532 Mr. Shelford Bidivell [Marcli 11, 



intensity in correspondence with the varying phases of the complex 

 soimd-waves produced by the human voice. It is evident that if a 

 beam so regulated were allowed to fall upon the selenium cell, the 

 exact words spoken, with their articulation unimpaired, would be re- 

 produced in the telephone. Professor Bell adopted a device which is 

 equally marvellous for its extraordinary simplicity and for its perfect 

 efficiency. The beam of light is made to fall upon the face of a small 

 flexible mirror, whence it is reflected to the distant selenium cell, 

 lenses being used for the purpose of rendering the rays parallel and 

 condensing them where required. The speaker directs his voice upon 

 the back of the mirror, which takes up the sound-waves and is thrown 

 into a state of vibration, thus becoming alternately concave and 

 convex. Now, when it is concave the light reflected by it is more 

 concentrated, and the selenium cell more brightly illuminated. On 

 the other hand, when it is convex, the opposite effect is produced : the 

 rays are more dispersed and the illumination of the cell less intense. 

 And since the movements of the mirror are in exact correspondence 

 with the sound-waves of the voice, so also will be the intensity of the 

 illumination of the selenium cell. The strength of the current pass- 

 ing through it will vary in the same proportion, and will cause the 

 telephone plate to vibrate in consonance with the mirror, and thus to 

 reproduce the exact sounds by which the mirror was set in motion. 



In the small exi3erimental photophone which is before you, the 

 receiving station is within 20 feet of the transmitter, and any sounds 

 heard in the telephone would of course be utterly drowned by the 

 actual voice of the sj^eaker at the mirror. It is necessary, therefore, 

 to prolong the telephone wires, and carry them to a distant room, 

 where the sounds that have travelled along the beam of light can be 

 heard without interruption. Professor Bell, using instead of a lens 

 ^ large reflector for receiving the beam of light, has heard words 

 which were spoken when the mirror was 700 feet away from the 

 selenium cell. 



It is impossible to exhibit the photophone in action to an 

 audience, because the effects can only be heard by a single person 

 at a time. But I may mention, that in the course of some experi- 

 paents with this little instrument which Professor Tyndall very 

 kindly permitted me to make here on the 7th of December last, every 

 word transmitted by it was perfectly understood. 



I propose now to say a few words upon another and very different 

 application of selenium. In point of interest and imj^ortance it 

 cannot be compared with the photoi^houe : but since it is a child of 

 my own I naturally regard it with a certain amount of affection. It 

 occurred to me a few months ago that the wonderful jDroperty of 

 selenium, which we have been discussing this evening, might be 

 applied in the construction of an instrument for transmitting pictures 

 of natural objects to a distance along a telegraph wire. I have 

 constructed a rough experimental api^aratus in order to ascertain 

 whether my ideas could be carried out in practice, and it is so far 



