VISION

, is the act of seeing, or of perceiving external objects by the organ of sight.

When an object is so disposed, that the rays of light, coming from all parts of it, enter the pupil of the eye, and present its image on the retina, that object is then seen. This is proved by experiment; for if the eye of any animal be taken out, and the skin and fat be carefully stripped off from the back part of it, till only the thin membrane, which is called the retina, remains to terminate it behind, and any object be placed before the front of the eye, the picture of that object will be seen figured as with a pencil on that membrane. There are thousands of experiments which prove that this is the mechanical effect of Vision, or seeing, but none of them all appear so conveniently as this, which is made with the very eye itself of an animal; an eye of an ox newly killed shews this happily, and with very little trouble. It will indeed appear singular in this, that the object is inverted, in the picture thus drawn of it, in the eye; and the case is the same in the eye of a living person.

Various other opinions however have been held concerning the means of Vision among philosophers.

The Platonists and Stoics held Vision to be effected by the emission of rays out of the eyes; conceiving that there was a sort of light thus darted out; which, with the light of the external air, taking hold as it were of the objects, rendered them visible; and thus returning back again to the eye, altered and new modified by the contact of the object, made an impression on the pupil, which gave the sensation of the object.

Our own countryman, Roger Bacon, distinguished as he was in many respects, also assents to the opinion that visual rays proceed from the eye; giving this reason for it, that every thing in nature is qualified to discharge its proper functions by its own powers, in the same manner as the sun, and other celestial bodies. Opus Majus, pa. 289.

The Epicureans held, that Vision is performed by the emanation of corporeal species or images from objects; or a sort of atomical effluvia continually flying off from the intimate parts of objects, to the eye.

The Peripatetics hold, with Epicurus, that Vision is performed by the reception of species; but they differ from him in the circumstances; for they will have the species (which they call intentionales) to be incorporeal. It is true, Aristotle's doctrine of Vision, delivered in his chapter De Aspectu, amounts to no more than this, that objects must have some intermediate body, that by this they may move the organ of sight. To which he adds, in another place, that when we perceive bodies, it is their species, not their matter, that we receive; as a seal makes an impression on wax, without the wax receiving any thing of the seal.

But this vague and obscure account the Peripatetics have thought sit to improve. Accordingly, what their master calls species, the disciples, understanding of real proper species, assert, that every visible object expresses a perfect image of itself in the air contiguous to it; and this image another, somewhat less, in the next air; and the third another; and so on till the last image ar- rives at the crystalline, which they hold for the chief organ of sight, or that which immediately moves the soul. These images they call intentional species.

The modern philosophers, as the Cartesians and Newtonians, give a better account of Vision. They all agree, that it is performed by rays of light reflected from the several points of objects received in at the pupil, refracted and collected in their passage, through the coats and humours, to the retina; and this striking, or making an impression, on so many points of it; which impression is conveyed, by the correspondent capillaments of the optic nerve, to the brain, &c.

Baptista Porta's experiments with the camera obscura, about the middle of the 16th century, convinced him that vision is performed by the intermission of something into the eye, and not by visual rays proceeding from the eye, as had been the general opinion before his time; and he was the first who fully satisfied himself and others upon this subject; though several philosophers still adhered to the old opinion.

As for the Peripatetic series or chain of images, it is a mere chimera; and Aristotle's meaning is better understood without than with them. In fact, setting these aside, the Aristotelian, Cartesian, and Newtonian doctrines of Vision, are very consistent with one another; for Newton imagines that Vision is performed chiefly by the vibrations of a fine medium (which penetrates all bodies) excited in the bottom of the eye by the rays of light, and propagated through the capillaments of the optic nerves, to the sensorium. And Des Cartes maintains, that the sun pressing the materia subtilis, with which the whole universe is every where filled, the vibrations and pulses of that matter reflected from objects, are communicated to the eye, and thence to the sensory: so that the action or vibration of a medium is equally supposed in all.

