A HISTORY OF SCIENCE 



The idea of transverse vibrations carried with it an- 

 other puzzle. Why does not the ether, when set 

 aquiver with the vibration which gives us the sensa- 

 tion we call light, have produced in its substance sub- 

 ordinate quivers, setting out at right angles from the 

 path of the original quiver? Such perpendicular vi- 

 brations seem not to exist, else we might see around a 

 corner ; how explain their absence ? The physicist could 

 think of but one way : they must assume that the ether is 

 incompressible. 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. By 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 re- 

 quired 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 



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