THE ET1IER AND PONDERABLE MATTER 



other puzzle. Why does not the ether, when set aquiver 

 with the vibration which gives us the sensation we call 

 light, have produced in its substance subordinate quiv- 

 ers, setting out at right angles from the path of the 

 original quiver? Such perpendicular vibrations seem 

 not to exist, else we might see around a corner ; how 

 explain their absence? The physicists could think of 

 but one way: they must assume that the ether is in- 

 compressible. It must fill all space at any rate, all 

 space with which human knowledge deals perfectly 

 full. 



These properties of the ether, incompressibility and 

 elastic rigidity, are quite conceivable by themselves; 

 but difficulties of thought appear when we reflect upon 

 another quality which the ether clearly must possess 

 namely, frictionlessness. Per hypothesis this rigid, in- 

 compressible body pervades all space, imbedding every 

 , particle of tangible matter; yet it seems not to retard the 

 movements of this matter in the slightest degree. This 

 is undoubtedly the most difficult to comprehend of the 

 alleged properties of the ether. The physicist explains 

 it as due to the perfect elasticity of the ether, in virtue 

 of which it closes in behind a moving particle with a 

 push exactly counterbalancing the stress required to 

 penetrate it in front. 



To a person unaccustomed to think of seemingly 

 solid matter as really composed of particles relatively 

 wide apart, it is hard to understand the claim that 

 ether penetrates the substance of solids of glass, for 

 example and, to use Young's expression, which we 

 have previously quoted, moves among them as freely 

 as the wind moves through a grove of trees. This 

 thought, however, presents few difficulties to the mind 



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