THE STORY OF NINETEENTH-CENTURY SCIENCE 



something which undulates; and this something could 

 not be air, for air exists only in infinitesimal quantity, if 

 at all, in the interstellar spaces, through which light 

 freely penetrates. But if not air, what then? Why, 

 clearly, something more intangible than air; something 

 supersensible, evading all direct efforts to detect it, yet 

 existing everywhere in seemingly vacant space, and also 

 interpenetrating the substance of all transparent liquids 

 and solids, if not, indeed, of all tangible substances. 

 This intangible something Young rechristened the Lu- 

 miniferous Ether. 



In the early days of his discovery Young thought of 

 the undulations which produce light and radiant heat as 

 being longitudinal a forward and backward pulsation, 

 corresponding to the pulsations of sound and as such 

 pulsations can be transmitted by a fluid medium with 

 the properties of ordinary fluids, he was justified in 

 thinking of the ether as being like a fluid in its proper- 

 ties, except for its extreme intangibility. But about 

 1818 the experiments of Fresnel and Arago with polar- 

 ization of light made it seem very doubtful whether the 

 theory of longitudinal vibrations is sufficient, and it was 

 suggested by Young, and independently conceived and 

 demonstrated by Fresnel, that the luminiferous undula- 

 tions are not longitudinal, but transverse; and all the 

 more recent experiments have tended to confirm this 

 view. But it happens that ordinary fluids gases and 

 liquids cannot transmit lateral vibrations ; only rigid 

 bodies are capable of such a vibration. So it became 

 necessary to assume that the luminiferous ether is a body 

 possessing elastic rigidity a familiar property of tangi- 

 ble solids, but one quite unknown among fluids. 



The idea of transverse vibrations carried with it an- 



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