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ohms in the light. All former experimenters seemed to have used platinum 
for the conducting part of their selenium cells, excepting Werner Siemens, 
•who found that iron and copper might he employed. We have also dis- 
covered that bra^s, although chemically acted upon by selenium, forms an 
excellent and convenient material ; indeed we are inclined to believe that the 
chemical action between the brass and selenium has contributed to the low 
resistance of our cells by forming an intimate bond of union between the 
selenium and the brass. We have observed that melted selenium behaves to the 
other substances as water to a greasy surface, and we are inclined to think that 
when selenium is used in connexion with metals not chemically acted upon 
by it, the points of contact between selenium and the metal offer a consider- 
able amount of resistance to the passage of a galvanic current. By using 
brass we have been enabled to construct a large number of selenium cells of 
different forms. The mode of applying the selenium is as follows : The cell 
is heated, and, when hot enough, a stick of selenium is rubbed over the 
surface. In order to acquire conductivity and sensitiveness, the selenium 
must next undergo a process of annealing. 
1 We simply heat the selenium over a gas stove and observe its appearance. 
When the selenium attains a certain temperature, the beautiful reflecting 
surface becomes dimmed. A cloudiness gradually extends over it, somewhat 
like the film of moisture produced by breathing upon a mirror. This ap- 
pearance gradually increases, and the whole surface is soon seen to be in the 
metallic, granular, or crystalline condition. The cell may then be taken off 
the stove, and cooled in any suitable way. 
1 We have devised about fifty forms of apparatus for varying a beam of 
light in the manner required, but only a few typical varieties need be shown. 
The source of light may be controlled, or a steady beam may be modified at 
any point in its path. The beam may be controlled in many ways. The 
best and simplest form of apparatus for producing the effect consists of a 
plane mirror of flexible material — such as silvered mica or microscope glass. 
Against the back of this mirror the speaker’s voice is directed. The light 
reflected from this mirror is thus thrown into vibrations corresponding to 
those of the diaphragm itself. 
1 In arranging the apparatus for the purpose of reproducing sound at a 
distance, any powerful source of light may be used, but we have experi- 
mented chiefly with sunlight. For this purpose a large beam is con- 
centrated by means of a lens upon the diaphragm mirror, and, after reflection, 
is again rendered parallel by means of another lens. The beam is received 
at a distant station upon a parabolic reflector, in the focus of which is placed 
a sensitive selenium cell, connected in a local circuit with a battery and tele- 
phone. A large number of trials of this apparatus have been made with the 
transmitting and receiving instruments so far apart that sounds could not be 
heard directly through the air. In illustration, I shall describe one of the 
most recent of these experiments. Mr. Tainter operated the transmitting 
instrument, which was placed on the top of the Franklin sGhoolhouse in 
Washington, and the sensitive receiver was arranged in one of the windows 
of my laboratory, 1325 L Street, at a distance of 213 metres. Upon placing 
the telephone to my ear I heard distinctly from the illuminated receiver the 
words : u Mr. Bell, if you hear what I say, come to the window and wave 
