. " With the assistance of a microphone one can hear the footsteps of a fly as loudly as if it were the tramping of a horse on a wooden bridge ; but it strikes me as much more wonderful that by means of a telephone I can hear a ray of light falling on a metal ; When light falls upon selenium prepared in a special way, a change takes place in its conductivity, and this change occurs almost instantaneously. It is the rays at the red end of the spectrum which are most efficient in bringing about this change. The conducti\dty is nearly double in sunUght what it is in the dark. When
. " With the assistance of a microphone one can hear the footsteps of a fly as loudly as if it were the tramping of a horse on a wooden bridge ; but it strikes me as much more wonderful that by means of a telephone I can hear a ray of light falling on a metal ; When light falls upon selenium prepared in a special way, a change takes place in its conductivity, and this change occurs almost instantaneously. It is the rays at the red end of the spectrum which are most efficient in bringing about this change. The conducti\dty is nearly double in sunUght what it is in the dark. When the source of light is removed, the drop in con- ductivity occurs very rapidly. To get the maximum effect from a small amount of material, the selenium must be prepared in a special form. Great ingenuity has been displayed in attack- ing this problem. Graham Bell, the inventor of the telephone, was the first to make a reliable selenium cell, as it is called, though Werner Siemens three years earlier, in 1875, had made an attempt. Presser in THE OPTOPHONE WITH BOOK-REST REMOVED. easily be damaged ; furthermore, owing to their small size, they can be suitably mounted in any way for the purpose in hand. It is a modification of these Httle tablets that is the actuating principle of the optophone. Now, if an ordinary telephone receiver be connected in series with an electric battery and one of these selenium tablets, a current will pass through the tablet, and the current will vary as the light falling on the tablet varies, since this alters its conductivity. The tablet is called a " selenium ; When flashes of light are thrown on to the selenium bridge at a rate of 256 per second, the current will rise and fall at that rate, and the telephone will sing out the note middle C of the piano. If the pulsations of the light are at half that frequency, that is, 128 per second, the telephone will sing out C an octave lower, and, with double the f^equenc3^ 512 pulsations pe
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