Vision (Lat. Visio From Videre To See), a term employed to denote, in different relations, the power, the act or process, or the object of sight. The behavior of rays of light reflected or transmitted through various media, among which are the lens and humors of the eye, and the manner in which images come to be formed at the retina, are considered under Optics. For the parts of the eye, and the action of the humors and iris, see Eye. - The mere action of rays of light upon the retina is not sufficient to secure the actual perception of the objects they proceed from. Of the other conditions, the most indispensable for distinct vision is that the divergent pencil of rays emanating from any one point of the object shall be brought to its proper focus at the level of the retina. But if the object be so near the eye, say within 6 in., that the convergent power of the lens fails to bring the pencils of light to a focus, or if the pencils from very distant objects in eyes too powerfully convergent are focalized and again dispersed before reaching the retina, no distinct perception takes place, but only a sense of light or brightness in greater or less degree. The least distance at which objects can be distinctly seen is termed the limit of distinct vision; it is for different eyes between 6 and 12 in.

At remoter distances the several pencils admitted by the small opening of the pupil, about in in diameter for an object at 6 in., may be regarded as consisting each of nearly parallel rays. For an object at 10 in., the angular divergence of each pencil i3 little over half a degree; and generally, it may be said that distinct vision is due to bundles of rays very slightly divergent or sensibly parallel. Hence, the object brought much nearer the eye is still distinctly visible through a pin hole, the edges of which cut off the too divergent rays; or through a lens sufficiently convex to complement, in converging the pencils, the effect of the eye. Thus, for distinct vision, the image must be at the retina, and distinctly formed there. Again, since there are degrees of faint illumination not allowing of sight, it must be sufficiently bright or intense. Thus, in cases of cataract or opacities of the organ, distinct vision gradually becomes impossible, except by the stronger degree of brightness of the objects. Many stars are wholly unperceived until the light emitted by each of them has been collected and concentrated from over a larger space than that afforded by the pupil, as by use of a large convex lens.

On the other hand, excessive brilliancy dazzles the eye, the impression not resulting in perception of the object, but often in pain, or in positive injury to the structure and the sensibility of the retina. Again, the image must have a sensible magnitude. When a single shoot of young corn at a distance is quite invisible, the several shoots of like size in a hill, impressing a larger space on the retina, may come clearly into view; but the rows are, on the same principle, more distinctly seen than the single hill could be. Ehrenberg finds that the smallest square magnitude visible near at hand to the naked eye, white on a black ground, or the reverse, is about 1/405 of an inch. With effort, a less spot may be seen; and if, like gold leaf, the particle reflects light powerfully, it may be seen down to a fineness of 1/1125 of an inch. On the principle of extension already referred to, lines of much greater fineness are visible - if opaque, and viewed toward the light, down to the 1/4900 diameter, or about half the thickness of the silkworm's fibre. The magnitude and extreme limit of distance and visibility of an object vary also with its brightness, and hence with its color.

A white object in sunlight is seen at a distance of 17,250 times its diameter; in the same light a red object of like size only about one half as far; a blue object at somewhat less distance than a red one. But all small or distant objects become more readily visible when the background or surrounding objects present to it the strongest contrast, as in the case of a black object on a white ground, or a white object on a black ground. Though the human eye is capable of seeing objects both at great and small distances, yet most persons, wishing minutely to examine an object, place it at 8 to 10 in. before the eyes. But one whose eyes lack convergent power must remove the object further away to obtain distinct vision, and such a one is said to be longsighted; this condition is presbyopia, and is remedied by use of convex eye glasses. On the other hand, when from any cause too rapid convergence takes place, the object must be brought close to the eye, so as to carry its image back to the retina, and one thus affected is said to be near-sighted; this condition is myopia, and is remedied by concave eye glasses. Like any other lens, the crystalline has its optic centre; this coincides nearly with its centre of form.

The direction of every ray passing through this is nearly the same after as before transmission. Consequently, every such ray is an axis of a pencil of rays from some point of the object. The pencil of rays coming from the point directly before the eye has its axis corresponding with the principal or " optic axis " of the eye, passing through very nearly the centres of the pupil and lens, and meeting the retina in a direction perpendicular to its surface, at the middle point of the small circular area which is the seat of most distinct vision. - In providing for accurate vision under different circumstances, a very important element is the alternate contraction and dilatation of the pupil. When exposed to a strong light so intense that it would dazzle the eye and thus produce indistinctness of vision, the pupil contracts, shutting out a portion of the luminous beam, and thus reducing the amount of light to that which is readily borne by the eye. On the other hand, on first entering a darkened apartment with the pupils in a contracted condition, we may be incapable of perceiving surrounding objects; but within a few seconds the pupils enlarge, admitting a greater amount of light to the eye, and everything in the apartment soon becomes distinctly perceptible.

