202 IOWA ACADEMY OF SCIENCE 



tensity over its cross section. We are thus led to suspect that 

 this same condition, present in Bell's work, played its part in 

 the effect on the selenium cell. It is more difficult, however, 

 to accept such a conclusion in the case of the cell's response, 

 than in that of the production of the stroboscopic effect. In 

 the former the total amount of light falling upon the cell must 

 remain very nearly the same, even though at any particular 

 point on the cell the intensity changes by the lateral displace- 

 ment, while in the latter case it is a region of small area that 

 is chiefly concerned. 



In a consideration of the stroboscopic effect the fact must 

 not be overlooked that the eye is extremely sensitive to small 

 changes of light intensity, so that the changing area of the 

 beam on the screen may contribute its effect, which, however, 

 appears to be at most only a minor effect. The decisive experi- 

 ment must have achieved a separation of the two mechanical 

 conditions, that of periodic expansion and contraction of the 

 beam with the accompanying changes in light intensity due to 

 this expansion and contraction alone, and secondly, the periodic 

 lateral displacement of the beam. It is easy to have the second 

 condition alone present, as has been shown, but it is difficult 

 to see how one could have changing curvature without lateral 

 displacement. 



"While the idea of reflecting light directly from a vibrating 

 membrane was arrived at independently by the author it was 

 later learned that the idea was anticipated by Professor Bell 

 about 1880, who, however, used the reflected light for a differ- 

 ent purpose. The production of the stroboscopic effect by this 

 method appears to be new. 



In this method various kinds of diaphragms can be used. 

 Satisfactory results have been attained with silvered mica 

 membranes, as well as paper and rubber dam membranes (with 

 mica mirrors attached to them either directly with paste or with 

 bits of cork or cardboard between mirror and membrane.) 

 Paper membranes seem to give as good results as any. 



A of figure 21 shows an arrangement of a paper membrane 

 over the end of a short glass tube, with a lever system and a 

 small mirror (microscope cover glass) that could periodically 

 be given an angular displacement by the vibrating membrane 

 actuated by a sounded tone at the open end of the tube. This 



