THE PHOTOPHONE.
The Photophone is the latest development of Professor Graham Bell's ingenuity, and for its scientific novelty, if not for its practical utility, well deserves a brief description. One of the elementary bodies, named selenium, and allied to sulphur, is known to undergo certain 'changes in its molecular structure when light falls upon it. These changes cause the very high resistance it offers to the passage of an electric current to vary slightly, and this curious effect, hitherto believed to be unique, has lately been the subject of investigation by various English physicists. It occurred to several that this substance might be employed as a sort of telephone, a beam of light being used to replace the conducting wires of the usual forms of these in- struments. Professor Graham Bell, the discoverer of the tele- phone, to whom, amongst others, this idea occurred, has had the good-fortune to throw that thought into practical shape.
A mirror, from which is reflected a powerful beam of light, may be caused to vibrate by means of the voice. These vibra- tions toss the beam of light slightly to and fro, and this vibrat- ing beam falls upon a selenium receiver, through which an electric current is passing, thereby creating slight variations in the resistances the current encounters. These tiny variations in electric resistance can be detected and rendered audible by that wonderfully sensitive little instrument, the Bell telephone. This was the conception which led Professor Bell to announce, in a lecture delivered before the Royal Institution so long ago as 1878, the " possibility of hearing a shadow fall upon a piece of selenium." Within the last few months, he has succeeded in putting this into practical execution.
In the articulating photophone, a beam of light, derived either from an artificial source or from the Sun, is thrown by a mirror on to the transmitter, which is a small disc of silvered glass, with a tube and mouthpiece attached. The beam of light reflected from the transmitter is focussed as nearly as pos- sible upon the distant receiver. When, therefore, words are spoken into the month-piece, the disc becomes agitated, alters slightly in shape, and, therefore, in its focal length, and thus • affects the receiving-station, by throwing upon it a greater or less amount of light, according as the beam is in or out of focus. If absolutely accurate adjust- ment were possible, and all disturbing elements could be eliminated, the varying amount of illumination received at the distant end would wholly depend upon the variations in sound at the transmitting end, and an exact reproduction of the original sounds would be obtained. This we cannot expect yet but the results already obtained lead one to hope that in time even this may be achieved.
The receiver of the photophone, as at present arranged, con- sists of a large concave mirror, which reflects and focusses the light upon a selenium cell ; this is connected with a battery, and a couple of ordinary telephones are included in the circuit. The selenium cell is very ingeniously adapted by Professor Bell to its purpose. It consists of alternate discs of brass and mica the edges of which are coated with selenium, pared to make it as thin as possible, whilst yet exposing a sufficiently large surface to the action of the light. Any increase of light, falling upon this selenium cell, lessens its electric resistance ; hence the vibrations of the mirror (caused by the words spoken into the mouthpiece by the transmitter), altering somewhat the amount of light received upon the cell, reproduce themselves audibly, by means of the greater or less amount of electricity thereby transmitted through the telephone. Both transmitter and receiver must, of course, be so supported as to be free to move, according to the direction in which the beam has to be sent or received.
There are many difficulties in the practical working of this little instrument, but though entirely satisfactory results have not yet been obtained, the principle is beyond dispute that sound and light can act upon one another in the manner described. Articulate speech has been transmitted by means of the telephone to a distance of some 230 yards, the voice being heard sometimes almost as loudly as in talking through an ordinary telephone, though the sound varies in intensity in an unaccountable manner.
Professor Bell has arrived at many interesting results while experimenting upon this instrument. He has found that curious molecular changes take place notonly in selenium, but also in thin surfaces of almost any substance ; so that they respond, by audible vibrations, to the action of an intermittent beam of light. There is a great difference, however, in the sensitiveness of the different substances ; vulcanite is one of the best, carbon is very good, but water is absolutely irresponsive, and glass, unsilvered, is also bad. Upon this discovery, Professor Bell has constructed a simple form of photophone for transmitting musical tones.
A beam of light is thrown upon a mirror, and focussed by a lens as before ; at the focus is a disc, perforated round its cir- cumference with numerous holes. From this disc, which can be rotated so that the beam passes through a varying number of holes, according to the speed of rotation, the light passes on to a receiving disc of ebonite, from whence the sounds are con- veyed by a tube to the listener. That these musical sounds— which are much louder than the spoken words—are really due to the action of light or radiant energy of some form, may be easily proved, for when the beam is interrupted by means of a disc of some opaque body, though the perforated disc is still rotating, nothing is heard at the receiver. No wires are needed as conductors between the transmitter and the receiver; the beam of light forms the only necessary connection, and this beam of light, with the simple apparatus described, has been the means of conveying distinct musical sounds to a distance of more than a mile. Not that even this distance is a necessary limit, for there is no reason why the sound should not be carried as far as the light can be thrown. We have here, in fact, a musical heliostat.
The real cause of the molecular changes accompanying this action of an intermittent beam of light upon different sub- stances is not yet certain. It appears probable, however, that the varying electric resistances of selenium are directly due to light; whilst, as with the radiometer, radiant heat is probably the real source of those molecular changes which produce the audible vibrations of vulcanite and other bodies. Whether, however, it be heat or light which is the original source of these vibrations, the wonder is equally great; for, if it be heat, the molecules composing the substance must be cooled and heated with sufficient rapidity to respond to vibrations, of which there may be many hundreds in a second. Science is every day showing us that we are only beginning to discern the subtler potencies of matter and energy, and we find that the goal of to- day becomes the starting-point of to-morrow, and that a barrier is no sooner reached, than it becomes a gateway to new and wider views of truth.
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