568 BIBLIOGRAPHY OF HERSCHEl's WRITINGS. 



Herschel, W.: Hynopsis of the Writixgs of— Continncd. 



A. D. Vol. P. 



1800 90 2G2 JExperimenfs on the illuminating Poiver of coloured Baj/s. 



262-270 Teu cxperiraoiits on tho visibility of objects ilhirainated by dif- 

 ferently coloured light. 

 268 "The maximum of illumination lies in the briglitest yellow or palest 

 green. " 



270 "May not the chemical properties of the prismatic coloiirs be as dif- 



ferent as those which relate to light ^and heat? Adequate methods 



271 for an investigation of them may easily be found ; and we cannot 

 too minutely enter into an analysis of light, which is the most subtle 

 of all the active ])rinciples that are concerned in the mechanism of 

 the operations of nature." * 



Eadiant Heat is of different Rcfrangihility. 

 ^ "I must now remark that my foregoing experiments ascertain beyond 



a doubt that radiant heat, as well as light, whether they he the 

 same or diiferent agents, is not only refrangible, but is also subject 



272 to the laws of dispersion arising from its different refraugibility ; 

 and, as this subject is new, I may be permitted to dwell a few mo- 

 ments upon it. The prism refracts radiant heat, so as to separate 

 that which is less efficacious from that which is more so. Tho 

 whole quantity of radiant heat contained in a sunbeam, if this dif- 

 ferent refrangibility did not exist, must inevitably fall iiniformly 

 on a space equal to the area of the prism ; and if radiant heat were 

 not refrangible at all, it would fall upon an equal space in the place 

 where tho shadow of the prism, when covered, may be seen. But, 

 neither of these events taking place, it is evident that radiant heat 

 is subject to the laws of refraction, and also to those of the different 

 refrangibility of light. May not this lead us to surmise that radiant 

 heat consists of pai'ticles of light of a certain range of momenta, 

 and which range may extend a little farther on each side of refrangi- 

 bility than that of light ? We have shewn that in a gradual ex- 

 posure of the thermometer to the rays of the prismatic spectrum, 

 beginning from the violet, we come to the maximum of light long 

 before we come to that of heat, which lies at the other extreme. By 

 several experiments, which time Avill not allow me now to report, 

 it appears that the maximum of illumination has little more than 

 half the heat of the full red rays; and, from other experiments, I 

 likewise concluded that the full red falls still short of the maxi- 

 mum of heat, which perhaps lies even a little beyond visible re- 

 fraction. In this case, radiant heat will at least partly, if not 

 chiefly, consist, if I may be permitted the expression, of invisible 

 light; that is to say, of rays coming from the snn, that have such 



^ a momentum as to be unfit for vision. And, admitting, as is highly 

 probable, that the organs of sight are only adapted to receive im- 



273 pressions from j^articles of a certain momentum, it explains why 

 the maximum of illumination should be in the middle of the re- 

 frangible rays, as those which have greater or less momenta are 

 likely to become equally unlit for impressions of sight. Whereas, 

 in radiant heat, there may be no such limitation to the momentum 

 of its particles. From the powerful effects of a burning lens, how- 

 ever, we gather the information that the momentum of terrestrial 

 radiant heat is not likely to exceed that of the sun, and that, con- 

 sequently, the refraugibility of calorific rays cannot extend much 

 beyond that of colourific light. Hence, we may also infer that the 



