THE SIZE OF ATOMS. 159 



blackened behind, I see coloured rings ; the 

 phenomenon will be shown to you on the screen 

 by means of the electric light reflected from the 

 space of air between the two pieces of glass. 

 This phenomenon was first observed by Sir 

 Isaac Newton, and was first explained by the 

 undulatory theory of light. [Newton's rings are 

 now shown on the screen before you by reflected 

 electric light.] If I press the glasses together, 

 you see a dark spot in the centre ; the rings 

 appear round it, and there is a dark centre with 

 irregularities. Pressure is required to produce 

 that spot. Why ? The answer generally given 

 is, because glass repels glass at a distance of two 

 or three wave-lengths of light ; say at a distance 

 of i/5oooth of a centimetre. I do not believe 

 that for a moment. The seeming repulsion comes 

 from shreds or particles of dust between them. 

 The black spot in the centre is a place where 

 the distance between them is less than a quarter 

 of a wave-length. Now the wave-length for 

 yellow light is about i/i/,oooth of a centimetre, 

 and the quarter of i/i7,oooth is about i/7O,oooth. 



