ON THE PRODUCTION OF SOUND BY LIGHT. 819 



over the surface. In order to acquire conductivity and sensitiveness, 

 the selenium must next undergo a process of annealing. 



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 breath- 

 ing upon a mirror. This appearance gradually increases, and the whole 

 surface is soon seen to be in the metallic, granular, or crystalline condi- 

 tion. The cell may then be taken off the stove, and cooled in any 

 suitable way. When the heating process is carried too far, the crys- 

 talline selenium is seen to melt. Our best results have been obtained 

 by heating the selenium until it crystallizes, and continuing the heat- 

 ing until signs of melting appear, when the gas is immediately put 

 out. The portions that had melted instantly recrystallize, and the 

 selenium is found upon cooling to be a conductor, and to be sensitive 

 to light. The whole operation occupies only a few minutes. This 

 method has not only the advantage of being expeditious, but it proves 

 that many of the accepted theories on this subject are fallacious. Our 

 new method shows that fusion is unnecessary, that conductivity and 

 sensitiveness can be produced without long heating and slow cooling ; 

 and that crystallization takes place during the heating process. We 

 have found that, on removing the source of heat immediately on the 

 appearance of the cloudiness, distinct and separate crystals can be 

 observed under the microscope, which appear like leaden snow-flakes 

 on a ground of ruby red. Upon removing the heat, when crystalliza- 

 tion is further advanced, we perceive under the microscope masses of 

 these crystals arranged like basaltic columns standing detached from 

 one another, and at a still higher point of heating the distinct columns 

 are no longer traceable, but the whole mass resembles metallic pud- 

 ding-stone, with here and there a separate snow-flake, like a fossil, 

 on the surface. Selenium crystals formed during slow cooling after 

 fusion present an entirely different appearance, showing distinct 

 facets. 



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. For instance, it may be polarized, and then affected 

 by electrical or magnetic influences in the manner discovered by Fara- 

 day and Dr. Ker. The beam of polarized light, instead of being passed 

 through a liquid, may be reflected from the polished pole of an electro- 

 magnet. Another method of affecting a beam of light is to pass it 

 through a lens of Tariable focus. I observe that a lens of this kind has 

 been invented in France by Dr. Cusco, and is fully described in a re- 

 cent paper in " La Nature " ; but Mr. Tainter and I have used such a 

 lens in our experiments for months past. The best and simplest form of 



