i 88 i .] The Photophone. 21 1 
images by electricity. For the purpose 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 illumination, 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 the 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 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 
current increases in strength and the needle of the galvano- 
meter indicates the change upon a graduated scale. The 
deflection on the scale thus becomes a measure of the dimi- 
nution of resistance, and hence of the intensity of the ray of 
light. There is a wonderful, nay fearful, likeness between 
this curious little instrument 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 con- 
ducting wires, voltaic battery, and galvanometer, are paral- 
leled 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 galvanometer does not 
respond so strongly as before. The selenium, in faCt, 
becomes 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 Professor Bell had to 
contend with in adapting selenium to the photophone. 
Another drawback which he had to overcome was to 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 crys- 
talline into the amorphous form, so that its resistance is apt 
to vary in an unexpected manner. Professor Bell found it 
best to crystallise it by annealing it in a crucible at a tem- 
perature of 2io° centigrade for 24 hours, and then allowing 
