Simple sensations of noise, thus produced, may vary in intensity and duration, but apart from these differences they are qualitatively alike. It is possible that small qualitative differences also exist among them, due to the conditions of their rise, but such differences are too small to be marked by distinguishing names. The noises commonly so called are compound ideas made up of such simple noise-sensations and of a great many irregular tonal sensations (cf. § 9, 7). The homogeneous system of simple noise-sensations is probably the first to develop. The auditory vesicles of the lower animals, with their simple otoliths, could hardly produce anything but these. In the case of man and the higher animals it may be surmised that the structures found in the vestibule of the labyrinth receive only homogeneous stimulations, corresponding to simple sensations of noise. Finally, experiments with animals deprived of their labyrinths, make it probable that even direct stimulations of the auditory nerve can produce such sensations (p. 41).
In the embryonic development of the higher animals, the cochlea develops from an original, vestibular vesicle, which corresponds exactly to a primitive auditory organ. We are, therefore, justified in supposing that the complex system of tonal sensations is a product of the differentiation of the homogeneous system of simple noise-sensations, but that in every case where this development, has taken place, the simple system has remained along with the higher.
10. The system of simple tone-sensation is a continuity of one dimension. We call the quality of the single simple tones pitch. The one-dimensional character of the system finds expression in the fact that, starting with a given pitch, we can vary the quality only in two opposite directions: one we call raising the pitch, the other lowering it . In actual experience simple sensations of tone are never presented alone, but always united with other tonal sensations and with accompanying simple sensations of noise. But since, according to the scheme given above (§ 5, 1), these concomitant elements can be varied indefinitely, and since in many cases they are relatively weak in comparison with one of the tones, the abstraction of simple tones was early reached through the practical use of tonal sensations in the art of music. The names c, c #, d #, and d stand for simple tones, though the clangs of musical instruments or of the human voice by means of which we produce these different pitches, are always accompanied by other, weaker tones and often, too, by noises. But since the conditions for the rise of such concomitant tones can be so varied that they become very weak, it has been possible to produce really simple tones of nearly perfect purity.
The simplest means of doing this is by using a tuning-fork, and a resonator tuned to its fundamental tone. Since the resonator increases the intensity of the fundamental only, the other, accompanying tones are so weak when the fork sounds, that the sensation is generally apprehended as simple and irreducible. If the sound-vibrations corresponding to such a tonal sensation are examined, they will be found to correspond to the simplest possible form of vibration, the pendulumoscillation, so called because the vibrations of the atmospheric particles follow the same laws as a pendulum oscillating in a very small amplitude. [ 1 ] That these relatively simple sound-vibrations correspond to sensations of simple tones, and that we can even distinguish the separate tones in compounds, can be explained, on the basis of the physical laws of sympathetic vibration, from the structure of the organs in the cochlea. The basilar membrane in the cochlea is in its different parts tuned to tones of different pitch, so that when a simple oscillatory sound-vibration strikes the ear, only the part tuned to that particular pitch will vibrate in sympathy. If the same rate of oscillation comes in a compound sound-vibration, again only the part tuned to it will be affected by it, while the other components of the wave will set in vibration other sections of the membrane, which correspond in the same way to their pitch.
11. The system of tonal sensations shows its character as a continuous series in the fact that it is always possible to pass from a given pitch to any other through continuous changes in sensation. Music has selected at option from this continuity single sensations separated by considerable intervals, thus substituting a tonal scale for the tonal line. This selection, however, is based on the relations of tonal sensations themselves. We shall return to the discussion of these relations later, in taking up the ideational compounds arising from these sensations (§
9). The natural tonal line has two extremities, which are conditioned by the physiological capacity of the ear for receiving sounds. These extremities are the lowest and highest tones; the former corresponds to 8-10 double vibrations per second, the latter to 40,000-50,000. C. SENSATIONS OF SMELL AND TASTE.
12 . Sensations of smell form a complex system whose arrangement is still unknown. All we know is that there is a very great number of olfactory qualities, between which there are all possible transitional forms. There can, then, be no doubt that the system is a continuity of many dimensions.
12a. Olfactory qualities may be grouped in certain classes, each of which contains classes sensations which are more or less related.
This fact may be regarded as an indication of how these sensations may perhaps be reduced to a small number of principal qualities. Such classes are for examples, sensations like those from ether, balsam, musk, benzine, those known as aromatic, etc.
It has been observed in a few cases that certain olfactory sensations which come from definite substances, can also be produced by mixing others.