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 

 re transferred again to those of the ether itself is., 

 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 the term radiation was not yet 



coined by Sir G. Stokes by analogy generally used to embrace the 



with opalescence as involving no ! invisible chemical (ultra-violet) and 



theoretical suggestion, in place of caloric (infra-red) rays; that photo- 



the earlier names of " internal graphy, which more than any other 



dispersion " or " epipolised light " , process has familiarised us with 



used by Brewster and Herschel. ; chemical radiation, was a compara- 



He, however, very soon favoured i tively recent invention ; that the 



the term " degraded light," sug- ideas of conservation, conversion, 



gested by William Thomson (Lord 

 Kelvin) (see the second memoir, 

 1853, p. 387). The latter was at 

 that time occupied with his cele- 



and degradation of energy were 

 quite new ; that the general term 

 energy had not even been invented, 

 we must indeed regard the words 



brated and not less epoch-making I of Sir G. Stokes as containing a 



researches referring to the dissipa- prophetic programme of the ideas 



tion or degradation of energy, of \ and problems of the whole subse- 



which more in the next chapter. If quent period down to quite recent 



we remember that fifty years ago times. ? 



