THE COMPOSITION OF MIND 



in reality a compound of the fourth or fifth, or 

 even of some higher, order. 



In the case of visual sensations, the same con- 

 clusion is reached by a precisely similar argu- 

 ment, sensations of colour differing from those 

 of sound only as answering to wave lengths 

 immeasurably shorter and more rapid in suc- 

 cession. It is unnecessary to insist upon the 

 manifold analogies between sound and light, 

 which are each day brought more vividly before 

 the attention of the physical inquirer, as, for ex- 

 ample, in the wonderful but plausible hypothe- 

 sis lately propounded, that all the lines in the 

 spectrum are simply the harmonic over-tones of 

 a fundamental colour, which, being a couple of 

 octaves below red, is itself invisible. Restrict- 

 ing our statement to ascertained points of re- 

 semblance, it may be said that the argument 

 from the phenomena of musical pitch applies 

 step by step to the phenomena of colour as we 

 rise in the scale from red to violet ; the only 

 difference being that, as the slowest vibrations 

 which the eye receives occur at the rate of about 

 458,000,000,000,000 in a second, we cannot 

 experimentally distinguish, as in the case of the 

 lowest sounds, the seemingly elementary sensa- 

 tion which answers to each couple of vibrations. 

 Nevertheless, from experiments with the elec- 

 tric spark it has been shown that a sensation of 



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