Harmony and Discord.—When several tones sound together we may get peculiar feelings of pleasure or displeasure designated as consonance and dissonance respectively. A note sounds most consonant with its octave. When with the octave the 'third' and the 'fifth' of the note are sounded, for instance c—e—g—c´, we get the 'full chord' or maximum of consonance. The ratios of vibration here are as 4:5:6:8, so that one might think simple ratios were the ground of harmony. But the interval c—d is discordant, with the comparatively simple ratio 8:9. Helmholtz explains discord by the overtones making 'beats' together. This gives a subtle grating which is unpleasant. Where the overtones make no 'beats', or beats too rapid for their effect to be perceptible, there is consonance, according to Helmholtz, which is thus a negative rather than a positive thing. Wundt explains consonance by the presence of strong identical overtones in the notes which harmonize. No one of these explanations of musical harmony can be called quite satisfactory; and the subject is too intricate to be treated farther in this place.
Discriminative Sensibility of the Ear.—Weber's law holds fairly well for the intensity of sounds. If ivory or metal balls are dropped on an ebony or iron plate, they make a sound which is the louder as they are heavier or dropped from a greater height. Experimenting in this way (after others) Merkel found that the just perceptible increment of loudness required an increase of 3/10 of the original stimulus everywhere between the intensities marked 20 and 5000 of his arbitrary scale. Below this the fractional increment of stimulus must be larger; above it, no measurements were made.
Discrimination of differences of pitch varies in different parts of the scale. In the neighborhood of 1000 vibrations per second, one fifth of a vibration more or less can make the sound sharp or flat for a good ear. It takes a much greater relative alteration to sound sharp or flat elsewhere on the scale. The chromatic scale itself has been used as an illustration of Weber's law. The notes seem to differ equally from each other, yet their vibration-numbers form a series of which each is a certain multiple of the last. This, however, has nothing to do with intensities or just perceptible differences; so the peculiar parallelism between the sensation series and the outer-stimulus series forms here a case all by itself, rather than an instance under Weber's more general law.
CHAPTER V.
TOUCH, THE TEMPERATURE SENSE, THE MUSCULAR SENSE, AND PAIN.
Nerve-endings in the Skin.—"Many of the afferent skin-nerves end in connection with hair-bulbs; the fine hairs over most of the cutaneous surface, projecting from the skin, transmit any movement impressed on them, with increased force, to the nerve-fibres at their fixed ends. Fine branches of axis-cylinders have also been described as penetrating between epidermic cells and ending there without terminal organs. In or immediately beneath the skin several peculiar forms of nerve end-organs have also been described; they are known as (1) Touch-cells; (2) Pacinian corpuscles; (3) Tactile corpuscles; (4) End-bulbs."[23]
Fig. 24.—End-bulbs from the conjunctiva of the human eye, magnified.
These bodies all consist essentially of granules formed of connective tissue, in which or round about which one or more sensory nerve-fibres terminate. They probably magnify impressions just as a grain of sand does in a shoe, or a crumb does in a finger of a glove.
Touch, or the Pressure Sense.—"Through the skin we get several kinds of sensation; touch proper, heat and cold, and pain; and we can with more or less accuracy localize them on the surface of the body. The interior of the mouth possesses also three sensibilities. Through touch proper we recognize pressure or traction exerted on the skin, and the force of the pressure; the softness or hardness, roughness or smoothness, of the body producing it; and the form of this when not too large to be felt all over. When to learn the form of an object we move the hand over it, muscular sensations are combined with proper tactile, and such a combination of the two sensations is frequent; moreover, we rarely touch anything without at the same time getting temperature sensations; therefore pure tactile feelings are rare. From an evolution point of view, touch is probably the first distinctly differentiated sensation, and this primary position it still largely holds in our mental life."[24]