222 RECENT PROGRESS OF THE THEORY OF VISION. 



certain peculiarities which have been observed in the perception 

 of musical notes, peculiarities as enigmatical as those we have 

 been considering in the eye. In the cochlea of the internal ear 

 the ends of the nerve fibres lie regularly spread out side by side, 

 and provided with minute elastic appendages (the rods of Corti) 

 arranged like the keys and hammers of a piano. My hypothesis 

 is, that here each separate nerve fibre is constructed so as to 

 take cognizance of a definite note, to which its elastic fibre 

 vibrates in perfect consonance. This is not the place to describe 

 the special characters of our sensations of musical tones which 

 led me to frame this hypothesis. Its analogy with Young's 

 theory of colours is obvious, and it refers the origin of overtones, 

 the perception of the quality of sounds, the difference between 

 consonance and dissonance, the formation of the musical scale, 

 and other acoustic phenomena, to as simple a principle as that 

 of Young. But in the case of the ear, I could point to a much 

 more distinct anatomical foundation for such a hypothesis, and 

 since that time, I have been able actually to demonstrate the 

 relation supposed; not, it is true, in man or any vertebrate 

 animals, whose labyrinth lies too deep for experiment, but in 

 some of the marine Crustacea. These animals have external 

 appendages to their organs of hearing which may be observed 

 in the living animal, jointed filaments to which the fibres of the 

 auditory nerve are distributed ; and Hensen, of Kiel, has satis- 

 fied himself that some of these filaments are set in motion by 

 certain notes, and others by different ones. 



It remains to reply to an objection against Young's theory 

 of colour. I mentioned above that the outline of the colour- 

 disc, which marks the position of the most saturated colours 

 (those of the spectrum), approaches to a triangle in form ; but 

 our conclusions upon the theory of the three primary colours 

 depend upon a perfect rectilinear triangle inclosing the complete 

 colour-system, for only in that case is it possible to produce all 

 possible tints by various combinations of the three primary 

 colours at the angles. It must, however, be remembered that 

 the colour-disc only includes the entire series of colours which 

 actually occur in nature, while our theory has to do with the 



