/S. P. Langley — The History of a Doctrine. 5 



are apt to think we have made our own ; and it seems to me 

 that any one who consults the originals will admit, that, though 

 its full consequences have not been wrought out till our own 

 time, yet the fundamental idea of heat as a mode of motion is 

 so far from being a modern one, that it was announced in 

 varying forms by Newton's immediate predecessors, by Des- 

 cartes, by Bacon, by Hobbes, and in particular by Boyle, while 

 Hooke and Huyghens merely continue their work, as at first 

 does Newton himself. 



If, however, Newton found the doctrine of vibrations already, 

 so to speak, " in the air," we must, while recognizing that in the 

 history of thought the new always has its root in the old, and 

 that it is not given even to a Newton to create an absolutely 

 new light, still admit that the full dawn of our subject properly 

 begins with him, and admit, too, that it is a bright one, when 

 , we read in the " Optics" such passages as these : 



"Do not all fixed bodies, when heated beyond a certain 

 degree, emit light and shine, and is not this emission performed 

 by the vibrating motions of their parts?" And again: "Do 

 not several sorts of rays make vibrations of several bignesses ?" 

 And still again : " Is not the heat conveyed by the vibrations 

 of a much subtler medium than air?" 



Here is the undulatory theory ; here is the connection of the 

 ethereal vibrations with those of the material solid ; here is 

 " heat as a mode of motion ;" here is the identity of radiant 

 heat and light ; here is the idea of wave-lengths. What a step 

 forward this first one is ! And the second ? 



The second is, as we know, backward. The second is the 

 rejection of this, and the adoption of the corpuscular hypoth- 

 esis, with which alone the name of Newton (a father of the 

 undulatory theory) is, in the minds of most, associated to day. 



Do not ,let us forget, however, that it was on the balancing 

 of arguments from the facts then known, that he decided, and 

 that perhaps it was rather an evidence of his superiority to 

 Huyghens, that apprehending equally clearly with the latter 

 the undulatory theory, he recognized also more clearly that 

 this theory, as then understood, utterly failed to account for 

 several of the most important phenomena. With an equally 

 judicial mind, Huyghens would perhaps have decided so too, 

 in the face of difficulties, all of which have not been cleared 

 up even to-day. These two great men, then, each looked 

 around in the then darkness as far as his light carried him. 

 All beyond that was chance to each ; and fate willed that 

 Newton, whose light shone farther than his rivals, found it 

 extend just far enough to show the entrance to the wrong way. 

 He reaches the conclusion that we all know ; one not only 

 wrong in regard to light, but which bears pernicious results 