In some of the lower animals, whose vision adapts itself to extreme conditions of light and darkness, as in the cat, the variations in size of the pupil are exceedingly marked; this opening being reduced in broad sunlight to a nearly imperceptible slit, while at night it is so expanded that the borders of the iris almost disappear. - Notwithstanding the perfection of our visual apparatus, there is still only a small space directly in front of each eye within which objects can be distinctly seen. Outside of this space there is a field or circle of considerable extent within which we can perceive, though indistinctly, the presence of luminous objects; and this is called "the field of vision.1' In man, when using both eyes immovably fixed, the field of vision within its extreme limits subtends an angle of about 180°; everything outside of this circle being invisible to us, because the rays of light coming from the sides and behind cannot enter the pupil. In many of the lower animals, particularly in birds and fishes, there is reason to believe that the field of vision is very much more extensive than this, owing to the lateral position of the two eyes, one being placed on each side of the head, and to the great prominence of the eyeball and its high refractive power, in consequence of which rays of light coming in an extremely oblique direction are bent inward and made to enter the pupil.

It is certain, for example, that the ostrich can see objects directly behind him; for a spectator placed in this position can see both the pupils, and is of course himself visible to the animal. In some fishes, if not in most of the class, there appears to be really no limit to the field of vision in any direction; so that the animal can perceive rays of light coming from every point of the sphere, excepting that actually occupied by his own body. But even within the field of human vision there is only a single spot in its centre at which objects can be seen distinctly, that is, where their form and outlines are accurately perceived. Thus, when we stand in front of a row of upright stakes or poles, we can see those placed directly in front of the eye with perfect distinctness, while those placed a little on either side, though perceptible, are indistinct. When we look steadily at the middle of a printed page we see the distinct form of each separate letter only immediately in front; above, below, and on each side of this spot, at successive distances, the letters first become confused, then run into each other, and finally the distinction of letters and words is lost, and we perceive only the lines and spaces.

The straight line extending directly in front of each eye, upon which alone objects are distinctly perceived, is called the "line of distinct vision." Everything above or below this line, to the right or the left, is perceived only in an imperfect manner. This is practically compensated for by the mobility of the eyeballs, by which we are enabled to direct the line of distinct vision to all parts of the space in front, in rapid succession. When reading, for instance, the eyes follow the printed lines from left to right, seeing each letter and word distinctly in succession. Consequently an object can be distinctly perceived by both eyes simultaneously only when placed at a certain distance, namely, at that point where the two visual axes, or the lines of distinct vision for the two eyes, cross each other. As the eyes are situated two or three inches apart in their orbits, when they are both directed toward the same object their visual axes converge and meet at the situation of the object. Thus, although we look with two eyes, we see but a single object; because, as the two lines of vision meet at a single point the two distinct images exactly cover each other and so form but one. But either within or beyond this point vision becomes both double and indistinct.

Thus, on holding up a slender rod at a distance of one or two feet in front of the face, and in the same range with any similar object, such as a door knob, on the opposite side of the room, if we direct both eyes to the rod we see it distinct and single, while the door knob appears as two indistinct images, one on each side. If we now direct the eyes to the door knob, it in turn becomes distinct and single, while the rod is doubled and confused in outline. This is because when we look at the nearer object so as to see it distinctly, the further one is still within the field of vision, and is therefore perceived; but it is perceived with imperfect outlines, since it is outside the line of distinct vision. But for the right eye it is to the right of this line, and for the left eye to its left; and the two images therefore no longer occupy the same spot, and the object accordingly appears double. On this account we never see all parts of a landscape, even directly in front, with distinctness at the same time.

The fore ground, the middle ground, and the distance are each examined separately; and when one is distinct, the others are always more or less confused. - One great advantage of the simultaneous action of the two eyes, as above described, is that we are thus enabled to judge in great measure of the distance of an object. As the angle of convergence of the two eyes varies according to the distance at which an object is placed, we instinctively appreciate the amount of this convergence and consequently the distance of the object. A still greater advantage is, that by using two eyes we appreciate the qualities of solidity and projection of foreign bodies. Vision with but one eye presents a simple expanse or variously shaded and colored picture in front of the visual organ. But since the two eyes are placed at a certain distance from each other, and their visual axes, as above mentioned, are convergent, it follows that for near objects it is not precisely the same figure which is perceived by each eye. If we look, for example, at a square box at the distance of a few feet, both eyes will see the front of the box equally well; but in addition the left eye will see a little of its left side in perspective, and the right eye will see a little of its right side.

These two images, occupying the same spot at the convergence of the visual axes, are perceived as one; and it is by this combination of two different figures that we acquire the perception of the solidity of the object examined. "We feel that it is not a flat picture, because in that case the same image would be presented to each eye; and however well it might be painted, such a picture could never deceive us, if looked at with both eyes simultaneously. It is on this principle that the stereoscope is constructed. (See Stereoscope.) - A change is also required in transferring the sight from near to remote objects, or vice versa, which is called the accommodation of the eye to distinct vision at different distances. The eye is a natural optical instrument, consisting essentially of a sensitive nervous expansion, the retina, intended to receive the impression produced by luminous rays, and of various refracting media, destined to converge these rays to a focus at the surface of the retina. The distinctness of vision depends upon, the accuracy with which the rays of light, diverging from all parts of a luminous object, are brought to a focus exactly at the level of the sensitive membrane destined to receive them.

