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of light, and to what are called the phenomena of 

 interference ; showing that these are much less in- 

 telligible on the corpuscular than on the undulatory 

 theory. At the same time they suggested ways of 

 surmounting many of the difficulties which had 

 prevented the acceptance of the latter ; hence it 

 came, in its turn, to afford a clearer mental picture 

 of relations among all the phenomena than the 

 other. Nothing beyond this is required of the 

 theory, which is accepted as being scientifically 

 true in spite of the difficulty of imagining clearly 

 any kind of medium which should be capable of 

 transmitting undulations such as those required 

 under the given conditions. 



Take, lastly, the illustration offered by the two 

 theories of heat. Bacon, in his " Xovum Organum," 

 had given his opinion that "the very essence of 

 heat, or the substantial self of heat, is motion and 

 nothing else." But he failed to show how any 

 relation among the phenomena of heat could be 

 clearly represented to the mind on these lines, so 

 that his proposition remained a mere form of words 

 expressing no usable syntax. In Newton's time 

 and down to the end of the eighteenth century the 

 clearest idea of relationship among the phenomena 

 of heat was derived from the hypothesis of an im- 

 ponderable fluid, termed "caloric," which entered 

 or left ponderable bodies according as they were 

 heated or cooled. Benjamin Thompson, better 



