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together. Beats and intermissions are thus identical, and both, when succeeding each other fast enough, produce a harsh discordant jar, or rattle.
Two questions now suggest themselves ; first, why should such an intermittent soundówhy should rapid beatsóbe unpleasant ? and secondly, why should beats cease to be unpleasant when they become sufficiently rapid ?
With regard to the first question, beats produce intermittent excitement of certain auditory nerve fibres. Now any excitement of a nerve fibre deadens its sensibility, and thus during a continuance of the excitement, the excitement itself deadens the sensibility of the nerve, and in this way protects it against too long and too violent excitement. But during an interval of rest, the sensibility of the nerve is quickly restored. Therefore if the excitement instead of being continuous is intermittent, the nerve has tune to regain its sensibility more or less, during the intervals of rest; thus the excitement acts much more intensely than if it had been continuous, and of the same uniform strength.
In the analogous case of light, for example, every one must have experienced the unpleasant sensation of walking along the shady side of a high picket fence, with the evening sun shining through. Here the fibres of the optic nerve are alternately excited and at rest. During the short intervals of rest, the nerve regains more or less its sensibility, and thus the excitements due to the sunlight are much more intense than they would have been, had the irritation been continuous; for in this case, the continuous irritation would have produced a continuous diminution in the sensibility of the nerve. It is precisely the same cause, which renders the flickering of a gas jet, when water has got into the pipe, so unpleasant.
An intermittent tone is to the nerves of hearing, what a flickering light is to the nerves of sight, or scratching to the nerves of touch. A much more unpleasant and intense excitement is produced thaii would be occasioned by a continuous tone. The following simple experiment is instructive on this point. Strike a tuning-fork and hold it farther and farther from the ear, till its tone can just not be heard. Now if the fork, while still faintly vibrating, be revolved, it will become audible. For as we have seen, during its revolution, it is brought into positions such, that it alternately can and cannot transmit its sound to the ear, and this alternation of strength is immediately perceptible to the ear. As Helmholtz has pointed out, this fact supplies us with a delicate means of detecting