Графични страници
PDF файл
ePub

tions of some elastic and peculiar ether, we must be content to use words which signify facts that we know, though they may leave much unknown respecting the cause of the facts. That we may see, there must be light. That we may see some particular object, it must be possible for light, moving according to its established laws, to proceed from it to the eye. The motion of light, whatever it is, stream or undulation, is a well-established fact. Its calculated velocity is nearly two hundred thousand miles in a second, about eleven and a half millions in a minute; so that it is somewhere about eight minutes in travelling from the sun to the earth. Its leading laws, so far as vision is concerned, are these. It travels in straight lines. If it passes into a denser medium, it becomes proportionably bent; and so the opposite way, in passing from denser to rarer. This is refraction. Striking against an object through which it cannot pass, it is reflected ; and the angle at which the ray is reflected, is equal to that formed by the ray in striking the object. But the most important law is that by which its separate rays can be collected together in what is called a focus. The actual performance of human vision depends on this ; so that it is necessary to attend to it somewhat particularly. Rays passing through a convex lens are refracted, or bent from the perpendicular, so as to tend to a point behind, where they all meet: this is called the focus; and the distance is greater or less in proportion to the convexity of the surface through which they pass. An image of the object from which the rays proceed may thus be obtained on a proper surface, held behind the glass at the regular focal distance.

For the present general sketch of the subject it is only necessary to observe that experiments have proved that a ray of light is not homogeneous ; that is, not all of one kind. It is composed of several colours, which, united, form colourless light. This, by being passed through a prism, may be resolved into seven colours --red, orange, yellow, green, blue, indigo, and violet. The colours of bodies depend on this. Different surfaces reflect to the eye different colours from the light that shines on them.

As to the objects of vision, it is sufficient here to say that they are bodies of various kinds, considered as visible; considered in their figure, colour, position, and distance. Only it ought to be remarked, that of distance there is a real judgment formed, though the eye was so early trained, and the habit formed so soon and so completely, that the mind is conscious of nothing but what appears to be an instantaneous perception. The two chief elements of the judgment are size, together with brightness of light, and depth of shadow. A circular plane, half a hollow sphere, with the hollow part opposite the eye, and another half, with the round part opposite the eye, if of the same diameter, fill the same space in the eye, but present different appearances in regard to light and shadow; and by these appearances, which the mind has learned to decide upon with what seems like intuitive rapidity, the real shape of the object is known. A smaller body near the eye makes a larger image than a greater one at a distance: yet we seem to see them according to their real size ; judging of them, though with inconceivable quickness, partly by intervening objects, partly by their greater or less distinctness, as well as other circumstances connected with the light which flows from them. We stand in a room, and through a pane of glass less than a foot square, we see a whole window across the street with many such panes. And yet the window, the outlines of which we could mark on the pane, does not, when we look at it without particular thought, look less than the pane through which we see it. Our idea is in reality formed both of perception and judgment; but we have acquired the habit of referring the whole to the former alone.

Vision itself is performed by means of the eye; for though the organ is double, we may, in such a sketch as this, speak of it as in the singular number. How it is that two organs only convey one impression is a mystery, of which, perhaps, we can give no other account than that so the Creator has willed that it should be.

Every one knows that in man the eyes are fixed in the upper part of the face, one each side of the nose. Nor is it necessary to do more than to remind the reader that, for the safety of this important organ, each eye is fixed in a suitably-sized cavity of bone; or that, to prevent certain inconveniences, (from dust, for instance, or too much light,) there are provided, the eyebrows, two arches, consisting of adipose tissue, covered with hairs, on the ridges of what is called the frontal bone, immediately above the eyes; the eyelids, in front of the eyeball, the upper being larger than the lower, so that they meet below the middle line, and, when they meet, completely close in the eye; and the eyelashes, a row of fine hairs on the edge of each lid, keeping from the eye small particles of dust, &c., as well as shading it from too intense light. Provision is also made for spreading over the surface of the eye, by the lachrymal glands and their secretory ducts, a saline, muco-serous fluid. While this moistens the eye, and in a manner washes it, certain mental emotions will occasion its larger flow. We are then said to shed tears. So the Creator has appointed, and so it is: but who can fathom the mystery? who say how it is ?

