160 POPULAR LECTURES AND ADDRESSES. 



The place where you see the middle of that black 

 circle is an air-space, with the distance apart less 

 than i/7O,oooth of a centimetre. Passing from 

 this black spot to the first ring of maximum 

 light, add half a wave-length to the distance, 

 and we can tell what is the distance between 

 the two pieces of glass at this place ; add another 

 half wave-length, and we come to the next 

 maximum of light again ; but the colour pre- 

 vents us speaking very definitely because we 

 have a number of different wave-lengths con- 

 cerned. 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 unmistakeable 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 in- 

 numerable little fulcrums constituted by the mole- 

 cules of dust ; and the pieces of glass are pressed 



