46 



SCIENTIFIC THOUGHT. 



culty arose of explaining how in refracting substances, 

 be they fluid, amorphous (singly refracting), or crystalline 

 (including doubly refracting), these different rays, with 

 different wave - lengths, come to travel with different 

 velocities, and hence take different courses ; how, further, 

 some of these rays come to be extinguished or reflected 

 (or both) in varying degrees. 



Now, although the complete answer to this general 

 question has not yet been given, a principle has been 

 recognised which gives us a clue to the possible explana- 

 tion of a large class of phenomena, and which is thus of 

 remarkable fruitfulness. It was first laid down by 

 Euler, 1 a pure mathematician, whose physical reasoning 

 was frequently suggestive but never particularly clear and 

 definite; it was probably first applied to optical phenomena 

 by Sir George Stokes ; 2 and it was later on used by him 



1 In the last section of his treatise 

 on light and colours (' Berlin Me- 

 moirs,' 1745 ; published in Latin, 

 1746), Euler treats of luminous, 

 reflecting, refracting, and opaque 

 bodies, and he there mentions the 

 analogy which exists with musical 

 resonance. ' ' The smallest particles 

 [of opaque bodies] are similar to 

 stretched strings, which are, as it 

 were, specially receptive for certain 

 vibrations, which they can assume 

 without being struck, if only they 

 are affected by the undulatory 

 movement of the air. "In his 

 expositions upon light and colours, 

 Euler always starts with the analogy 

 of sound and light ; he follows it 

 with absolute consistency " (Cher- 

 buliez, 'Euler's physicalische Ar- 

 beiten,' p. 44). This analogy was 

 exactly what was absent in the in- 

 vestigations of Brewster, who re- 

 mained to the end an adherent of the 



projectile theory. Balfour Stewart 

 came nearest to the true explana- 

 tion in his memoir of 1858 (' Trans, 

 of the Royal Society of Edin- 

 burgh,' 1861) ; but this referred to 

 radiant heat and to Pre" vest's theory 

 of exchanges. It contains the 

 words : " The absorption of a plate 

 equals its radiation, and that for 

 every description of heat" (p. 13). 

 Had this statement been distinctly 

 applied to luminous rays, spectrum 

 analysis would have been his dis- 

 covery, although his theoretical 

 proof might be regarded as in- 

 sufficient (see Scheiner's treatise 

 on Astronomical Spectroscopy, 

 trausl. by Frost, 1894, p. 112 ; also 

 Rosenberger's ' Geschichte der 

 Physik,' vol. iii., 1890, p. 482 sq.) 



' J See the references given on 

 p. 277 of the first volume of this 

 history. 



