410

FUNCTIONS OF THE INTERNAL EAR.

character, however, there can be no doubt, from its being divided, like the cochlea of Mammals and of Man, by a membranous partition on which the nerve is spread out.

520. From the circumstance that in almost every instance in which the semicircular canals exist at all, they are three in number, and lie in three different directions, corresponding to those of the bottom and two adjoining sides of a cube, it has been supposed (and with much probability) that they assist in producing the idea of the direction of sounds. It has been also supposed that the cochlea is the organ by which we judge of the pitch of sounds; and this would seem to be not improbable, especially when we compare the development of the cochlea in different animals, with the variety in the pitch of the sounds which it is important they should hear distinctly, especially the voices of their own kind. The compass of the voice (that is, the distance between its highest and its lowest tones) is much greater in Mammals than in Birds; as is also the length of the cochlea. In Reptiles, which have little true Vocal power, the cochlea is reduced to its lowest form; and in the Amphibia, it disappears altogether.

521. That the Vestibule, and the passages proceeding from it, constitute even in Man the essential part of the organ of hearing, is evident from the fact, that when (as happens not unfrequently) the membrana tympani has been destroyed by disease, and the chain of bones has been lost, the faculty is not by any means abolished, though it is deadened. In this state, the vibrations of the air must act at once upon the membrane of the fenestra ovalis, as in the lower animals which possess no external or middle ear; instead of striking the membrane of the tympanum, and being transmitted along the chain of bones.

522. It has been stated (§ 510) that the sensation of hearing is produced by the successive undulations or vibrations communicated to the Ear from the sonorous body, either by the air, or by a liquid or solid medium. This is the case with all continuous sounds or tones; but single momentary sounds, such as those produced by the discharge of a pistol, the blow of a hammer, the ticking of a watch, or the beat of a clock, make their impression on the ear by a single shock. All continuous tones are in fact caused by a succession of such shocks, communicated to the ear with sufficient rapidity

NATURE OF CONTINUOUS TONES.

411

for the interval between them not to be distinguished. Thus, if we cause a tight string to vibrate by pulling or striking it, we occasion, not one vibration only, but a long succession of vibrations (MECHAN. PHILOS. § 187); every one of which gives a new impulse to the air, and produces a new impression on the organ of hearing. These vibrations we can see, when they are sufficiently extensive; and we can always feel them, by placing the finger on the string. In the same manner, the vibrations of a bell or of a tuning-fork continue long after the first blow; and these, though we cannot see them, may be readily felt with the finger. It is, in fact, in their power of continuing to vibrate after they have been struck, that the peculiarity of these resonant bodies consists. In other instances in which continuous tones are produced, the vibrations are kept-up by the continued application of the original cause, and cease as soon as it is withdrawn: this is the case, for instance, in the string of the violin when set in vibration by the bow, and in the flute and organ-pipe when caused to sound by the passage of air through them.

523. In all these cases, then, the continuous tones are due to a succession of impulses given by the sounding body to the air; and according to the rapidity with which the impulses succeed one another, will be the pitch of the sound. It is not difficult to ascertain by experiment the number of such impulses required to produce particular tones. The lowest note (C) given by any musical instrument (that which is produced by an open organ-pipe of 32 feet long, or by a stopped pipe of 16 feet) requires 16 impulses per second;1 but continuous tones have been produced by impulses occurring at the rate of only 7 or 8 per second; so that the impression produced upon the ear by each must have lasted at least 1-7th or 1-8th of a second. On the other hand, it has been ascertained that the ear can appreciate tones produced by 24,000 impulses in a second; so that the limit already adverted to (§ 517) must be above this tone, the pitch of which is about 4 octaves above the highest F of the pianoforte.

524. The strength or loudness of musical tones depends

1 A backward as well as forward vibration must take place with every impulse; consequently the number of the vibrations is twice that of the impulses.

412

DIFFERENCES IN TONES, AND IN SENSE OF HEARING.

upon the force and extent of the vibrations communicated by the sounding body to the air. Thus, when we draw the middle of a tight string far out of the straight line, and then let it go, a loud sound is produced, and we can see that the space through which the string passes from side to side is considerable. As the extent of the vibrations of the string diminishes, the sound becomes less powerful; and when we can no longer see the vibrations, but can only feel them, the sound is faint. The length of the undulations in the air corresponds with that of the vibrations in the sounding body; and consequently they will strike upon the tympanum with more or less force, according as these are longer or shorter. The cause of the differences in the timbre or quality of musical tones,-such, for instance, as those which exist between the tones of a flute, a violin, and a trumpet, all sounding a note of the same pitch,—are unknown; but they probably depend upon the different form of the vibrations.

