PHYSICS OF THE NINETEENTH CENTURY. LIGHT. 449 



it would bend into the shadow, for pression or motion cannot be pro- 

 pagated in a fluid in right lines beyond an obstacle which stops part 

 of the motion, but will bend and spread every way into the quies- 

 cent medium which lies beyond the obstacle. The waves on the sur- 

 face of stagnant water, passing by the sides of a broad obstacle which 

 stops part of them, bend afterwards and dilate themselves gradually 

 into the quiet water behind the obstacle. The waves, pulses, or vibra- 

 tions of the air, wherein sounds consist, bend manifestly, though not 

 so much as the waves of water, for a bell or a cannon may be heard 

 beyond a hill which intercepts the sounding body . . . but light is 

 never known to bend into the shadow." Some expressions to which we 

 would specially direct the reader's attention we have placed in italics, 

 because the bending of light round an object was now experimentally 

 proved to be precisely what occurred. The bending is less in air than 

 in water, and extremely small in the case of light, because of the extreme 

 minuteness and immense velocity of the undulations. 



It is especially for his development of the undulatory theory of light 

 that Young's name will always be remembered in the history of science. 

 It has been already mentioned that Newton preferred what is called 

 the corpuscular theory of light, which explains the phenomena by sup- 

 posing that very minute particles are actually projected from luminous 

 bodies, and that they continue to move with the velocity which light 

 possesses. The alternative hypothesis, which was originally proposed 

 by Hooke and Huyghens, attributed light to undulations of a highly 

 elastic medium. It might, at first sight, appear that the corpuscular 

 theory had the advantage of its rival in simplicity. But this simplicity 

 is only apparent, for when the facts are studied closely it is found that 

 the emission theory requires special modifications to explain them ; 

 and, further, that the modifications required by the different classes of 

 facts are independent of and unconnected with each other. On the 

 other hand, when the undulatory theory came to be applied to newly- 

 observed phenomena, the modifications it required had, in many cases, 

 a connection with others, or were mutually confirmative. These re- 

 marks may be illustrated by Newton's proposed explanation of his 

 "rings" (page 223) by "fits of easy reflection and transmission;" his 

 explanation of diffraction by an attraction between the particles of 

 light and the solid body near which they pass (page 226) ; while double 

 refraction required that the particles of light should be assumed to 

 possess yet another property unconnected with either of the former 

 namely, polarity, which purely arbitrary assumption has given a con- 

 venient name to certain conditions of light. The manner in which the 

 undulatory hypothesis lends itself to the explanation of very different 

 phenomena will be seen in the accounts of certain experiments proving 

 the interference, diffraction, and polarization of light, which will shortly 

 be brought under notice. 



The undulatory theory was suggested to Hooke by the colours of 



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