The most important and efficient of the refracting parts of the eye is the crystalline lens, a doubly convex transparent body, of considerable density, immediately behind the pupil and some distance in front of the retina. As the original divergence of the luminous rays passing through the pupil varies with the distance of the object from which they emanate, the crystalline, lens, if both its refracting power and its position remained the same, could not bring the rays to a focus at the same point behind it, for luminous objects at different distances. For example, suppose the lens to be so arranged that the rays from an object at the distance of 6 in. may be accurately focussed at the retina. If the object be now removed to a distance of 30 in., the divergence of its rays at their entrance through the pupil will be diminished; but, the refractive power remaining unaltered, they will now be converged more rapidly than before. They will accordingly meet and cross each other; and when they reach the lens they will have already become partially dispersed, the effect of this being to produce partial indistinctness of vision.

This difficulty is met with in telescopes and spy glasses, and is obviated in them by shifting the relative distance of the object glass and the eye piece, when the instrument is turned from a near object to a remote one, or vice versa. In the eye the correction might be accomplished either by moving the lens backward and forward, so as to vary its distance in front of the retina, or by changing its refractive power, to correspond with the varying distances of visual objects. The experiments of Donders and others have shown conclusively that the latter method is that which is really employed by nature. If the eye be accommodated for vision of a distant object, and a lighted candle be then held in front of it and a little on one side, an observer will perceive three reflected images of its flame in the eyeball; namely, one from the anterior surface of the cornea, one from the anterior surface of the lens, and a third from the posterior surface of the lens. If the sight be now changed from the distant to a near object, the reflected image upon the cornea remains unaltered, but that from the anterior surface of the lens diminishes and moves slightly forward. The third reflection also diminishes a little, but is not altered in position.

These changes show that in shifting the accommodation of the eye from distant to near objects, the lens increases the curvature of its surfaees and of course its refractive power, the bulging taking place principally in front. When the sight is shifted from near to remote objects, of course these changes are reversed. Thus, when we look at a near object, the refractive power of the lens is increased, and the diverging rays are more powerfully refracted; when we look at an object comparatively remote, the refractive action is diminished, and the slightly divergent rays are still brought to a focus exactly at the surface of the retina. This alteration in the form of the lens is accomplished by means of the internal muscular apparatus of the eye. It is much more marked when the changes, are made between objects at short than at long distances, since the difference in the angular divergence of the rays is greater in the former case than in the latter. Accordingly, much greater accommodating power is required for shifting the sight from a distance of 6 in. to 12 in., than from 12 in. to 24 in.; and for all distances beyond 50 ft. the change required is comparatively trifling. - A remarkable fact in regard to vision is that of the temporary persistence of visual impressions.

If a bright object be presented to the eye, and then suddenly obscured, the impression of light remains upon the retina for a short interval of time after the luminosity has actually disappeared; and if the luminous body be again restored to its position before this interval has elapsed, we fail to see that it has disappeared at all. Thus the act of winking, by which the light is momentarily excluded from the eye, does not interfere with vision, because it is performed and terminated so rapidly that the images of external ojects appear to remain upon the retina until they are again actually perceptible. A lighted stick revolving with rapidity presents the appearance of a luminous circle; and the successive sparks thrown off from a knife grinder's wheel produce the impression of a continuous stream of fire. - Closely connected with this part of the subject is the question of the amount of time required by the eye for the perception of light and the distinct vision of luminous objects. It is well known that if a darkened apartment, containing objects in rapid movement, as for instance revolving wheels, be suddenly illuminated by an electric spark of sufficient intensity, the wheels will be perceived, but will be seen as if at rest.

The duration of the spark is so short that the spokes of the wheel do not move far enough in the interval of illumination to confuse each other's outlines upon the retina; and yet the illuminated bodies are perceived with perfect distinctness. Prof. O. N. Rood ("American Journal of Science and Arts," September, 1871) has shown that visual perception may take place within an interval of time exceedingly minute. In his experiments the illuminating agent was the electric, spark from a Leyden jar, passed between platinum points separated by a distance of 1/25 of an inch. The illuminated object was a glass plate ruled with parallel black and white lines, each 1/600 of an inch in thickness. The image of this plate was thrown, through an achromatic lens, upon a mirror revolving upon its axis at the rate of 340 times a second, and thence reflected upon a surface of plain glass, where it was brought to a focus and viewed by a telescopic eye piece. The images of the black and white lines, moving with the revolution of the mirror, would thus become mingled and confused if the illumination lasted long enough to allow them to pass over a space equal to their own diameters; and the duration of the electric spark was thus determined to be in some instances hardly more than second.

Yet with this illumination the letters on a printed page were plainly to be seen, and with a polariscope the cross and rings around the axes of crystals could be observed with all their peculiarities. An illumination continuing forty billionths of a second is therefore sufficient for distinct vision.