The Eye a Camera Obscura. 595 contract, the pupil is lessened, when the radiating contract, it is enlarged. These changes happen according to the intensity of light and the state of sensibility of the retina, as may at any time be proved by closing the eyelids for a moment to make the pupil dilate, and then opening them towards a strong light, to make it contract. Behind the pupil is seen the lens, D, with its circumference attached to the ciliary processes, E: it is more convex behind than before. The disease of the eye called cataract (from a Greek word implying obstruction), exists when the lens becomes opaque, and the cure is to extract the lens entirely, or to depress it to the bottom of the eye, and then to substitute for it externally a powerful artificial lens or spectacle-glass. The three lines marking here the external wall or boundary of the eye stand for its three coats, as they have been called, the strong sclerotic, and the double lining of the choroid and retina. The figure of a cross is represented upon the back part of the retina as formed by the light entering from the cross without (which cross has to appear here small and near, although representing one large and distant). The image of the cross is inverted, for the same reason as it is in the camera obscura: but we learn that the perception of an object may be equally distinct in whatever position the image may fall upon the retina. It has been explained above, that a lens cannot well form imagery of great extent except on a concave surface,—and the retina is such a surface. The anterior and posterior chambers of the eye are the compartments which are before and behind the crystalline lens, D. The nature of the eye as a camera obscura may be studied by taking the eye of a recently-killed bullock, and after carefully cutting away the back part of the two outer coats, going with it to a dark place and directing the pupil towards any brightly illuminated objects. There may, then, be seen through the semi-transparent retina, left as a screen at the back of the eye, a minute but perfect picture of all the objects in front—a picture, therefore, formed on the back of the little apartment or camera obscura, by the agency of the convex cornea and lens in front, just as occurs in our artificial camera. The picture is inverted, for reasons explained above. The sclerotic coat of the eye of a rabbit is comparatively thin. When a rabbit's eye is held before a candle in a dark room, with the cornca in front, a beautiful inverted image of the candle is seen. 596 Phenomena of Vision. Phenomena of Vision. 825. Upright Vision from Inverted Images.-Because the images formed on the retina are always inverted as respects the true position of the objects producing them-just as happens in a simple camera obscura,-persons have wondered that things should appear upright, or in their true situations. The explanation is simple. It is known that a man in bed with his cheek on the pillow judges as correctly of the position of the objects around him, as any other person-never deeming them to be inclined or crooked, because their images on his retina are inclined in relation to the natural perpendicular when the head is erect. Boys who at play bend themselves down to look backwards from between their knees, although a little puzzled at first, because the usual position of the images on the retina is reversed, soon see as correctly in that way as in any other. It appears, therefore, that while the mind studies the form, colour, etc., of external objects in their images as pictured on the retina, it judges of their position, not by the accidental position of the image on the retina, or by receiving it as a whole, but by the direction in which the light comes from the object and its parts towards the eye-no more deeming an object to be placed low because its image is low in the eye, than a man in a room into which a sunbeam enters by a hole in the windowshutter, deems the sun low because its image is on the floor. In a preceding article (p. 576) it has been demonstrated, under the head of Refraction, that a coin placed in a basin of water is not seen in its true position, but raised above it, the eye judging the object to be in the direction of the line by which the light arrives. An arrow placed with its point upwards in front of the eye, is seen with the point uppermost in accordance with the direction in which the rays of light reach the retina or sensitive membrane. If the arrow were placed horizontally in front of the eye, with its point to the right, there would be no difficulty in comprehending that as the rays of light pass from the point on the right they must impinge on the retina to the left, and it could be seen only in its true position on the right, i.e., in the direction taken by the rays. A similar observation applies to an arrow placed perpendicularly or in any other position before the eye. The eye does not see the image, but sees by means of the image. 826. Other illustrations present themselves. Thus a candle carried past a key-hole throws its light on the opposite wall, but the Inversion of Objects on the Retina. 597 luminous spot moves in a direction the opposite of that in which the candle is carried; and a child must be very young who has not learned to judge at once of the true motion of the candle by the contrary apparent motion of the image. A boatman, who, being accustomed to his oar, can direct its point against any object with great precision, has long ceased to deem it strange that when he desires to move the point of an oar in some one direction, his hand must move in the contrary direction. Now the seeing of things upright by images which are inverted, is a fact of the same nature. 