A HISTORY OF SCIENCE 



preaching a demonstration. It was while pondering 

 over the familiar but puzzling phenomena of colored 

 rings into which white light is broken when reflected 

 from thin films Newton's rings, so called that an ex- 

 planation occurred to him which at once put the entire 

 undulatory theory on a new footing. With that sagac- 

 ity of insight which we call genius, he saw of a sudden 

 that the phenomena could be explained by supposing 

 that when rays of light fall on a thin glass, part of the 

 rays being reflected from the upper surface, other rays, 

 reflected from the lower surface, might be so retarded 

 in their course through the glass that the two sets 

 would interfere with one another, the forward pulsa- 

 tion of one ray corresponding to the backward pulsa- 

 tion of another, thus quite neutralizing the effect. 

 Some of the component pulsations of the light being 

 thus effaced by mutual interference, the remaining 

 rays would no longer give the optical effect of white 

 light; hence the puzzling colors. 



Here is Young's exposition of the subject: 



Of the Colors of Thin Plates 



"When a beam of light falls upon two refracting 

 surfaces, the partial reflections coincide perfectly in 

 direction; and in this case the interval of retardation 

 taken between the surfaces is to their radius as twice 

 the cosine of the angle of refraction to the radius. 



" Let the medium between the surfaces be rarer than 

 the surrounding mediums; then the impulse reflected 

 at the second surface, meeting a subsequent undula- 

 tion at the first, will render the particles of the rarer 

 medium capable of wholly stopping the motion of the 



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