from Bradley to FremeL 105 



beginning with red and ending in white. The testimony of 

 practical astronomers was soon given that such appearances are 

 not observed ; and the hypothesis was accordingly abandoned. 



The fortunes of the wave-theory began to brighten at the 

 end of the century, when a new champion arose. Thomas 

 Young, born at Milverton in Somersetshire in 1773, and 

 trained to the practice of medicine, began to write on optical 

 theory in 1799. In his first paper* he remarked that, according' 1 

 to the corpuscular theory, the velocity of emission of a 

 corpuscle must be the same in all cases, whether the projecting 

 force be that of the feeble spark produced by the friction of two Q 

 pebbles, or the intense heat of the sun itself a thing almost 

 incredible. This difficulty does not exist in the undulatory 

 theory, since all disturbances are known to be transmitted^ 

 through an elastic fluid with the same velocity. The reluctance 

 which some philosophers felt to filling all space with an elastic 

 fluid he met with an argument which strangely foreshadows 

 the electric theory of light : " That a medium resembling in 

 many properties that which has been denominated ether does 

 really exist, is undeniably proved by the phenomena of 

 electricity. The rapid transmission of the electrical shock 

 shows that the electric medium is possessed of an elasticity as 

 great as is necessary to be supposed for the propagation of light. 

 Whether the electric ether is to be considered the same with 

 the luminous ether, if such a fluid exists, may perhaps at some 

 future time be discovered by experiment : hitherto I have not 

 been able to observe that the refractive power of a fluid 

 undergoes any change by electricity." 



Young then proceeds to show the superior power of the^ 

 wave-theory to explain reflexion and refraction. In the 

 corpuscular theory it is difficult to see why part of the light 

 should be reflected and another part of the same beam reflected ; 

 but in the undulatory theory there is no trouble, as is shown 

 by analogy with the partial reflexion of sound from a cloud or _, 

 denser stratum of air : " Nothing more is necessary than to 



* Phil. Tni., 1800, p. 106. 