827. It is only in the centre of the retina that vision is perfectly distinct. This is felt at once by looking at a printed page, and observing that only the two or three letters to which the axis of the eye is directed, are clearly seen; and, consequently, aithough the whole page is depicted on the retina at once, the eye, in reading, has to direct its centre successively to every part. Then it may be remarked, in viewing the diagram of the eye, that the centre of the retina is more distant from the lens than any other part of it, and, consequently, that when the centre is at the true focal distance required for the formation of a perfect image, no other point of the retina can be so at the same time. On examining a dead eye, the point of distinct vision is distinguishable from the retina around, by being rather more transparent. It might have been expected that this point would be where the optic nerve enters the eye; but, in fact, the optic nerve enters considerably nearer to the nose than the point of distinc: vision; and the part is altogether blind or insensible. Had the two optic nerves therefore entered at points of the retina corresponding (in the sense explained above), there would have been an invisible spot cn every object, cppɔsite to the insensible points; but as the case really stands, the part of any object from which the light passes to the insensible or blind part of one ere, cannot be opposite to the insensible part of the other. The existence of the blind spot (or punctum cæcum, to give its Latin name), where the nerve of the eye enters, is discoverable by placing in a row on a table some small objects, as coins or wafers, about three inches apart, and one eye being closed, by looking with the other at a middle object of the row: the object next to that, on the outside, will then be invisible, although those st.ll farther off will remain in sight. Another mode of proof is to shut one eye while looking with the other at the nails of the two thumbs held together at arm's length before the face; on then moving the outside thumb sideways away from the other, while the 598 Cause of Short Sight. eye continues directed to the other, the moved one, when at the distance of about three inches, will disappear, but will come intɔ view again when still farther removed. Since the distance of perfect images behind a lens varies, there cannot be perfect sight unless where a perfect image is formed on the retina; and according to the various distances of the objects in front, that is to say, according as the pencils of light which fall upon it have more or less of divergence in them, it follows, that the eye, in being able, as it is, to see distinctly objects at different distances (the shortest is about five inches), possesses a power of altering the relation of its parts to accommodate itself to the circumstances. This is called the adjustment of the eye to distance. Among the eyes of the myriads of mankind, however, it happens that all do not originally possess such powers exactly in the requisite degree, and that many lose them from a natural decay as life advances. 828. Persons are called short-sighted, whose eyes, from too great convexity of the corona or lens, have so strong a bending or converging power that the rays of light entering them are brought to a focus Fig. 199. before reaching the retina -at a, fig. 199, for instance, instead of at b; so that the rays when spreading again beyond the focus, where they cross one ananother, produce on the retina that sort of indis tinct image which is seen in the camera obscura, of which the screen is too distant from the lens. This defect of sight obliges the individual, when using the naked eye, to hold objects very near to it, that the consequent greater divergence of the rays may be proportioned to the unusual refracting power of the eye ;-or the person may find a remedy in placing concave lenses between the object and the eyes these lenses lessen the convergence of the rays from objects at the usual distance, and cause the perfect images in the eye to be formed farther from the lens, and thereby on the retina itself. Without concave spectacles—as the lenses are called when fixed together in a frame,—persons with the defect now under consideration cannot see objects distinctly from a distance exceeding ten or twelve feet. This defect often diminishes with years, so that the person who in youth necded strong spectacles, in old age sees well without them. Cause of Long Sight. 599 There is the opposite defect of deficient convergent power in the eye, dependent on a too great flatness of the cornea or lens, and which is much more common than the last-mentioned defect. The great majority of persons after middle age begin to experience this in some degree. In such cases the rays of light are not collected quite into a focus when they reach the retina; they would meet only at b, for instance (fig. 200), instead of, as they should do, at c, and hence the image is indistinct, in the same manner as in the camera obscura, when the screen is held too near to the lens. Persons suffering from this defect cannot, when using the naked eye, see distinctly any object very near to it, because the deficient conerging power of the eye cannot conquer the great divergence of rays coming from a near point; hence, they will remove objects Fig. 200. under examination, as a book or newspaper, to a considerable distance, even to that of arm's length, so as to receive from them only rays nearly parallel. These persons, in contradistinction to the last described, are called long-sighted persons. This defect is remedied by the common convex spectacles, which do part of the converging work, so to speak, before the light enters the eye, leaving undone only that which the weakly converging eye can easily accomplish. As this defect, like the last, is met with in all degrees, spectacles must be chosen accordingly. 829. Up to a recent time it was believed that the change in the state of the eyes, which comes on about middle age, obliging most persons to use spectacles, was simply a weakening of the converging power of the eyes; but the writer found, in his own case, that the defect arose chiefly from double images of the objects being formed in each eye, one image being strong, as usual, and the other being more faint, overlapping the first, and jutting beyond it towards the right hand. For a time he deemed this a peculiarity in his own case, but accident leading him to examine further, he found that a great majority of the persons using spectacles had the same defect. As the age of these was greater, the double vision was more marked, and the displacement to the right of the fainter image was greater. Old people found that while a broad object had only a shadowy projecting edge on the right side, tall narrow objects, like a flagstaff, a long chimney, or a slender stecple, appeared two, standing |