52 



SCIENTIFIC THOUGHT. 



amination into the question how vibrations of the 

 luminiferous medium can be mechanically transferred to 

 the compound molecules of a transparent body, and 

 retransferred again to those of the ether itself i.e., 

 the question of the absorption and emission of light. He 

 showed that vibrations of a certain period, corresponding 

 to a definite tint of colour, could eventually give rise to 

 vibrations of altered period in the emitted light ; that 

 this period, however, must always be longer i.e., that 

 the new colour must always be of a lower order in the 

 35. scale of refrangibility. He was thus not onlv able to 



Explana- 

 tion of explain mechanically the peculiar luminosity which he 



fluorescence. 



termed fluorescence, 1 and which had been observed by 

 Herschel and Brewster in certain minerals and solutions, 

 and independently studied by E. Becquerel in France, 

 but he also showed how, by means of such substances, 

 rays of light which, owing to the frequency of their 

 vibrations, transcend the perceptive powers of the 

 human eye, can be made visible by giving rise to 

 secondary waves of less frequency. The line of reason- 



1 The term fluorescence was 

 coined by Sir G. Stokes by analogy 

 with opalescence as involving no 

 theoretical suggestion, in place of 

 the earlier names of " internal 

 dispersion " or " epipolised light " 

 used by Brewster and Herschel. 

 He, however, very soon favoured 

 the term " degraded light," sug- 

 gested by William Thomson (Lord 

 Kelvin) (see the second memoir, 

 1853, p. 387). The latter was at 

 that time occupied with his cele- 

 brated and not less epoch-making 

 researches referring to the dissipa- 

 tion or degradation of energy, of 

 which more in the next chapter. If 

 we remember that fifty years ago 



the term radiation was not yet 

 generally used to embrace the 

 invisible chemical (ultra-violet) and 

 caloric (infra-red) rays; that photo- 

 graphy, which more than any other 

 process has familiarised us with 

 chemical radiation, was a compara- 

 tively recent invention ; that the 

 ideas of conservation, conversion, 

 and degradation of energy were 

 quite new ; that the general term 

 energy had not even been invented, 

 we must indeed regard the words 

 of Sir G. Stokes as containing a 

 prophetic programme of the ideas 

 and problems of the whole subse- 

 quent period down to quite recent 

 times. 



