256 CONNECTION OF ELECTETCITY AND MAGNETISM. 



deep m the interior of the earth, and the other a hundred thousand 

 miles deep in the body of the sun, act on one another with precisely the 

 same force as if the strata beneath which each is buried had been non- 

 existent. If any medium takes part in transmitting this action, it must 

 surely make some difference whether the space between the bodies con- 

 tains nothing but this medium, or whether it is occupied by the dense 

 matter of the earth or of the sun. 



But the advocates of direct action at a distance are not content with 

 instances of this kind, in which the phenomena, even at first sight, 

 appear to favor their doctrine. They push their operations into the 

 enemy's camp, and maintain that even when the action is apparently 

 the pressure of contiguous portions of matter, the contiguity is only 

 apparent — that a space always intervenes between the bodies which act 

 on each other. They assert, in short, that so far from action at a 

 distance being impossible, it is the only kind of action which ever occurs, 

 and that the favorite old vis a tergo of the schools has no existence in 

 nature, and exists only in the imagination of schoolmen. 



The best way to prove that when one body pushes another it does not 

 touch it is to measure the distance between them. Here are two glass 

 lenses, one of which is pressed against the other by means of a weight. 

 By means of the electric light we may obtain on the screen an image of 

 the place where the one lens presses against the other. A series of 

 colored rings is formed on the screen. These rings were first observed 

 and first explained by Newton. The particular color of any ring 

 depends on the distance between the surfaces of the piece's of glass. 

 Newton formed a table of the colors corresponding to different dis- 

 tances, so that by comparing the color of any ring with Newton's table 

 we may ascertain the distance between the surfaces of that ring. The 

 colors 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 centers^ 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 four thou- 

 sandth part of a millimeter 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 color 

 appears at the central spot, and the diameters of all the rings increase. 

 This shows that the surfaces are now nearer than at first, but they are 

 not yet in optical contact, for, if they were, the central spot would be 

 black; 1 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 

 indicates only that the distance between the surfaces is much less than 

 a wavelength of light. To show that the surfaces are not in real con- 



