1883.] on the Size of Atoms. 189 



Now in that beautiful experiment well known as Newton's rings 

 we have at once a measure of length in the distance between two 

 pieces of glass to give any particular tint of colour. The wave-length 

 you see, in the distance from crest to crest of the waves travelling up 

 the long model when I commence giving a simj)le harmonic oscilla- 

 tion to the lowest bar. I have here a convex lens of very long focus, 

 and a piece of plate glass with its back blackened. When I press the 

 piece of glass against the glass 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 w^ave-lengths of light ; say at a distance of l-5000th 

 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 l-17,000th of a centimetre. The quarter of l-17,000th 

 is about 1-70, 000th. The place where you see the middle of that black 

 circle corresponds to air at a distance of less than l-70,000th of a 

 centimetre. Passing from this black spot to the first ring of maxi- 

 mum light, add half a wave-length to the distance, and we can tell 

 what the distance between the two pieces of glass is at this place ; 

 add another half wave-length, and we come to the next maximum of 

 light again ; but the colour prevents us speaking very definitely 

 because we have a number of different wave-lengths concerned. I 

 will simplify that by reducing it all to one colour, red, by interposing 

 a red glass. You have now one colour, but much less light altogether, 

 because this glass only lets through homogeneous red light, or not 

 much besides. Now look at what you see on the screen, and you 

 have unmistakable evidence of fulcrums of dust between the glass 

 surfaces. When I put on the screw, I whiten the central black spot 

 by causing the elastic glass to pivot, as it were, round the innumerable 

 little fulcrums constituted by the molecules of dust ; and the pieces 

 of glass are pressed not against one another, but against these fulcrums. 

 There are innumerable — say thousands — of little particles of dust 

 jammed between the glass, some of them of perhaps l-3000th of a 

 centimetre in diameter, say 5 or 6 wave-lengths. If you lay one piece 

 of glass on another, you think you are pressing glass on glass, but it 

 is nothing of the kind ; it is glass on dust. This is a very beautiful 

 phenomenon, and my first object in showing this experiment was 

 simply because it gives us a linear measure bringing us down at once 

 to i-100,000th of a centimetre. 



