SCIENCE.' 



m 



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 conception, 

 and of having devised apparatus — though of a crude nature — lor 

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

 severance of my friend, Mr. Sumner Tainter, of Watertown, 

 Mass., 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 

 limits. The resistance of selenium cells employed by former ex- 

 perimenters was measured in millions of ohms, 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 dark, and 155 ohms in 

 the light. All former experimenters seemed to have used platinum 

 for the conducting part ot their selenium cells, excepting Werner 

 Siemens, who found that iron and copper might be employed. 

 We have also discovered thit brass, although chemically acted 

 upon by selenium, forms an excellent and convenient material; 

 indeed, we are inclined to believe that the chemical action be- 

 tween the brass and selenium has contributed to the low resistance 

 ot our cells by forming an intimate bond of union between the 

 selenium and brass. We have observed that melted selenium be- 

 haves 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 consiruct 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 con- 

 ductivity and sensitiveness, 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 

 gradually extends over it, somewhat like the film of moisture pro- 

 duced 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 heating 

 process is carried too far, the crystalline selenium is seen to melt. 

 Our best results have been obtained by heating the selenium 

 until it crystalizes, and continuing the heating until signs of melt- 

 ing appear, when the gas is immediately put out. '1 he por.ions 

 that had melted instantly re-crystallize, and the selenium is found 

 upon cooling to be a conductor, and to be sensitive to light. The 

 whole operation occupies only a few minutes. This method has not 

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

 ot the accepted theories on this subject are fallacious. Our new 

 method shows that tusion is unnecessary, that conductivity and 

 sensitiveness can be produced without long heating and slow cool- 

 ing; and that crystalization takes place during the heating pro- 

 cess. We have found that on removing the source of heat imme- 

 diately 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 crystalization is further advanced, we perceive under 

 the microscope masses of these crystals arranged like basaltic 

 columns standing detached from one another, and at a still higher 

 point of heating the distinct columns are no longer traceable, 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 en- 

 tirely different appearance, showing distinct facets. 



We have devised about fifty torms of apparatus for varying a 

 beam of light in the manner required, but only a few typical vari- 

 eties need be shown. The source of light may be controlled, or a 

 steady beam may be modified at any point in its path. The beam 

 may De controlled in many ways. For instance, it may be polar- 

 ized, and then affected by electrical or magnetic influences in the 

 manner discovered by Faraday and Dr. Ker. The beam of polar- 

 ized light, instead of being passed through a liquid, may be re- 

 flected from the polished pole of an ,-lcCtro-magnet. Another 

 method of affecting a beam ol light is to pass it through a lens of 

 vaiiable focus. I obseive that a lens of this kind has been invent- 

 ed in France by Dr. Cusco, and is fully described in a recent paper 

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

 our experiments for months past. The best and simplest form of 

 apparatus for producing the effect remains to be described. This 

 consists of a plain miiror of flexible material — such as silvered 

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

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

 thus thrown into vibration corresponding to those of the diaphragm 

 itielf. 



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 purpose a large 

 beam is concentrated by means of a lens upon the diaphragm mir- 

 ror, and, after reflection, is again rendered parallel by means of 

 another lens. The beam is received at a distant station upon a par- 

 abolic reflector, in the focus of which is placed a sensitive selenium 

 cell, connected in a local circuit with a battery and telephone. 

 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 instrument, 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 labora- 

 tory, 1325 L street, at a distance of 213 metres. Upon placing the 

 telephone to my ear I heard distinctly from the illuminated re- 

 ceiver 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 instruments are necessarily within ear- 

 shot of one another, and we have therefore been accustomed to 

 pooling the electric circuit connected with the selenium receiver, 

 so as to place the telephones in another room. By such experi- 

 ments we have found that articulate speech can be reproduced by 

 the oxy-hydrogen light, and even by the light of a kerosene lamp. 

 The loudest effects obtained from light are produced by rapidly in- 

 terrupting the beam by the perforated disk. The great advantage of 

 this form of apparatus for experimental work is the noiselessness 

 of its rotation, admitting the close approach of the receiver without 

 interfering with the audibility of ihe effect heard trom 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 mo- 

 tion thus produces a sound. In this way musical tones have been 

 heard even from the light of a candle. When distant effects are 

 sought another apparatus is used. By placing an opaque screen 

 near the rotating disk the beam can be entirely cut off by a slight 

 motion of the hand, and musical signals, like the dots and dashes 

 of the Morse telegraph code, can thus be produced at the distant 

 receiving station. 



We have made experiments, 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 absorbing sub- 

 stances. Professor Cross has been kind enough to give me his as- 

 sistance in conducting these experiments. When a solution of 

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

 the sound produced by the intermittent beam is very slightly 

 diminished ; but a solution of iodine in bisulphide of carbon 

 cuts off most, but not all, ot the audible effect. Even an ap- 

 parently opaque sheet of hard rubber does not entirely do this. 

 When the sheet of hard rubber was held near the disk interrupter 

 the rotation of the disk interrupted what was then an invisible 

 beam, which passed over a space of about twelve feet before it 

 reached the lens which finally concentrated it upon the selenium 

 cell. A famt but perfectly perceptible musical tone was heard trom 

 the telephone connected with the selenium. This could be inter- 

 rupted at will by placing the hand in the path of the invisible 

 beam. It would be premature, without further experiments, to 

 speculate too much concerning the nature of these invisible rays ; 

 but it is difficult to believe that they can be bent rays, as the effect 

 is produced through two sheets of hard rubber containing between 

 them a saturated solution of alum. Although effects are produced 

 as above shown by forms of radiant energy which are invisible, 

 we have named the apparatus for the production and reproduction 

 of sound in this way " The Photophone," because an ordinary beam 

 of light contains the rays which are operative. 



It is a well-known fact that the molecular disturbance produced 

 in a mass of iron by the magnetizing influence of an intermittent 

 electrical current can be observed as sound by placing the ear in 

 close contact with the iron. It occurred to us that the molecular 

 disturbance produced in crystalline selenium by the action of an 

 intermittent beam of light should be audible in a similar manner 

 without the aid of a telephone or battery. Many experiments 

 were made to verify this theory without definite results. The 

 anomalous behavior of the hard rubber screen suggested the 

 thought of listening to it also. This experiment was tried with 

 extraordinary success. I held the sheet in close contact with my 

 ear, while a beam of intermittent light was focussed upon it by a 

 lens. A distinct musical note was immediately heard. We found 

 the effect intens.fied by arranging the sheet of hard rubber as a 

 diaphragm, and listening thiough a hearing-tube. We then tried 

 crystalline selenium in the form of a thin disk, and obtained a 

 similar but less intense effect. The other substances which I 

 enumerated at the beginning of my address were now successively 

 tried in the form of thin disks, and sounds were obtained from all 

 but carbon and thin glass. We found hard rubber to produce 

 a louder sound than any other substance we tried, excepting an- 

 timony, and paper and mica to produce the weakest sound. On 

 the whole, we feel warranted in announcing as our conclusion that 

 souuds can be produced by the action oj a variable light from substances 

 of all kinds, when in the form of thin diaphragms. We have heard 

 trom interrupted sunlight very perceptible musical tunes through 

 tubes of ordinary Vulcanized rubber, of brass and of wood. These 

 were all the materials at hand in tubular lorm, and we have had 

 no opportunity since of extending the observations to other sub- 

 stances. 



