132 



SCIENCE. 



a residue which proved upon examination to be what he had been 

 in search of — a new elementary substance. The chemical proper- 

 ties of this new element were found to resemble those of tellurium 

 in so remarkable a degree that Berzeliusgave to the substance the 

 name of "Selenium," from the Greek word selene, the moon — 

 (" tellurium," as is well known being derived from tellus, the earth, 



Although tellurium and selenium are alike in many respects, they 

 differ in their electrical properties ; tellurium being a good conduc- 

 tor of electricity, and selenium, as Berzelius showed, a non-con- 

 ductor. Knox discovered, in 1837, that selenium became a conduc- 

 tor when fused ; and Hittorff, in 1852, showed that it conducted, at 

 ordinary temperatures, when in one of its allotropic forms. When 

 selenium is rapidly cooled from a fused condition, it is a non-con- 

 ductor. In this, its vitreous form, it is of a dark-brown color, 

 almost black by reflected light, having an exceedingly brilliant 

 surface. In thin films it is transparent, and appears of a beautiful 

 ruby red by transmitted light. When selenium is cooled fn m a 

 fused condition with extreme slowness, it presents an entirely dif- 

 ferent appearance, being a dull lead color, and having throughout 

 a granulated or crystalline structure, and looking like a metal. In 

 this form it is perfectly opaque to light, even in very thin films. 

 This variety of selenium has long been known as " granular" or 

 "crystalline" selenium, or, as Regnault called it, "metallic" sele- 

 nium. It was selenium of this kind that Hittorff found to be a 

 conductor of electricity at ordinary temperatures. He also found 

 that its resistance to the passage of an electrical current diminished 

 continuously by heating up to the point of fusion, and that the 

 resistance suddenly increased in passing from the solid to the 

 liquid condition. It was early discovered that exposure to sunlight 

 hastens the change of selenium from one allotropic form to an- 

 other ; and this observation is significant in the light of recrnt dis- 

 coveries. 



Although selenium has been known for the last sixty years it has 

 not >et been utilized to any extent in the arts, and it is still consid- 

 ered simply as a chemical curiosity. It is usually supplied in the 

 form of cylindrical bars. These bars are sometimes found to be in 

 the metallic condition ; but more usually they are in the vitreous or 

 non-conducting form. It occurred to Willoughby Smtih that, on 

 account of the high resistance of crystalline selenium, it might be 

 usetully employed at the shore-end of a submarine cable, in his 

 system of testing and signalling during the process of submersion. 

 Upon experiment, the selenium was found to have all the resist- 

 ance required — some of the bars employed measuring as much as 

 1400 megohms — a resistance equivalent to that which would be of- 

 fered by a telegraph wire long enough to reach from the earth to 

 the sun ! But the resistance was found to be extremely variable. 

 Experiments were made to ascertain the cause of this variability. 

 Mr. May, Mr. Willoughby Smith's assistant, discovered that the 

 resistance was less when the selenium was exposed to light than 

 when it was in the dark. 



In order to be certain that temperature had nothing to do with 

 the effect, selenium was placed in a vessel of water, so that the 

 light had to pass through from one to two inches of watt r in order 

 to reach the selenium. The approach of a lighted candle was 

 found to be sufficient to cause a marked deflection of the needle of 

 the galvanometer connected with the selenium, and the lighting of 

 a piece of magnesium wire caused the selenium to measure less 

 than half the resistance it did the moment before. 



