12 A'ANSAS CITY REVIEW OF SCIENCE. 



the battery flows through the selenium cell and the galvanometer at once, so that 

 when a ray of light falls on the selenium and diminishes its resistance, the cur- 

 rent increases in strength, and the needle of the galvanometer indicates the 

 change upon a graduated scale. The deflection on the scale thus becomes a 

 measure of the diminution of resistance, and hence of the intensity of the ray of 

 light. There is a wonderful, nay, fearful likeness between this curious little in- 

 strument and the human eye. The movable eye lids or shutters are present in 

 both, as well as the crystalline lens, while the selenium cell with the conducting 

 wires, voltaic battery, and galvanometer, are paralleled by the optic nerves and 

 the brain. Moreover, there is another similarity which is fatal to the use of the 

 "selenium eye " as an exact photometer. After exposure to the light repeatedly 

 or for some length of time, the selenium cell loses its sensibility to light, and the gal- 

 vanometer does not respond so strongly as before. The selenium, in fact, be- 

 comes fatigued, and, like the living eye, requires to rest ere it regains its former 

 power. This tendency to physical fatigue was one great difficulty which Prof. 

 Bell had to contend with in adapting selenium to the photophone. Another 

 drawback which he had to overcome was the variable and uncertain nature of 

 that body. Selenium occurs in two forms, the amorphous or vitreous, and the 

 crystalline or metallic form. In the vitreous condition it is a non-conductor of 

 electricity, in the crystalline state it is a conductor, but it has a fickle way of 

 passing from the crystalline into the amorphous form, so that its resistance is apt 

 to vary in an unexpected manner. Professor Bell found it best to crystallize it by 

 annealing it in a crucible at a temperature of 210° centigrade, for 24 hours, and 

 then allowing it to cool for 60 hours. With conductive selenium thus prepared, 

 he constructed his photophonic cell for receiving the transmitted light. 



This device combines a low resistance of the selenium with a large receptive 

 surface. It consists of a number of round disks or "washers" of brass, about 

 two inches in diameter, arranged side by side on axles, but separated from each 

 other by disks of mica of slightly less diameter. These are clamped together in 

 close file, and the grooves formed between the edges of the mica and brass are 

 filled up with melted selenium, which is then annealed in the manner described, 

 and the surface of the whole turned smooth in a lathe. The alternate brass discs 

 are connected together, the first to the third and fifth, the second to the fourth 

 and sixth, and so on, so as to give a circuit through the selenium surface, the bat- 

 tery and a receiving telephone. A double cell of this construction is then placed 

 in the focus of a silvered reflector of parabolic contour, and the photophone re- 

 ceiver is complete. 



The illustration represents this reflector, c c, with the cell, s, in the focus, 

 and the requisite attachments of battery and telephones. The transmitting ap- 

 paratus consists of a mirror, m, reflecting a beam of sunlight through a lens, and 

 (if for the sake of experiment it is desired to cut off the heat rays) likewise through 

 a cell. A, of alum water upon the transmitter, b. This is simply a diaphragm of 

 thin flexible glass, silvered on the outside to reflect the light, and fitted into a 

 frame which carries an india-rubber tube and mouth-piece, permitting a person to 



