502 



NA TURE 



\_Scpt. 



1880 



minds in diflerent parts of the world somewhat similar ideas to 

 my own. 



Although the idea of producing and reproducing sound by the 

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

 independent conception of my own, I recognise the fact that 

 the knowledge necessary for its conception has been dis- 

 seminated throughout the civilised world, and that the idea 

 may therefore have occurred to many other minds. The funda- 

 mental idea, on which rests the possibility of producing speech 

 by the action of light, is the conception of what may be 

 termed an undnlatory 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 lie 

 similar in shape to that representing tlie movement of the air. 

 I do not know whether this conception had been clearly real- 

 ised by "J. F. W.," of Kew, or by Mr. Sargent, of Phila- 

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

 due the honour of having distinctly and independently for- 

 mulated tlie conception, 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. Summer Tainter, of Watertown, Mass., that the problem 

 of producing and reproducing sound by the agency of liglit 

 has at last been successfully solved. The first point to 

 which we devoted our attention was the reduction of the resis- 

 tance of crystalline selenium within manageable limits. The 

 resistance of selenium cells employed by former e-xperimenters 

 was measured in millions of ohm=, and we 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 measuring only 300 ohms in the dnrh, and 155 ohms in tite 

 light. All former experimenters seem to have used platinum 

 for the conducting 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 contributed to the low i-esist- 

 anceof 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 inchned to think that when selenium is used in con- 

 nection with metals not chemically acted upon by it, the poinis 

 of contact between selenium and the metal offer a considerable 

 amount of resi-tance to the pas-age of a galvanic current. By usini; 

 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 over the surface. In order 

 to acquire conductivity and sensitiventss, the selenium must next 

 undergo a process of annealing. 



We simply heat the selenium over a gas stove and observe its 

 appearance. When the selenium attains a certain temperature, the 

 beautiful reflecting surface becomes dimmed. A cloudiness gradu- 

 ally extends over it, somewhat like the film of moisture produced by 

 breathing upon a mirror. This appearance gradually increases, and 

 the whole surface is soon seen to be in the metallic, granular, or 

 crystalline condition. The cell may then be taken off the stove, and 

 cooled in any suitable way. When the healing process is carried 

 too far, the crystalline selenium is seen to melt. Our best results 

 have been obtained by heating the selenium until it crystallises, 

 and continuing the heating until signs of melting appear, when 

 the gas is immediately put out. The portions that had melted 

 instantly recrystallise, and the selenium is found upon cooHng to 

 be a conductor, and to be sensitive to light. The whole opera- 

 tion occupies only a few minutes. This method has not only the 

 advantage of being expeditious, but it proves that many of the 

 accepted theories on this subject are fallacious. Our new method 

 shows that fusion is unnecessary, that conductivity and sensitive- 

 ness can be jiroduced w ithout long heating and slow cooling ; 

 and that crystallisation takes place during the heating process. 

 We have found that on removing the source of heat immediately 

 on the appearance of the cloudiness, distinct and separate crystals 

 can be observed under the microscope, which appear like leaden 

 snow-flakes on a ground of ruby red. Upon removing the heat. 



when crystallisation is further advanced, we perceive under the 

 microscope masses of these crystals arranged like b.isaltic 

 columns standing detached from one another, and at a still 

 higher point of heating the distinct columns are no longer trace- 

 able, but the whole mass resembles metallic pudding-stone, with 

 here and there a separate snow-flake, like a fossil, on the surface. 

 Selenium crystals formed during slow cooling after fusion present 

 an entirely different appearance, sho» ing distinct facets. 



We have devised, about fifty forms of apparatus for vary- 

 ing a beam of light in the manner required, but only a 

 few typical varieties need be shown. The source of light 

 may be controlled, or a steady beam may be modified at 

 any point in ils path. The beam may be controlled in many 

 ways. For instance, it may be polaristd, and then affected 

 by electrical or magnetic influences in the manner discovered 

 by Faraday and Dr. Kerr. 'Ihe beam of polarised light, 

 instead of being passed through a liquid, may be reflected from 

 the polished pole of an electro-magnet. Another method of 

 affecting a beam of light is to pass it throu.;h a lens of variable 

 focus. I observe that a lens of this kind has been invented in 

 France by Dr. Cusco, and is fully de.-cribed in a recen paper in 

 La Nature ; but Mr. Tainter and I have used such a lens in our 

 experiments for months past. The best and simjjlest form of 

 apparatus for producing the effect remains to be described. This- 

 consists of a plane mirror of flexible material — such as silvered 

 mica or microscope glass. Against the back of this mirror the 

 speaker's voice is directed. The light reflected from this mirror 

 is thus thrown into vibrations corresponding to those of the 

 diaphragm itself. 



In arranging the apparatus for the purpose of reproducing 

 sound at a distance any powerful source of light may be used, 

 but we have experimented chiefly with sunlight. For this pur- 

 p ise a large beam is concentrated by means of a lens upon the 

 diaphragm mirror, and, after reflection, is again lendered parallel 

 by means of another lens. The beam is received at a distant station 

 upon a parabolic reflector, in the focus of which is placed a sensitive 

 selenium cell, connected in a local circuit with a battery and tele- 

 phone. A large number of trials of this apparatus have been 

 made with the transmitting and receiving instruments so far 

 apart that sounds could not be heard directly through the air. 

 In illustration I shall describe one of the most recent of these 

 experiments. Mr. Tainter operated the transmitting in trument, 

 which was placed on the top of the Franklin schoolhouse in 

 Washington, and the sensitive receiver was arranged in one of 

 the windows of my laboratory, 1325 L street, at a di-tance of 

 213 metres. Up' u placing the telephone to my ear I heard 

 distinctly from the illuminated receiver the words : " Mr. Bell, 

 if you hear what I say come to the window and wave your hat." 

 In laboratory experiments the transmitting and receiving instru- 

 ments are necessarily within earshot of one another, and we 

 have, therefi re, been accustomed to pooling the electric circuit 

 connected ih the selenium receiver, so as to place the tele- 

 phones in another room. By such experiments we have found 

 that articulate speech can be reproduced by the oxyhydrogen 

 light, and even by the light of a kerosene lamp. The loudest 

 efl'ects obtained from light are produced Dy rajJidly interrupting 

 the beam by the perforated disk. The great advant.nge of this 

 form of apparatus for experimental work is the noiselessness of 

 its rotation, admitting the close approach of toe receiver without 

 interfering with the audibility of the effect heard from the latter ; 

 for it will be understood that musical tones are emitted from the 

 receiver when no sound is made at the transmitter. A silent 

 motion thus produces a sound. In this way musical tones have 

 been heard even from the light of a candle. When distant 

 elfects are sought another apparatus is used. By placing an 

 opaque screen near the rotating disk the beam can he entirely 

 cut off by a slight motion of the band, and musical signals, like 

 the dots and dashes of the Morse telegra])h code, can thus be 

 produced at the distant receiving station. 



We have made ex| eriments, with the object of ascertaining 

 the nature of the rays that affect selenium. For this purpose we 

 have placed in the path of an intermittent beam various absorb- 

 ing substances. I'rof. Cross has been kind enough to give me 

 his assistance in conducting these experiments. When a solu- 

 tion of alum or bisulphide of carbon, is employed, the loudness 

 of the sound produced by the intermittent beam is very slightly 

 diminished ; but a snlution of iodine in bisulphide of carbon cuts 

 off most, but not ;dl, of the audible eft'ect. Even an a] parently 

 opaque sheet ot hard rubber does not entirely do this. When 

 the .'.heet of hard rubber was held near the disk interrupter the 



