266 Lord Rayleigh on the History of 



the yellow, and the red rays of light, and the rays of invisible 

 heat, constitute seven different degrees of the same scale, dis- 

 tinguished from each other into this limited number, not by 

 natural divisions, but by their effects on our senses : and we 

 may also conclude that there is some similar relation between 

 heated and luminous bodies of different kinds/^ 



And, again, on p. 654 : " If heat is not a substance, it must 

 be a quality; and this quality can only be motion. It was 

 Newton^s opinion that heat consists in a minute vibratory 

 motion of the particles of bodies, and that this motion is com- 

 municated through an apparent v^acuum by the undulations 

 of an elastic medium, which is also concerned in the phenomena 

 of light. If the arguments which have been lately advanced 

 in favour of the undulatory theory of light be deemed valid, 

 there will be still stronger reasons for admitting this doctrine 

 respecting heat; and it y/ill only be necessary to suppose the 

 vibrations and undulations princ'pally constituting it to be 

 larger and stronger than those of light, while at the same 

 time the smaller vibrations of light, and even the blackening 

 rays, derived from still more mirute vibrations, may perhaps, 

 when sufficiently condensed, concur in producing the effects 

 of heat. These effects, beginning from the blackening rays, 

 which are invisible, are a little more perceptible in the violet, 

 which still possess but a faint power of ilkimination ; the 

 yellow-green affcd the most light ; the red gives less light, 

 but much more heat; while the still larger and less frequent 

 vibrations, which have no effect upon the sense of sight, may 

 be supposed to give rise to the least refrangible rays, and 

 to constitute invisible heat." 



It is doubtless true that Young's views did not at the time 

 of the publication of these lectures * command the authority 

 which now attaches to them. But when the undulatory theory 

 gained acceptance, there was no room left for the distinct 

 entities. 



J. B. Reade, one of the pioneers of photography, in a letter 

 to R. Hunt t, of date Feb. 1854, thus speaks of Young : — 

 " Dr. Young's propositions are, that radiant light consists in 

 undulations of the luminiferous sether, that light differs from 

 heat only in the frequency of its undulations, that undulations 

 less frequent than those of light produce heat, and that undu- 

 lations more frequent than those of light produce chemical and 

 photographic action, — all proved by experiments.^^ 



* I may remark, in passing, that Brougliam knew a little of experi- 

 menting, as of everything else, except law ! 



t Hunt's ' Researches on Light,' Longmans, 1854, p. 374. Hunt himself, 

 not being an imdulationist, was upon the other side. 



