SELENIUM AND ITS USES. 235 



ity in the results he obtained. Persistent efforts in tracing the cause of these dis- 

 crepancies resulted in the extremely valuable discovery that the electrical resist- 

 ance of selenium was less while exposed to sun light than when in the dark. 

 This valuable discovery was made in May, 1872, and was communicated to the 

 Society of Telegraph Engineers on the 17th of February, 1873. His apparatus 

 consisted of platinum wires, inserted into the opposite ends of a bar of selenium, 

 hermetically sealed in a glass tube. This was placed in a dark box and suitably 

 connected with instruments by means of which its electrical resistance could be 

 measured. When so arranged, the exposure to the light, consequent on open- 

 ing the lid of the box, caused an instantaneous reduction in the resistance. So 

 sensitive was this arrangement to the action of light that merely placing the hand 

 between the selenium bar and the light of a distant gas burner, instantly affected 

 the instrument. 



The discovery of this curious property of selenium naturally attracted con- 

 siderable attention from scientific men in different parts of the world, and nu- 

 merous investigations were made concerning its properties, for example : Profes- 

 sor Adams, of Kings' College, showed that the greenish-yellow rays were the 

 most effective in varying the resistance of selenium, and Professor Siemens pro- 

 duced a variety of selenium that was fifteen times as great a conductor of elec- 

 tricity in the light as in the dark. 



Among some of the more interesting applications of the variations in the 

 electrical resistance of selenium, by the action of light we rriay mention the 

 " selenium eye," in which an attempt was made to imitate the action of light on 

 the iris of the human eye. By an ingenious use of selenium resistances, intro- 

 duced into the circuit of a battery, the inventor caused the intensity of the light 

 that entered a lens to be maintained approximately constant by the action of an 

 electro-magnet that varied the size of the opening of a diaphragm, in this manner 

 roughly imitating the contraction and dilatation of the pupil of the eye. 



Perhaps one of the most famous uses of this property of selenium, was made 

 by Alexander Graham Bell, of telephone celebrity. This inventor, in an instru- 

 ment he termed a telephote, has so utilized the variations in the electrical resist- 

 ance of selenium while under the influence of light, as to be able actually to kold 

 telephonic communication along a ray of light instead of along the customary 

 metallic wire. The telephote is now generally known under the general name 

 of radiophone. 



Numerous attempts have been made, in different parts of the world, to utilize 

 this variation in the electrical resistance of selenium under the influence of light 

 for the transmission of photographs, or, outline sketches, by electricity. Some of 

 these attempts have been comparatively successful, and we may in a subsequent 

 issue discuss them in detail. 



We shall not be found wanting in the Exhibition in a display of this curious 

 element. A single exhibitor has applied for space for an exhibit of selenium 

 resistances that he values at $10,000. An application has also been made by a 

 French savant who contemplates exhibiting an instrument called the telectro- 



