332 THE PRINCIPLES OF SCIENCE. 



it preserves throughout its course until extinguished. This 

 is indeed but one case of the general principle of undula- 

 tory movement, which Sir John . Herschel has stated in 

 the most complete manner under the title, 'Principle of 

 Forced Vibrations' (vol. ii. p. 65), and has asserted to be 

 absolutely universal and without exception. But Sir John 

 Herschel himself described in the * Philosophical Transac- 

 tions' for 1845 a curious appearance in a solution of qui- 

 nine ; as viewed by transmitted light the solution appeared 

 colourless, but in certain aspects it possessed a beautiful 

 celestial blue tint. Curiously enough the coloured light 

 comes only from the first portion of liquid which the 

 light enters. Similar phenomena in fluor-spar had been 

 described by Sir D. Brewster in 1838. Professor Stokes, 

 having minutely investigated the phenomena, discovered 

 that they were more or less present in almost all vegetable 

 infusions, and in a number of mineral substances. He 

 came to the conclusion that this phenomenon, called by 

 him Fluorescence, could only be explained by a degrada- 

 tion or alteration in the refrangibility of the rays of light ; 

 he asserts, in fact, that light- rays of very short length of 

 vibration in falling upon certain atoms excite undulations 

 of greater length, in total opposition to the principle of 

 forced vibrations. No complete explanation of the mode 

 of change is yet possible, because it evidently depends 

 upon the intimate constitution of the atoms of the sub- 

 stances concerned ; but Professor Stokes believes that the 

 principle of forced vibrations is true only so long as the 

 excursions of an atom are very small compared with 

 the magnitude of the complex molecules 11 . It is now also 

 well known that in Calorescence the refrangibility of rays 

 may be increased and the wave-length diminished. Rays 

 of obscure heat and low refrangibility may be concentrated 

 so as to heat a solid substance, and make it give out rays 

 n 'Philosophical Transactions' (1852), vol. cxlii. pp. 465, 548, &c. 



