664 THE PRINCIPLES OF SCIENCE. [CHAP. 



solution of quinine ; as viewed by transmitted light the 

 solution appeared colourless, but in certain aspects it exhi- 

 bited a beautiful celestial blue tint. Curiously enough the 

 colour is seen only in the first portion of liquid which the 

 light enters. Similar phenomena in fluor-spar had been 

 described by 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 Fluores- 

 cence, could only be explained by an alteration in the 

 refrangibility of the rays of light ; he asserts that light-rays 

 of very short length of vibration in falling upon certain 

 atoms excite undulations of greater length, in opposition to 

 the principle of forced vibrations. No complete explana- 

 tion of the mode of change is yet possible, because it 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. 1 



It is well known that in Calorescence the refrangibility 

 of rays is 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 

 belonging to any part of the spectrum, and it seems pro- 

 bable that this effect arises from the impact of distinct but 

 conflicting atoms. Nor is it in light only that we discover 

 limiting exceptions to the law of forced vibrations ; for if 

 we notice gentle waves lapping upon the stones at the edge 

 of a lake we shall see that each larger wave in breaking 

 upon a stone gives rise to a series of smaller waves. Thus 

 there is constantly in progress a degradation in the magni- 

 tude of water-waves. The principle of forced vibrations 

 seems then to be too generally stated by Herschel, but it 

 must be a difficult question of mechanical theory to dis- 

 criminate the circumstances in which it does and does not 

 hold true. 



We sometimes foresee the possible existence of exceptions 

 yet unknown by experience, and limit the statement of our 

 discoveries accordingly. Extensive inquiries have shown 

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



