818 THE POPULAR SCIENCE MONTHLY. 



action of light, as described above, was an entirely original and inde- 

 pendent conception of my own, I recognize the fact that the knowl- 

 edge necessary for its conception has been disseminated throughout 

 the civilized world, and that the idea may therefore have occurred to 

 many other minds. The fundamental idea, on which rests the possi- 

 bility of producing speech by the action of light, is the conception of 

 what may be termed an undulatory beam of light in contradistinction 

 to a merely intermittent one. By an undulatory beam of light, I 

 mean a beam that shines continuously upon the selenium receiver, but 

 the intensity of which upon that receiver is subject to rapid changes, 

 corresponding to the changes in the vibratory movement of a particle 

 of air during the transmission of a sound of definite quality through 

 the atmosphere. The curve that would graphically represent the 

 changes of light would be similar in shape to that representing the 

 movement of the air. I do not know whether this conception had 

 been clearly realized by " J. F. W.," of Kew, or by Mr. Sargent, of 

 Philadelphia ; but to Mr. David Brown, of London, is undoubtedly due 

 the honor of having distinctly and independently formulated the con- 

 ception, and of having devised apparatus though of a crude nature 

 for carrying it into execution. It is greatly due to the genius and 

 perseverance of my friend Mr. Sumner Tainter, of Watertown, Mas- 

 sachusetts, that the problem of producing and reproducing sound by 

 the agency of light has at last been successfully solved. 



The first point to which we devoted our attention was the reduc- 

 tion of the resistance of crystalline selenium within manageable lim- 

 its. The resistance of selenium-cells employed by foi*mer experiment- 

 ers was measured in 'millions of ohms, and w r e do not know of any 

 record of a selenium-cell measuring less than 250,000 ohms in the 

 dark. We have succeeded in producing sensitive selenium-cells mea- 

 suring only 300 ohms in the dark, and 155 ohms in the light. All 

 former experimenters seemed to have used platinum for the conduct- 

 ing part of their selenium-cells, excepting Werner Siemens, who found 

 that iron and copper might be employed. We have also discovered 

 that brass, although chemically acted upon by selenium, forms an 

 excellent and convenient material ; indeed, we are inclined to believe 

 that the chemical action between the brass and selenium has contrib- 

 uted to the low resistance of our cells by forming an intimate bond 

 of union between the selenium and brass. We have observed that 

 melted selenium behaves to the other substances as water to a greasy 

 surface, and we are inclined to think that, when selenium is used in 

 connection with metals not chemically acted upon by it, the points of 

 contact between selenium and the metal offer a considerable amount 

 of resistance to the passage of a galvanic current. By using brass we 

 have been enabled to construct a large number of selenium-cells of 

 different forms. The mode of applying the selenium is as follows : 

 The cell is heated, and, when hot enough, a stick of selenium is rubbed 



