XXXV.] OOLOUES AND THEIR COMBINATION. 107 



There is what I regard as demonstrative evidence for believing Radiance 



that radiance consists of undulations in a universal ethereal consists of 



imdula- 

 medium. The radiance of the siin, and of all other sources tions. 



with which we are famdiar, consists of a variety of rays, which, ^.^^^ °^ 



' . . difterent 



SO far as we know, differ from each other in no physical character wave- 



except the length of the undulations of which they are composed, lengths 



° . "' ^ are mixed 



The velocity of all the rays is the same, and consequently the together 



number of undulations, or waves, to the second in any ray is "^ ^}'"^ 



sunbeam, 

 inversely proportional to the length of the waves that compose 



that ray. All the rays are capable of being refracted ; the 



refrangibility is the greatest where the wave-length is the least ; 



so that rays from the same source, as for instance the rays that 



constitute a sunbeam, may be separated by refraction through a Their 



prism ; the rays of unequal refrangibility being refracted at separaliou 



unequal angles, and consequently proceeding in different direc- prism. 



tions. The unequally refracted rays, when received on a screen, 



constitute the prismatic spectrum. 



As radiance is a form of energy, and as all energy is capable 

 of transformation into heat, it can scarcely be doubted that the 

 heating-power of any ray is a measure of the quantity of energy 

 due to it. When we apply this test, we find that the visible 

 brightness of a ray has nothing to do with the c[uantity of 

 energy which it contains ; for the rays which have the greatest 

 heating-power are scarcely if at all visible. This, of course, is The 

 not a physical but a physiological fact ; or, in other words, it places of 

 depends not on the nature of radiance, but on the constitution jjogt of 

 of our visual faculty. What is even more remarkable is that gi'eatest 

 the greatest chemical, or actinic, or photographic power coincides po^-er^ 

 neither with the greatest heating power nor with the brightest ^^'^ of 

 light ; the rays of greatest chemical power, on the contrary, are Chemical 

 almost if not quite invisible, and are at the opposite end of the power, 

 spectrum from the heating rays.^ coincide 



The rays of different wave-lengths, and consequently of dif- 

 ferent refrangibilities, produce different sensations of colour in raj-s have 

 the eye. Beginning at the rays of the greatest wave-length and cUfferent 

 least refrangibility, and going on to those of opposite character, 

 the succession is as follows : - — 



1 For the probable reason of the last mentioned fact, see a paper by 

 Professor Tyndall in the Fortnightly Eeview for February 1869. 



2 I take this enumeration from Professor Grassmann's paper on the 

 Theory of Compound Colours, in the Philosophical Magazine for April 1854. 