It is scarcely possible to give, in words only, (especially when only few words can be employed,) without the aid of drawings, such a description of this complicated and wonderfully-formed organ,—this admirable optical instrument, -as will convey a distinct idea of it to the mind. Even the best description will be imperfect; for, when composition and form have been most carefully stated, there is one thing that cannot be described, and that is, life. The various parts are all necessary, and so is the arrangement they have received; but their actual power is dependent on their being living substances. When the eye is anatomically examined, it has ceased to be a living eye; and one of the first lessons which physiology teaches us is this,—that there are functions which living structures perform that can only be performed by them as such. The vis vitæ (the power, energy, of life) is an ultimate fact in the science; and the anatomist, in studying structures and organs, is obliged to do so in the absence of that on which the performance of the function depends. The

eye is a globe, about an inch in diameter, rather projecting in front. This globe, with its contents, being fixed, though with a certain power of motion, in the orbit, or socket, and connected with the optic nerve, performs its appointed work of vision. The eyeball, removed from its socket, feels firm, even to hardness; but this is owing, not to its being a solid body, but to the firmness of its enclosing coats, or tunics, and the distension of the fluids, or humours, which it contains. Let us suppose the globe of the eye to be cut downwards, longitudinally, exactly in halves, and the flat section to be exposed to view. It would be seen to be surrounded, for about four-fifths of the circle, (leaving the remaining fifth in front,) by a dense, fibrous membrane, which receives its name from its hardness,--the sclerotic tunic. Its anterior surface is covered by a thin tendinous layer,—the tunica albuginea,which also is covered, for a part of its extent, by the conjunctiva, (the mucous membrane of the front of the eye,) and, by its whiteness, occasions the common expression, “the white of the eye." The front of the eye (the other fifth of the circle) is covered by the cornea, (horny,) a transparent, projecting layer, fitting in to the bevelled edge of the sclerotica. The cornea is composed of four layers,—the conjunctiva, the cornea proper, the cornea elastica, and the lining membrane of the anterior chamber of the eyeball. Within the globe thus formed by the sclerotica and cornea are several humours, contained in distinct chambers; but these are constituted by some other membranes, which must, therefore, first be mentioned, as they form coats of at least portions of the eyeball. The choroid membrane is situated immediately within the sclerotica, and terminates in front at the ciliary circle, a ring placed between the sclerotica, the choroid membrane, and the iris. This last is a circular membrane, a little behind the front of the eye, forming a septum (division) between the anterior and posterior chambers, and attached by its circumference to the ciliary circle and its processes, which are small white bodies projecting from the circle, and arranged in a radiated form behind the iris. In the centre of the iris there is a small circular aperture, called the pupil; and the structure of the iris is such as to be capable of enlarging or diminishing this aperture, so that under some circumstances the pupil dilates, under others, contracts.

The third of the principal tunics of the eye is composed of the retina, which, as such, terminates towards the front by an abrupt border at the commencement of the ciliary processes; and the xonula ciliaris, the prolongation of the vascular layer of the retina to the lens, which is constituted by a crystalline humour, encircled by what is called the canal of Petit, the zonula ciliaris being in front of it.

All who understand the powers of the telescope are aware that they result from the combination of glasses which refract the rays of light in different degrees; and when a number of glasses is necessary, the instrument must be of a larger size. So likewise when the object is to have glasses which shall receive and refract a large number of rays, and magnify distant objects more, as the glasses must be of a larger circle, so again must the telescope be larger. That by means of which the great astronomer Herschell performed his observations on the heavenly bodies, was forty feet long, and had to be moved by machinery. But all that is done by human vision (and how much is done!) is done by the eye, that natural telescope, a globe which only averages a diameter of one inch. This is effected, so far as the operations of living substances can be understood by the examination of their construction and arrangement when dead, by means of the law of refraction, (already referred to,) and certain fluids, or humours, secreted for the purpose, and arranged in certain cavities, or chambers, formed by the membranes already mentioned.

In the globe of the eye there are three humours, or refracting media; the aqueous, the crystalline, (lens,) and the vitreous. The aqueous is a perfectly colourless, and weakly albuminous, fluid, about five grains in weight, filling the anterior and posterior chambers in front of the eye; the first, the cavity between the cornea and iris; the other, the narrow space between the iris behind and the lens, which is just beyond the pupil. The anterior cavity is about two lines (or twelfths of an inch) deep; the posterior, half a line. The vitreous humour forms the principal bulk (perhaps two-thirds) of the globe of the eye, occupying the whole of the posterior cavity, except the space taken up by the lens. It is thin, and almost watery, though having a somewhat gelatinous consistence, and is enclosed in a delicate membrane, called the hyaloid. Opposite to the opening in the iris, (called the pupil,) and

« ПредишнаНапред »