THE PHOrOPHONE EXPLAINED. 11 



falls on it, an electric current will more readily flow through it than when it is 

 kept wholly in the dark. If then, we make the current to flow at the same time 

 through a telephone, the impact of the ray of light on the selenium will cause 

 such an increase of the current as will be audible in the telephone. Again, the 

 cutting off the light will so diminish the current as to sound the telephone, and 

 thus, as Professor Bell remarked, it will be possible 



" To hear a shadow fall 

 Athwart the stillness." 



Moreover, the stronger the ray of light, the less is the resistance which it 

 offers to the current , and hence it follows that an undulating beam of light will 

 set up corresponding undulations in the current, and these in turn will generate 

 vibrations in the telephone which may be heard aloud. Obviously, therefore, 

 if we could devise an apparatus by which the sound waves of the voice could 

 undulate a beam of light in sympathy with themselves, and project this beam to 

 a distant place where it could be received on a piece of selenium, through which 

 a current flowed on its way through a telephone, we should be able to reproduce 

 the original voice in the receiving telephone. 



At the very beginning of his attempt he encountered serious difficulties. The 

 intractable nature of selenium baffled all his efforts. This ambiguous material, 

 which, like phosphorus and sulphur, is neither metal nor non-metal, was acci- 

 dently discovered by Berzelius, the great Swedish chemist, when he was groping 

 for something else — tellurium — and the foundling has proved to be the more im- 

 portant substance of the two, for owing to its singular property of electric sensi- 

 bility to light, it has been chosen from among its humbler brethren and lifted into 

 honor. 



The striking effect was eagerly investigated by a number of scientists, who all 

 agreed in referring it to the action of light, and the yellowish-green rays of the 

 spectrum were found by Prof. W. Grylls Adams to be the most potent to produce 

 it. Mr. Robert Sabine demonstrated that there was a real diminution of the in- 

 ternal resistance of the selenium under the influence of light; but Prof. Adams 

 also showed that the observed increase of a current flowing through the substance 

 was not entirely due to its loss of resistance, but to the actual generation of a cur- 

 rent in the selenium. This fact is a very important one, and wiU, perhaps, find 

 its use hereafter in the transmission of optical images by electricity. For the pur- 

 pose of the photophone, however, the diminution of internal resistance under 

 light is the main consideration. Dr. Werner Siemens found the decrease to obey 

 a definite law. It is proportional to the square root of the intensity of illumina- 

 tion, and upon this basis he constructed a "photometer," or measurer of light. 



This interesting apparatus is known as the " Selenium Eye." It consists of 

 a hollow ball of blackened wood, formed of two halves which can be opened or 

 shut like the lids of an eye. A little within the ball is placed a glass lens to focus 

 the light which enters by the parted lids upon a sensitive selenium cell no bigger 

 than a wafer, fixed at the back of the ball. From this organ two fine platinum 

 wires lead to a voltaic battery and a delicate galvanometer. The current from 



