50 SCIENTIFIC THOUGHT. 



We have seen above how the vibratory theory of light 

 was arrived at mainly in the hands of Young through 

 dwelling on the analogy of certain optical phenomena, 

 notably those of interference, with the properties exhibited 

 by sound. Among the latter none were more remarkable 

 than those known popularly as consonance and resonance. 

 Sir George Stokes, on the appearance of Kirchhoff s memoir 

 on the relation of emission and absorption of certain rays 

 of light, gave the mechanical explanation in the following 

 words : 1 "In describing the result of a prismatic analysis 

 of the voltaic arc formed between charcoal poles, M. 

 Foucault ' found that the arc presents us with a medium 

 which emits the rays D on its own account, and which at 

 the same time absorbs them when they come from another 

 quarter.' . . . The remarkable phenomena discovered by 

 Foucault, and rediscovered and extended by Kirchhoff, 

 that a body may be at the same time a source of light, 

 giving out rays of a definite refrangibility, and an ab- 

 sorbing medium extinguishing rays of the same refrangi- 

 bility which traverse it, seems readily to admit of a 

 dynamical illustration borrowed from sound. We know 

 that a stretched string which on being struck gives out a 

 certain note, is capable of being thrown into the same 

 state of vibration by aerial vibrations corresponding to the 

 same note. Suppose now a portion of space to contain a 

 great number of such stretched strings, forming thus the 

 analogue of a ' medium.' It is evident that such a 

 medium, on being agitated, would give out the note above 

 mentioned, while on the other hand, if that note were 

 sounded in air at a distance, the incident vibrations would 



1 'Phil. Mag.,' March 1860, pp. 194, 196. 



