KINETIC OR MECHANICAL VIEW OF NATURE. 51 



throw the strings into vibration, and consequently would 

 themselves be gradually extinguished, since otherwise 

 there would be a creation of vis viva. The optical applica- 

 tion of this illustration is too obvious to need comment." 

 Already ten years before Kirchhoff gave to the 

 researches into the spectrum their popular celebrity and 

 practical importance, Stokes l had made an extensive ex- 



1 The memoir of Sir G. Stokes 

 " on the change of the refrangi- 

 bility of light," in the ' Philos. 

 Transactions' (May 1852), forms a 

 landmark in optical science, and 

 whilst dealing with the less obvious 

 though very frequent and general 

 phenomena of fluorescence and 

 phosphorescence, really indicated 

 the line of reasoning which has 

 become so fruitful and suggestive 

 in his own hands and in those 

 of other eminent natural phil- 

 osophers. On page 549 of that 

 memoir he wrote : " All believers 

 in the undulatory theory of light 

 are agreed in regarding the pro- 

 duction of light in the first instance 

 as due to vibratory movements 

 among the molecules of the self- 

 luminous body. . . . Nothing then 

 seems more natural than to suppose 

 that the incident vibrations of the 

 luminiferous ether produce vibra- 

 tory movements among the ultimate 

 molecules of sensitive substances, 

 and that the molecules in turn, 

 swinging on their own account, pro- 

 duce vibrations in the luminiferous 

 ether, and thus cause the sensation 

 of light. The periodic times of 

 these vibrations depend upon the 

 periods in which the molecules are 

 disposed to swing, not upon the 

 periodic time of the incident vibra- 

 tions." Referring, then, to the 

 dynamical difficulties which attach 

 to such a view, he proceeds to point 

 out " that we have no right to re- 

 gard the molecular vibrations as 



indefinitely small. The excursions 

 of the atoms may be, and doubtless 

 are, excessively small compared 

 with the linear dimensions of a 

 complex molecule. It is well 

 known that chemical changes take 

 place under the influence of light, 

 especially the more refrangible rays, 

 which would not otherwise happen. 

 In such cases it is plain that the 

 molecular disturbances must not be 

 regarded as indefinitely small. 

 But vibrations may very well take 

 place which do not go to the 

 length of complete disruption and 

 yet which ought by no means to be 

 regarded as indefinitely small. . . . 

 Certainly we cannot affirm that in 

 the disturbance communicated back 

 again to the luminiferous ether 

 none but periodic vibrations would 

 be produced having the same 

 period as the incident vibrations. 

 Rather, it seems that a sort of 

 irregular motion must be produced 

 in the molecules, periodic only in 

 the sense that the molecules retain 

 the same mean state ; and that the 

 disturbance which the molecules in 

 turn communicate to the ether 

 must be such as cannot be expressed 

 by circular functions of a given 

 period, namely, that of the incident 

 vibrations. " Stokes then refers to 

 the probable internal vibration of 

 the atoms in the compound mole- 

 cules, as " it is chiefly among organic 

 compounds . . . having a compli- 

 cated structure that internal dis- 

 persion (fluorescence) is found." 



