ON THE PRODUCTION OF SOUND BY LIGHT. 817 



saw that the effect could be produced at the extreme distance at which 

 selenium would respond to the action of a luminous body, but that this 

 distance could be indefinitely increased by the use of a parallel beam 

 of light, so that we could telephone from one place to another without 

 the necessity of a conducting wire between the transmitter and receiver. 

 It was evidently necessary, in order to reduce this idea to practice, to 

 devise an apparatus to be operated by the voice of a speaker, by which 

 valuations could be produced in a parallel beam of light, corresponding 

 to the variations in the air produced by the voice. 



I proposed to pass light through a large number of small orifices, 

 which might be of any convenient shape, but were preferably in the 

 form of slits. Two similarly perforated plates were to be employed. 

 One was to be fixed and the other attached to the center of a dia- 

 phragm actuated by the voice, so that the vibration of the diaphragm 

 would cause the movable plate to slide to and fro over the surface of 

 the fixed plate, thus alternately enlarging and contracting the free ori- 

 fices for the passage of light. In this way the voice of a speaker could 

 control the amount of light passed through the perforated plates with- 

 out completely obstructing its passage. This apparatus was to be 

 placed in the path of a parallel beam of light, and the undulatory beam 

 emerging from the apparatus could be received at some distant place 

 upon a lens, or other apparatus, by means of which it could be con- 

 densed upon a sensitive piece of selenium placed in a local circuit with 

 a telephone and galvanic battery. The variations in the light produced 

 by the voice of the speaker should cause corresponding variations in 

 the electrical resistance of the selenium employed : and the telephone 

 in circuit with it should reproduce audibly the tones and articulations 

 of the speaker's voice. I obtained some selenium for the purpose of 

 producing the apparatus shown ; but found that its resistance was al- 

 most infinitely greater than that of any telephone that had been con- 

 structed, and I was unable to obtain any audible effects by the action 

 of light. I believed, however, that the obstacle could be overcome by 

 devising mechanical arrangements for reducing the resistance of the 

 selenium, and by constructing special telephones for the purpose. I 

 felt so much confidence in this that, in a lecture delivered before the 

 Royal Institute of Great Britain, upon the 17th of May, 1878, I an- 

 nounced the possibility of hearing a shadow by interrupting the action 

 of lio-ht upon selenium. A few days afterward my ideas upon this 

 subject received a fresh impetus by the announcement made by Mr. 

 Willoughby Smith before the Society of Telegraph Engineers that he 

 had heard the action of a ray of light falling upon a bar of crystalline 

 selenium, by listening to a telephone in circuit with it. 



It is not unlikely that the publicity given to the speaking telephone 

 during the last few years may have suggested to many minds in differ- 

 ent parts of the world somewhat similar ideas to my own. 



Although the idea of producing and reproducing sound by the 

 vol. xvii. 52 



