SCIENCE IN THE INDUSTRIAL WORLD 



this question it may be necessary to explain what a 

 "standing" wave is. 



If light waves in motion are reflected back in the 

 direction from which they come, it is evident that the 

 returning ones will constantly meet those that are 

 advancing. At the points of contact, therefore, a 

 series of waves, whose crests rise and fall but cannot 

 move in either direction, owing to the counteracting 

 of the forward and backward impulses, will be pro- 

 duced. These are the standing waves. At such sta- 

 tionary points the light will be interfered with or 

 quenched, and thus there will result a "series of lay- 

 ers of light with intervals of darkness half a wave- 

 length apart." 



Now, several scientists, among them Zenker, of 

 Berlin, believed that this interference or quenching of 

 light was the explanation of the colors on Becquerel's 

 plates. The German physicist explained that "the 

 silver-chloride coating of the plate is so affected by 

 the light that metallic silver is produced in layers with 

 intervals of no chemical change which correspond to 

 the parts where the light is quenched by interference." 

 He assumed, and his assumption was afterwards 

 proved to be correct, that metallic silver was produced 

 because the successful reproduction of the colors re- 

 quired a strong reflecting surface, and he further 

 developed his theory by stating that these silver layers 

 of high reflecting power reflected "only, or chiefly, 

 light of the same wave-length (or color) as the light 

 which produced the layers when they are illuminated 

 by white light." In other words, Becquerel's colors 



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