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

Fig. 161.

15

1

A diagram of the Ear:-p. The pina. t. The tympanum. 1. The labyrinth. 1. The upper part of the helix. 2. The antihelix. 3. The tragus. 4. The antitragus. 5. The lobulus. 6. The concha. 7. The upper part of the fossa scaphoidea. 8. The meatus. 9. The membrana tympani, divided by the section. 10. The three little bones, crossing the area of the tympanum, malleus, incus, and stapes: the foot of the stapes blocks up the fenestra ovalis upon the inner wall of the tympanum. 11. The promontory. 12. The fenestra rotunda; the dark opening above the ossicula leads into the mastoid cells. 13. The Eustachian tube; the little canal upon this tube contains the tensor tympani muscle in its passage to the tympanum. 14. The vestibule. 15. The three semicircular canals, horizontal, perpendicular, and oblique. 16. The ampullæ upon the perpendicular and horizontal canals. 17. The cochlea. 18. A depression between the convexities of the two tubuli which communicate with the tympanum and vestibule: the one is the scala tympani, terminating at 12; the other is the scala vestibuli.

between the tread of friends and foes; whilst his white companion, who has lived among 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 the unsophisticated Indian 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 that lends its voice to the general chorus 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 sen

sation.

954. In all continued sounds or tones, there are several points to be attended to. In the first place, we take cognizance of their pitch; which depends upon the number of vibrations in a given time,-the high notes being produced by the most rapid vibrations, and the low notes by the slowest. The ear can appreciate tones produced by 24,000

impulses per second, the pitch of which is about four octaves above the highest F of the piano-forte. On the other hand, no sequence of vibrations fewer than 7 or 8 in a second, can produce a continuous tone, because the impression left by each impulse has passed away, before the next succeeds; and there is consequently nothing more than a succession of distinct beats. The strength or loudness of musical tones depends (other things being equal) on the force and extent of the vibrations, communicated by the sounding body to the medium which propagates them. This will diminish, however, with distance, which softens loud tones by lowering the intensity of the undulations, as a consequence of their more extensive diffusion. The causes of the difference in the timbre, or quality of musical tones,-such, for instance, as those which exist between the tones of a flute, a violin, a trumpet, and a human voice, all sounding a note of the same pitch, are unknown: but they probably depend upon differences of form in the undulations. Our ideas of the direction and distance of sounds, are for the most part formed by habit. Of the former we probably judge in great degree, by the relative intensity of the impressions received by the two ears; though we may form some notion of it by a single ear, if the idea just stated as to the use of the semicircular canals (§ 952), be correct.-Of the distance of the sounding body, 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; thus the effect of a full band at a distance may be given by the subdued tones of a concealed orchestra close by us; and the Ventriloquist produces his deception, by imitating as closely as possible, not the sounds themselves, but the manner in which they would strike our

ears.

6. Of the Sense of Sight.

955. By the faculty of Sight we are made acquainted in the first place, with the existence of Light; and by the medium of that agent we take cognizance of the form, size, color, position, &c., of bodies that transmit or reflect it. As to the mode in which luminous impressions are propagated through space, philosophers are at present undetermined; and the question is of no physiological importance, since all are agreed as to the laws which regulate their transmission. These laws, which will be found at large in any Treatise on Natural Philosophy,* may be briefly stated as follows.

I. Light travels in straight lines, so long as the medium through which it passes is of uniform density.

II. When the rays of light pass from a rarer medium into a denser one, they are refracted towards a line drawn perpendicular to the surface they are entering.

III. When the rays of light pass from a denser medium into a rarer one, they are refracted from the perpendicular.

IV. When rays proceeding from the several points of a luminous object, at a distance, fall upon a double convex lens, they are brought

* See Dr. Golding Bird's Manual, Chap. XXII.

to a focus upon the other side of it; in such a manner that an inverted picture of the object is formed upon a screen, placed in the proper position to receive it. Thus in Fig. 162, A B is the object, and EF the

Fig. 162.

lens; the rays issuing from the two extremities and the centre of the object, are brought to a corresponding focus at a less distance on the other side of it, so as to form a distinct picture; but as the rays from A are brought to a focus at D, and those from B at c, the picture will be inverted.

V. The further the object is removed from the lens, the nearer will the picture be brought to it, and the smaller will it be.

VI. If the screen be not held precisely in the focus of the lens, but a little nearer, or further off, the picture will be indistinct; for the rays which form it will either not have met, or they will have crossed each other.

