparatus constructed by the writer in 1874-5, for the purpose of receiving musical tones or compositions transmitted from a distance through a wire by electricity. (A cut of this apparatus is shown on page 875 of "Electricity and Electric Telegraph," by Prescott, issued in 1877.) It consists of a disk of metal rotated by a crank mounted on a suitable stand. The electric circuit passes through the disk to the hand of the operator in contact with it, thence running through the line-wire to the distant station. Now, if a tune is played at that station, upon an electrical key-board, as described in a previous chapter, and the disk rotated with the fingers in contact with it, the tune or other sounds will be reproduced at the ends of the fingers. After the telephone was invented and put into use I used this revolving disk as a receiver for speech as well as music, and by this means persons may carry on an oral conversation through the ends of their fingers. This apparatus has been confounded in the minds of some people with Edison's electromotograph. The phenomena of the electromotograph were produced by chemical effects, while that of the apparatus just described is electrostatic in its action. The electrostatic disk was made in the winter of 1873-4, while Edison's electrochemical discovery was made some time later.