435. THE PERCEPTION OF SOUND. [Nature, Vol. GIL p. 225, 1918.] I DO not think that Helmholtz's theory of audition, whatever difficulties there may be in it, breaks down so completely as Dr Perrett represents*. According to him, one consequence of the theory would be that " when a tuning-fork is made to vibrate, no note can be heard, but only an unimaginable din." I cannot admit this inference. It is true that Helmholfcz's theory contemplates the response in greater or less degree of a rather large number of " resonators" with their associated nerves, the natural pitch of the resonators ranging over a certain interval. But there would be no dissonance, for in Helmholtz's view dissonance depends upon intermittent excitation of nerves, and this would not occur. So long as the vibration is maintained, every nerve would be uniformly excited. Neither is there any difficulty in attributing a simple perception to a rather complicated nervous excitation. Something of this kind is involved in the simple perception of yellow, resulting from a combination of excitations which would severally cause perceptions of red and green. The fundamental question would appear to be the truth or otherwise of the theory associated with the name of J. Miiller. Whatever may be the difficulty of deciding it, the issue itself is simple enough. Can more than one kind of message be conveyed by a single nerve ? Does the nature of the message depend upon how the nerve is excited? In the case of sound—say from a fork of frequency 256—is there anything periodic of this frequency going on in the nerve, or nerves, which carry the message? It is rather difficult to believe it, especially when we remember that frequencies up to 10,000 per second have to be reckoned with. Even if we could accept this, what are we to think when we come to nerves conveying the sensation of light ? Can we believe that there are processes in action along the nerve repeated 101S times per second? I do not touch upon the anatomical matters treated by Sir T. Wrightson and Prof. Keith, or upon the phonetic evidence brought forward with authority by Dr Perrett. * Nature, Vol. cn. p. 184, 1918.n the direction of the primary light.