956. The Eye, in its most perfect form-such as it possesses in Man and the higher animals,-is an optical instrument of wonderful completeness; designed to form an exact picture of surrounding objects upon the Retina or expanded surface of the Optic nerve, by which the impression is conveyed to the brain. The rays of light, which diverge from the several points of any object, and fall upon the front of the cornea, are refracted by its convex surface, whilst passing through it into the eye, and are made to converge slightly. They are brought more closely together by the crystalline lens, which they reach after passing through the pupil; and its refracting influence, together with that produced by the vitreous humor, is such as to cause the rays, that issued from each point, to meet in a focus on the retina. In this manner, a complete inverted image is formed, as shown in Fig. 163; which represents a vertical section of the eye, and the general course

Fig. 163.

of the rays in its interior. As in the preceding figure, the rays which issue from the point A are brought to a focus at D; whilst those diverging from B are made to converge upon the retina at c.-The Retina, which is itself so thin as to be nearly transparent, is spread over the

layer of black pigment, which lines the choroid coat. The purpose of this is evidently to absorb the rays of light that form the picture, immediately after they have passed through the retina; in this manner, they are prevented from being reflected from one part of the interior of the globe to another; which would cause great confusion and indistinctness in the picture. Hence it is that, in those albino individuals (both of the Human race, and among the lower animals), in whose eyes this pigment is deficient, vision is extremely imperfect, except in a very feeble light; for the vascularity of the choroid and iris is such as to give to these membranes a bright red hue, which enables them powerfully to reflect the light that reaches the interior of the eye, when they are not prevented from doing so by the interposition of the pigmentary layer.

957. The Eye is so constructed, as to avoid certain errors and defects, to which all ordinary optical instruments are liable. One of these imperfections, termed spherical aberration, results from the fact, that the rays of light, passing through a convex lens whose curvature is circular, are not all brought to their proper foci, those which have passed through the exterior of the lens being made to converge sooner than those which have traversed its central portion. The result of this imperfection is, that the image is deficient in clearness, unless only the central part of the lens be employed.-The other source of imperfection is what is termed chromatic aberration; and it results from the unequal degree in which the differently-colored rays are refracted, so that they are brought to a focus at different points. The violet rays, being the most refrangible, are soonest brought to a focus; and the red being the least refrangible, have their focus at the greatest distance from the lens. Hence it is impossible to obtain an image by an ordinary lens, in which the colors of the object are accurately represented; for the foci of its differently-colored portions will be different; and its white rays will be decomposed, so that the outlines will be surrounded by colored fringes.-The Optician is enabled to correct the effects of these aberrations, by combining lenses of different densities and curvatures; so arranged as to correct each other's errors, without neutralizing the refractive power. This is precisely the plan adopted in the construction of the Eye; which, when perfectly formed, and in a healthy state, forms an accurate picture of the object upon the retina, free from either spherical or chromatic aberration. This is effected by the combination of humors of different densities, having curvatures precisely adapted to the required purpose.

958. There are certain variations, however, in the conformation of the eye which diminish the perfection of its result. Thus the Cornea may be too convex, and the whole refractive power too great; so that the image of an object at a moderate distance is formed in front of the retina, instead of upon it. When this is the case, a distinct image can only be formed, by bringing the object nearer to the eye; the effect of which will be, to throw the picture further back. Such an eye is said to be myopic, or short-sighted; and its imperfection may be corrected by placing a concave lens in front of the cornea, of a curvature adapted to neutralize what is superfluous in the convexity of the latter.-On

the other hand, if the cornea be too flat, and the refractive power of the humors be too low, the convergent rays proceeding from an object at a moderate distance will not meet upon the retina, but behind it (if they were allowed to pass on); consequently the picture is indistinct; and it can only be made clear, either by withdrawing the object to a greater distance, which will bring the focus of the eye nearer to its front, or by interposing a convex lens to increase the refractive power of the eye. Such a condition is termed presbyopic (from its being common in aged persons), or long-sighted. It may proceed to such an extent, that not even the removal of the object to any distance can permit the formation of a distinct picture; so that the assistance of a convex lens must be obtained even to see remote objects clearly; though a less degree of convexity will be required, than for the clear vision of nearer objects. This state is particularly well marked after the operation for cataract; for the removal of the crystalline lens so greatly diminishes the refractive power of the eye, as to render necessary the assistance of convex lenses of high curvature.

959. The power, by which a healthy, well-formed eye can accommodate itself to the distinct vision of objects at varying distances, is a very remarkable one; and its rationale is not yet properly understood. According to the laws already stated (§ 955, V. and VI.) the picture of a near object can only be distinct when formed more remotely from the lens than the picture of a distant object. Consequently when the eye, that has been looking at a distant object, and has seen it clearly, is turned to a near object, a distinct picture of the latter cannot be formed without some alteration, either in the distance between the refractive surfaces and the retina, or in the curvature of the former. It seems most probable that, in the Human eye, this adjustment is chiefly effected by the automatic contraction of the ciliary muscle; which, by drawing the lens nearer to the iris, will thus increase its distance from the retina. But this may not be the sole change.

960. The various humors and containing membranes of the Eye, thus answer the purpose of a most delicate and self-adjusting Optical

instrument; the sole part, which is immediately concerned in the act of sensation, being the Retina, or net-like expansion of the Optic nerve,