No. 7.] BELL — THE PHOTOPHONE. 397 



THE PHOTOPHONE, 



By Alexander Graham Bell. 



In bringing before you some discoveries made by Mr. Sumner 

 Tainter and myself, which have resulted in the construction of 

 apparatus for the production and reproduction of sound by means 

 of light, it is necessary to explain the state of knowledge which 

 formed the starting point of our experiments. I shall first des- 

 cribe the remarkable substance .selenium, and the manipulations 

 devised by various experiments ; but the final result of our re- 

 searches has evidenced the class of substances sensitive to light- 

 vibrations, until we can propound the fact of such sensitiveness 

 being a general property of all matter. We have found this 

 property in gold, silver, platinum, iron, steel, brass, copper, zinc, 

 lead, antimony, German silver, Jenkin's metal. Babbitt's metal, 

 ivory, celluloid, gutta percha, hard rubber, soft vulcanized rubber 

 paper, parchment, wood, mica and silvered glass; and the only 

 substances from which we have not obtained results are carbon 

 and thin microscopic glass. We find that when a vibratory beam 

 of light falls upon these substances they emit sounds, — the pitch 

 of which depends upon the frequency of the vibratory change in 

 the light. We find farther that, when we control the form or 

 character of the light-vibration on selenium, and probably on the 

 other substances, we control the quality of the sound and obtain 

 all varieties of articulate speech. We can thus, without a con- 

 ducting wire as in electric telephony, speak from station to station, 

 wherever we can project a beam of light We have not had 

 opportunity of testing the limit to which this photophonic influence 

 can be extended, but we have spoken to and from points 213 meters 

 apart ; and there seems no reason to doubt that the results will 

 be obtained at whatever distance a beam of light can be flashed 

 from one observatory to another. The necessary privacy of our 

 experiments hitherto has alone prevented any attempt at deter- 

 minins; the extreme distance at which this new method of vocal 

 communication will be available. I shall now speak of selenium. 

 In the year 1817 Berzelius and Gottlieb Gahn made an 

 examination of the method of preparing sulphuric acid in use at 

 Gripsholm. During the course of this examination they observed 

 in the acid a sediment of a partly reddish, partly clear brown 



