820 THE POPULAR SCIENCE MONTHLY. 



apj^aratus for producing the effect remains to be described. This con- 

 sists 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. 



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 telephone. 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 schoolhouse 

 in Washington, and the sensitive receiver was arranged in one of the 

 windows of my laboratory, 1325 L Street, at a distance of two hundred 

 and thirteen 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 labora- 

 tory experiments the transmitting and receiving instruments are neces- 

 sarily within ear-shot of one another, and we have, therefore, been 

 accustomed to prolong the electric circuit connected with the selenium 

 receiver, so as to place the telephones in another room. By such ex- 

 periments we have found that articulate speech can be reproduced by 

 the oxyhydrogen light, and even by the light of a kerosene-lamp. The 

 loudest effects obtained from light are produced by rapidly interrupt- 

 ing the beam by the perforated disk. The great advantage of this form 

 of apparatus for experimental work is the noiselessness of its rotation, 

 admitting the close approach of the receiver without interfering with 

 the audibility of the effect heard from the latter ; for it will be under- 

 stood that musical tones are emitted from the receiver when no sound 

 is made at the transmitter. A silent motion thus produces a sound. 

 In this way musical tones have been heard even from the light of a 

 candle. When distant effects are sought another apparatus is used. 

 By placing an opaque screen near the rotating disk the beam can be 

 entirely cut off by a slight motion of the hand, and musical signals, 

 like the dots and dashes of the Morse telegraph code, can thus be pro- 

 duced at the distant receiving station. 



We have made experiments with the object of ascertaining the 

 nature of the rays that affect selenium. For this purpose we have 

 placed in the path of an intermittent beam various absorbing sub- 

 stances. Professor Cross has been kind enough to give me his assist- 



