218 STUDIES IN ELECTRO-PHYSIOLOGY: 



is of a reddish-yellow colour, conducting heat badly and 

 electricity not at all, while the other variety known as 

 black or metallic selenium conducts heat, and under certain 

 conditions will form a good conductor of electricity. It is 

 with the latter only that we are concerned. 



In 1873 Mr. Willoughby Smith, then electrician-in-chief 

 to the Telegraph Construction and Maintenance Company, 

 discovered that this substance had a peculiar property in 

 that its electrical resistance varied with the amount of light 

 to which it was subjected ; the difference in these varia- 

 tions being very marked, and in the inverse ratio to the 

 degree of light. Later on Dr. Siemens, Professor Adams, 

 the Earl of Rosse, and other scientific men took up the 

 subject, but nothing practical was done until Professor 

 Graham Bell, in association with Mr. Sumner Tainter, 

 produced the photophone, an instrument in which light 

 was utilised for the transmission of sound. 



Of more interest to us, however, is the " Selenium 

 eye "of Dr. Siemens. It was in reality an artificial human 

 eye, with a lens in front, and lids to close when it was 

 weary ; for, curious as it may seem, it, like its perfect 

 prototype, became tired when exposed for a prolonged 

 period to bright light. 



The lens caused any light to which the " eye " was 

 subjected to be concentrated in the interior of the eyeball, 

 and at this spot a selenium grating was placed. This was 

 composed of two fine wires running together in zigzag 

 fashion, but not making actual contact. Upon these was 

 placed a melted drop of selenium, and the ends of the wires 

 were joined up with a galvanometer and battery. When 

 the " eye " had been closed and at rest for some little time, 

 it was found to be sensitive to the faintest gleam of light, 

 but after long exposure to bright light the lids closed for 

 a long time before it became again sensitive to feeble rays. 



Since then much experimental work has been done, and 



