108 HABIT AND INTELLKJENGE. [cHAP. 



Succession KeJ, orange, yellow, yellowish green, green, bluish green, 

 ot colours. a2ure, indigo, violet, purple. 



Tiie rays of greatest heating-power are beyond the brightest 

 red, and those of greatest cbeniical power nearly coincide with 

 the purple. The colours graduate into each other, so that any 

 accurate enumeration of tints is impossible. The fact that rays 

 of different wave-lengths, which is altogether a quantitative 

 Difference difterence, ■^vodLXX.ceqiialitative differences of colour, is a purely 

 is "^"^j^^"- physiological fact, and one of which no explanation appears 

 logical possible. It is, however, to be remarked, that the colour of 

 ' light is not in any true sense analogous to the tone of sound.^ 



and ana- The pitch of sound depends on the Avave-length of the sonorous 

 logous not undulation, and its tone depends on the combination of secondary 

 to pitch in waves with the fundamental. In the manner of its production, 

 sound. ^j^g colour of light is analogous, not to the tone of sound, but to 

 its i^itch ; for, as just stated, rays which have different wave- 

 lengths, and have, so far as we know, no other difference 

 whatever, excite totally different sensations of colour. 



There is strong reason for believing that the colour of light 

 corresponds to the pitch of sound in another and very re- 

 markable way. It is a law of sound (or rather of the hearing 

 faculty, for it is a physiological fact and not a physical one) — it 

 is a law of the hearing faculty, I saj^, that any two notes whereof 

 the sonorous waves producing the one are exactly twice as 

 numerous in a second as those producing the other, are in a 

 TliB manner recognised as the same note, the one being the octave of 



octave in ^^^ other. There is reason to think that the same is true of 

 in colour colours. What makes the subject obscure is the fact, which is 

 a purely physiological one, that the power of perceiving sound 

 extends over more than eleven octaves," while the power of 



1 It is, I tlunk, to be regretted that the Germans, and Dr. Tyndall in 

 imitation of tliem, have introduced the term sound-colour iu the sense of 

 tune. The word is not needed, for it exjiresses nothing that "tone " does 

 not express quite as well, both in German and in English ; and moreover, 

 as shown iu the text, the analogy it suggests is untrue. 



^ According to M. Dcspretz, " the number of vibrations required to pro- 

 duce an api)reciable musical sound, in persons endowed with an acute 

 sense of hearing, may vary from 16 [in a second] for the lowest, to 73,000 

 for the highest note." (Carpenter's Human Physiology, p. 660.) This is a 

 range of rather more than eleven octaves ; for the eleventh octave of a note 

 ol' lo honorous vibrations to the second contains 65, .536 to the secwud : 

 Uuit is lo say, 16 x 2i + " =€5,536. 