These results were naturally at first received by scientific men 

 with some incredulity, but they were verified by Sale, Draper, Moss 

 and others. When selenium is exposed to the action of the solar 

 spectrum, the maximum effect is produced, according to Sale, just 

 outside the red end of the spectrum, in a point nearly co-incident 

 with the maximum of the heat ra> s ; but, according to Adams, the 

 maximum effect is produced in the greenish-yellow or most lumin- 

 ous part of the spectrum. Lord Rosse expos-d selenium to the 

 action of non-luminous radiations from hot bodies, but could pro- 

 duce no effect ; wheieasa thermopile under similar circumstances 

 gave abundant indications of a current. He also cut off the heat- 

 rays from lumir.ous bodies by the interposition of liquid solutions, 

 such as alum, between the selenium and the source of light, with- 

 out affecting the power of the light to reduce the resistance of the 

 selenium; whereas the interposition of these same substances al- 

 most completely neutralize the effectupon the thermopile. Adams 

 found that selenium was sensitive to the cold light of the moon, and 

 Werner Siemens discovered that, in certain extremely sensitive 

 varieties of selenium, heat and light produced opposite effects. In 

 Siemens's experiments, special arrangements were made for the 

 purpose of reducing the resistanceof the selenium employed. Two 

 fine platinum wires were coiled together in the shape of a double 

 flat spiral in the zig-zag shape, and were laid upon a plate of mica 

 so that the discs did not touch one another. A drop of melted 

 selenium was then placed upon the platinum-wire arrangement, 

 and a second sheet of mica was pressed upon the selenium, so as 

 to cause it to spread out and fill the spaces between the wires. 

 Each cell was about the size of a silver clime. The selenium cells 

 were then placed in a parafnnc bath, ind exposed for some hours 

 to a temperature ol 210° C, after which they were allowed to cool 

 with extreme slowness. The results obtained with these cells were 

 very extraordinary ; in some cases the resistance of the cells, when 

 exposed to light, was only one-fif;eenth of their resistance in the 

 dark. 



Without dwelling farther upon the researches of others, I may say 



that the chief information concerning the effect of light upon the 

 conductivity of selenium will be found under the names of Wil- 

 loughby Smith, Lieutenant Sale, Draper and Moss, Professor W. 

 G. Adams, Lord Rosse, Day, Sabini, Dr. Werner Siemens and 

 Dr. C. W. Siemens. All observations by these various authors 

 had been made by means of galvanometers ; but it occurred to me 

 that the telephone, from its extreme sensitiveness to electrical in- 

 fluences, might be substituted with advantage. Upon considera- 

 tion of the subject, however, I saw that the experiments could not 

 be conducted in the ordinary way for the following reason : The 

 law of audibility of the telephone is precisely analogous to the law 

 of electric induction. No effect is produced during the passage of 

 a continuous and steady current. It is only at the moment of change 

 from a stronger to a weaker state, or vice versa, that any audible 

 effect is produced, and the amount of effect is exactly proportional 

 to the amount of variation in the current. It was, therefore, evi- 

 dent that the telephone could only respond to the effect produced 

 in selenium at the moment of change from light to darkness, or 

 vice versa ; and that it would be advisable to intermit the light 

 with great rapidity, so as to produce a succession of changes in 

 the conductivity of the selenium, corresponding in frequency to 

 musical vibrations within the limits of the sense of hearing. For 

 I had often noticed that currents of electricity, so feeble as to pro- 

 duce scarcely any audible effects from a telephone when the circuit 

 was simply opened or closed, caused very perceptible musical 

 sounds when the circuit was rapidly interrupted, and that the 

 higher the pitch of sound the more audible was the effect. I was 

 much struck by the idea of producing sound by the action of light 

 in this way. Upon farther consideration it appeared to me that 

 all the audible effects obtained from varieties of electricity could 

 also be produc d by variations of light acting upon selenium. I 

 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 variations 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 em- 

 ployed. One was to be fixed and the other attached to the centre 

 of a diaphragm 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 con- 

 tracting the free orifices for the passage of light. In this way the 

 voice of a speaker could control the amount ot light passed through 

 the perforated plates without 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 ap- 

 paratus, by means of which it could be condensed upon a sensi- 

 tive 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 articula- 

 tions ot the speaker's voice. I obtained some selenium for the 

 purpose of producing the apparatus shown ; but found that its re- 

 sistance was almost infinitely greater than that of any telephone 

 that had been constructed, and I was unable to obtain any audible 

 effects by the action of light. I believed, however, that the ob- 

 stacle 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 announced the possibility 

 of hearing a shadow by interrupting the action of light upon sel- 

 enium. A few days afterwards my ideas upon this subject re- 

 ceived a fresh impetus by the announcement made by Mr. Will- 

 oughby Smith betore the Society of Telegraph Engineers that he 

 had heard the action ot a ray of light tailing upon a bar of crys- 

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



It is not unlikely that the publicity given to the speaking tele- 

 phone during the last few years may have suggested to many 

 minds in different parts of the world somewhat sim, liar 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, 1 recognize the fact that the 

 knowledge necessary for its conception has been disseminated 

 throughout the civihzed world, and that the idea may therefore 

 have occurred to many other minds. The fundamental idea, on 

 •which rests the possibility of ptoducing 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 undu- 

 latory 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 o 



