No. 7.] BELL — THE PHOTOPHONE. 405 



reflected from the pollished pole of an electro-magnet. Another 

 method of affecting a beam of light is to pass it through a 

 lens of variable 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 apparatus for producing the eflfect remains to be 

 described. This consists of a plain mirror of flexible material — 

 such as silvered mica or microscopic glass. Against the back of 

 this mirror the speaker's voice is directed. The light reflected 

 from this mirror is thus thrown into vibration corresponding to 

 those of the diaphragm itself. 



In arranging the apparatus for the purpose of reproducing sound 

 at a distance, any powerful source of light may be used, but we 

 have experimented chiefly with sunlight. For this purpose a 

 large beam is concentrated 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 illustra- 

 tion, 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 schoolhouse 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 : "Mr Bell, if you hear what I 

 say, come to the window and wave your hat." In laboratory 

 experiments the transmitting aud receiving instruments are 

 necessarily within earshot of one anothar, and we have therefore 

 been accustomed to pooling the electric circuit connected with 

 the selenium receiver, so as to place the telephones in another 

 room. By such experiments we have found that articulate speech 

 can be reproduced by the oxy-hydrogen light, and even by the 

 light of a kerosene lamp. The loudest efi'ects obtained from light 

 are produced by rapidly interrupting the beam by the perforated 

 disk. The great advantage of this form of apparatus for experi- 

 mental work is the noiselessness of its rotation, admitting the 