525. The faculty of hearing, like that of sight, may be very much increased in acuteness by cultivation; but this increase depends rather upon the habit of attention to the faintest impressions made upon the organ, than upon any change in the organ itself. This habit may be cultivated in regard to sounds of some one particular class; all others being heard as by an ordinary person. Thus the watchful North American Indian recognises footsteps, and can even distinguish between the tread of friends or foes, whilst his companion who lives amid the busy hum of cities is unconscious of the slightest sound. Yet the latter may be a Musician, capable of distinguishing the tones of all the different instruments in a large orchestra, of following any one of them through the part which it performs, and of detecting the least discord in the blended effects of the whole,-effects which would be, to his coloured companion, but an indistinct mass of sound. In the same manner, a person who has lived much in the country is able to distinguish the note of every species of bird which lends its voice to the general concert of Nature; whilst the inhabitant of a town hears only a confused assemblage of shrill sounds, which may impart to him a disagreeable rather than a pleasurable sensation.-Of the direction and distance of sounds, our ideas are for the most part formed by habit. Of the former we probably judge, in great degree, by the

APPRECIATION OF SOUNDS -TRANSMISSION OF LIGHT. 413

relative intensity of the impressions received by the two ears; though we may form some notion of it by either singly (§ 520). Of the distance we judge by the intensity of the sound, comparing it with that which we know the same body to produce when nearer to us. The Ear may be deceived in this respect as well as the eye (§ 566); thus the effect of a full band at a distance may be given by the subdued tones of a concealed orchestra close to us; and the Ventriloquist produces his deceptions by imitating, as closely as possible, not the sounds themselves, but the manner in which they would strike the ear.

Sense of Sight.

526. By the faculty of Sight we are made acquainted, ir the first place, with the presence of light; and by the medium of that agent we take cognizance of the forms of surrounding bodies, their colours, dimensions, and positions. It is desirable that a short account should be here given of the laws of the transmission of light; since, without the knowledge of them, the beautiful action of the Eye cannot be understood.

527. The rays of light uniformly travel in straight lines, so long as they traverse the same medium (air, water, or glass, for instance), without obstruction. When issuing from a single luminous point into space, they diverge or separate, in such a manner as to cover a larger and larger surface as they proceed; and the intensity of the light diminishes in the same proportion. But when the rays pass from one medium to another either more or less dense,

they are bent out of their straight course, or refracted; unless they should happen to pass from the one to the other in a direction perpendicular to the plane which separates them. This may be made evident by a very simple experiment. Place a coin or any heavy body (a, fig. 205) at the bottom of a basin, and then retreat from it until the coin is hidden by the edge of the basin; if water be then poured-in, up to the level c, the coin will again become visible, although neither its own place nor that of the observer has undergone any change. The reason of this

Fig. 205.

414

COURSE OF REFRACTED RAYS.

is, that the rays of light, as they pass out of the water, are bent downwards, or from the perpendicular; so that they reach the eye of the observer when situated at a lower point than that at which the rays would have arrived if they had proceeded in a straight line. Thus the eye, situated at the end of the line a c, could not see the coin in a straight line, because rays passing in that line would be interrupted by the opaque sides of the basin; but it receives the ray which was passing through the water in the direction a d, and which was bent downwards at the moment of quitting it. If the eye had been placed directly over the coin, however, so that the ray passing through the latter to it would have emerged from the water in a direction perpendicular to its surface, no change in the apparent place of the object would have been made by pouring-in the water; since a ray that passes from one medium to another, however different, in a direction perpendicular to the surface which separates them, is not refracted. Those rays which pass-out most nearly in this direction are refracted least, whilst those which pass-out most nearly in the horizontal direction are refracted most.

528. The general law of refraction then is,-that all rays passing from a dense to a rare medium are refracted from the perpendicular, the degree of change being less as they are near the perpendicular, and greater as they depart from it. On the other hand, when rays pass from a rare medium into a dense one, they are bent towards the perpendicular; and this in a greater or less degree, according as their direction is more distant from the perpendicular, or nearer to it. Thus, in fig. 205, we will suppose the point a to be the position of the eye of a Fish; and the end of the line a c (previously occupied by the eye of the observer) to be the position of an Insect in the air. Now this insect will not be seen by the fish in its true place; for a ray passing from it to c would be so bent out of its course as not to reach the point a. The direction in which it is really seen is a d; for the ray proceeding from the object to the surface of the water, there undergoes such a refraction that it is bent downwards to a; and, as we always judge of the place of an object by the direction in which the rays last come to the eye, the insect is seen by the fish at d, that is, considerably above its real place (§ 476).