200 IOWA ACADEMY OP SCIENCE 



not vary with time, but is non-uniform over the cross-section of 

 the beam. The writer has no knowledge that this latter way 

 has heretofore been employed to produce the stroboscopic effect. 



In the case of a vibrating membrane which is itself reflecting 

 we have of course a mirror that changes periodically from con- 

 vex to plane to concave and back again, and hence a beam of 

 light reflected from its surface to a screen will periodically 

 change its total area of cross-section where it is intercepted 

 at the screen. Since the total quantity of light in the beam does 

 not change, the intensity of light in the spot on the screen varies 

 inversely as the area included in the spot. This would give then 

 on the screen a periodic illumination of the first general type 

 mentioned. 



Alexander Graham Bell 1 used such a reflecting membrane in 

 his photophone, permitting the beam of light to fall upon a 

 selenium cell. A telephone receiver in series with the cell re- 

 produced the tone actuating the membrane. Bell explained the 

 effect on the light sensitive cell by the changing curvature of 

 the mirror, as described. 



The explanation appears to be the obvious one, simple and 

 final. It would appear to be justified also in the light of other 

 experimental results obtained by Professor Bell. He constructed 

 a hollow convex lens with walls of mica or thin glass, and filled 

 it with a transparent liquid or gas. The walls of the lens could 

 be made to vibrate under the action of the voice, and thus the 

 lens curvature could be periodically changed. A beam of light 

 passing through the lens and falling upon the selenium cell pro- 

 duced the same effects in the telephone receiver as the vibrating 

 mirror. 



But, taking the case of the mirror, it must be remembered 

 that the amplitude of vibration is not large, and hence the cur- 

 vature changes over only a very narrow range. One would 

 hardly expect therefore that there could be a change in the in- 

 tensity of the beam of any consequence, at least for very short 

 distances between mirror and screen at which the stroboscopic 

 effect can be produced. With larger distances, as in Professor 

 Bell's work with the photophone, the effect is of course con- 

 siderably greater. 



1 BeIl, A. G. : "De la production et de la reproduction du son par la 

 lumiere," Annal. de Chimie et de Physique, t. XXI, 1880. 



