v THE SENSE OF HEARING 237 



Laueously for the whole series ol' tones and sounds, but, must have 

 evolved gradually, beginning with such as are indispensable to 

 the ear functioning as a resonator for the most ordinary sounds 

 of its environment. This, however, conflicts with the uniformity 

 with which the ear can appreciate all sounds within the limits of 

 acoustic perception. Though every one recognises the fact that 

 the ear becomes, musically, much more acute by practice, there are 

 normally no gaps in audition within the scale of perceptible 

 tones. 



Notwithstanding these and other more or less serious objections 

 to the theory of Helrnholtz it undoubtedly affords a ready explana- 

 tion of many acoustic phenomena, as well as of clinical and 

 experimental observations, although they cannot be said to prove 

 it directly. 



The capacity of the ear for distinguishing the different 

 qualities of tones, and the well-established fact (Hermann and 

 Lindig) that dislocation of the phases of two simultaneous tones 

 does not perceptibly alter timbre, even when it causes marked 

 changes in the form of the complex waves that result, are readily 

 explained by Helmholtz' theory, which assumes that the organ 

 of hearing analyses the tones, and separately excites the single 

 fibres corresponding to the component tones of which they are 

 built up, independently of the form of the resulting waves. 



By means of the resonance theory again it is easy to explain 

 the phenomenon of beats, though the explanation of the origin of 

 Tartini's tones in the same way is more difficult. In fact the 

 latter cannot be accounted for at all, if we admit with Lagrange 

 and Young that they are subjective interference phenomena, 

 because, as such, it is impossible to see how they can excite the 

 corresponding resonators of the organ of Corti. Helmholtz 

 endeavoured to prove their objective existence, and the more 

 recent work of K. Schafer and Zwaardemaker is in favour of 

 this view. 



We have said that normally there are no true auditory lacunae 

 in the scale of perceptible tones. Bezold, however, by means of 

 a contrivance which enabled him to vary the pitch of tones from 

 the highest to the lowest without interrupting the sequence, found 

 that in different individuals larger or smaller gaps occur at the 

 extremes of the series of perceptible tones. Some people show 

 a defect at the upper and lower ends of the tone-range, others 

 only at the lower, or only at the upper, auditory limit. In these 

 cases there is a more or less considerable restriction of perception 

 at the ends of the scale, that is, a diminished range of sensibility 

 to tones rather than a hiatus. 



But there are a number of otological observations, particularly 

 on deaf-mutes, which show the presence of true lacunae of hearing, 

 in other words of " acoustic islands." Some individuals are unable 



