ACTION AT A DISTANCE. 



one of which U prewed against the other by means of a weight. By n, 

 of the electric light we may obtain on the screen an image of the place via re 

 the one lens presses against the other. A series of coloured rings is formed on 

 the screen. These rings were first observed and first explained by Newton. 

 The particular colour of any ring depends on the distance between the surfaces 

 of the pieces of glass. Newton formed a table of the colours corresponding to 

 different distances, so that by comparing the colour of any ring with Newton's 

 table, we may ascertain the distance between the surfaces at that ring. The 

 colours are arranged in rings because the surfaces are spherical, and therefore 

 the interval between the surfaces depends on the distance from the line joining 

 the centres of the spheres. The central spot of the rings indicates the place 

 where the lenses are nearest together, and each successive ring corresponds to 

 an increase of about the 4000th part of a millimetre in the distance of the 

 surfaces. 



The lenses are now pressed together with a force equal to the weight of 

 an ounce; but there is still a measurable interval between them, even at the 

 place where they are nearest together. They are not in optical contact. To 

 prove this, I apply a greater weight. A new colour appears at the central 

 spot, and the diameters of all the rings increase. This shews that the sur 

 are now nearer than at first, but they are not yet in optical contact, for if 

 they were, the central spot would be black. I therefore increase the weights, 

 so as to press the lenses into optical contact. 



But what we call optical contact is not real contact. Optical contact indi- 

 cates ' only that the distance between the surfaces is much less than a wave- 

 length of light. To shew that the surfaces are not in real contact, I remove 

 the weights. The rings contract, and several of them vanish at the centre. 

 Now it is possible to bring two pieces of glass so close together, that they 

 will not tend to separate at all, but adhere together so firmly, that when torn 

 asunder the glass will break, not at the surface of contact, but at some other 

 place. The glasses must then be many degrees nearer than when in mere optical 

 contact. 



Thus we have shewn that bodies begin to press against each other whilst 

 still at a measurable distance, and that even when pressed together with u 

 force they are not in absolute contact, but may be brought nearer still, aii'l 

 that by many degrees. 



Why, then, say the advocates of direct action, should we continue to 



