ELECTRICAL INVENTIONS 217 



The photographic print is moved back and forth between two 

 terminal points of an electric circuit, one touching the upper 

 surface of the picture, the other the under surface. These points 

 move along a series of parallel lines, from one end of the print 

 to the other. The current that flows in the circuit varies accord- 

 ing to the amount of silver deposit at every point of the print. 

 Where the silver deposit is heavy so that the print is dark, the 

 metal acts as a good conductor and the current flows readily, but 

 where the print is light, the flow of the current is weak. At the 

 receiving station a piece of sensitized paper is made to move 

 mechanically in correspondence with the movement of the print. 

 A beam of light strikes at a point on this paper, and as the paper 

 moves this point of light runs over its surface in parallel lines 

 corresponding to those over which the terminal points are moving 

 upon the print. This beam of light is focused on the paper 

 through a piece of selenium, through which also flows the current 

 coming from the transmitting station. Selenium has this peculiar 

 property, that the stronger the electric current flowing in 

 it, the more readily it permits light to pass through it. When, 

 therefore, the terminal points are traveling over a dark part of 

 the print, the current transmitted is strong, the selenium permits 

 much light to pass through it, the sensitized paper is strongly 

 acted upon, and prints dark. Thus the sensitized paper repro- 

 duces point by point the dark and light areas of the original print. 



When the telegraph was invented, it seemed wonderful enough 

 that men could send intelligible messages over a wire for hundreds 

 of miles, but it seemed past belief when it was announced that 

 one could talk into a small instrument and be heard distinctly 

 miles away by another person who held to his ear a receiver 

 connected only by a wire with the sending instrument. 



As in the case of most inventions the possibility of the tele- 

 phone occurred to several persons, and rude attempts were made 

 to produce it years before the practical instrument was devised. 

 Credit is due to Page, an American (1837), to Froment (1850), 

 to Bour-Seul (1854), and to Philippe Reiss, a science teacher in a 