It is generally concluded then, that the images of objects are represented on the retina; which is only an expansion of the fine capillaments of the optic nerve, and from whence the optic nerve is continued into the brain. Now any motion or vibration, impressed on one extremity of the nerve, will be propagated to the other: hence the impulse of the several rays, sent from the several points of the object, will be propagated as they are on the retina (that is, in their proper colours, &c, or in particular vibrations, or modes of pressure, corresponding to them) to the place where those capillaments are interwoven into the substance of the brain. And thus is Vision brought to the common case of sensation.

Experience teaches us that the eye is capable of viewing objects at a certain distance, without any mental exertion. Beyond this distance, no mental exertion can be of any avail: but, within it, the eye possesses a power of adapting itself to the various occasions that occur, the exercise of which depends on the volition of the mind. How this is effected, is a problem that has very much engaged the attention of optical writers: but it is doubted whether it has yet been satisfactorily explained. The first theory for the solution of this problem is that of Kepler. He supposes that the ciliary processes contract the diameter of the eye, and lengthen its axis by a muscular power. But Mr. Thomas Young (in some ingenious Observations on Vision in the Philos. Trans. 1793) ob- | serves, that these processes neither appear to contain any muscular fibres, nor have any attachment by which they can be capable of performing this action.

Des Cartes ascribed this contraction and elongation to a muscularity of the crystalline, of which he supposed the ciliary processes to be the tendons: but he neither demonstrated this muscularity, nor sufficiently considered the connection with the ciliary processes.

De la Hire allows of no change in the eye, except the contraction and dilatation of the pupil: this opinion he founds on an experiment which Dr. Smith has shewn to be sallacious. Haller adopted his hypothesis, notwithstanding its inconsistency with the principles of optics and constant experience.

Dr. Pemberton supposes that the crystalline contains muscular fibres, by which one of its surfaces is flattened, while the other is made convex: but he has not demonstrated the existence of these fibres; and Dr. Jurin has proved that such a change as this is inadequate to the effect.

Dr. Porterfield conceives that the ciliary processes draw the crystalline forward, and make the cornea more convex. But the ciliary processes are incapable of this action; and it appears from Dr. Jurin's calculations, that a sufficient motion of this kind requires a very visible increase in the length of the axis of the eye; an increase which has never yet been observed.

Dr. Jurin maintains that the uvea, at its attachment to the cornea, is muscular; and that the contraction of this ring makes the cornea more convex. But this hypothesis is not sufficiently confirmed by observation.

Musschenbroek conjectures that the relaxation of this ciliary zone, which is nothing but the capsule of the vitreous humour where it receives the impression of the ciliary processes, permits the coats of the eye to push forward the crystalline and cornea. Such a voluntary relaxation however, Mr. Young observes, is wholly without example in the animal economy: besides, if it actually occurred, the coats of the eye could not act as he conceives; nor could they act in this manner without being observed. He adds, that the contraction of the ciliary zone is equally inadequate and unnecessary.

Mr. Young, having examined these theories, and some others of less moment, proceeds to investigate a more probable solution of this optical difficulty.—Adverting to the observation of Dr. Porterfield, that those who have been couched have not the power of accommodating the eye to different distances; and to the reflections of other writers on this subject; he was led to conclude that the rays of light, emitted by objects at a small distance, could only be brought to foci on the retina by a nearer approach of the crystalline to a spherical form; and he imagined that no other power was capable of producing this change, beside a muscularity of part, or of the whole of its capsule:—but, on closely examining first with the naked eye and then with a magnifier, the crystalline of an ox's eye turned out of its capsule, he discovered a structure which seemed to remove the difficulties that have long embarrassed this branch of optics.

“The crystalline of the ox, says he, is composed of various similar coats, each of which consists of six muscles, intermixed with a gelatinous substance, and attached to six membranous tendons. Three of the tendons are anterior, three posterior; their length is about two-thirds of the semidiameter of the coat; their arrangement is that of three equal and equidistant rays, meeting in the axis of the crystalline; one of the anterior is directed towards the outer angle of the eye, and one of the posterior towards the inner angle, so that the posterior are placed opposite to the middle of the interstices of the anterior: and planes passing through each of the six, and through the axis, would mark on either surface six regular equidistant rays. The muscular fibres arise from both sides of each tendon; they diverge till they reach the greatest circumference of the coat; and, having passed it, they again converge, till they are attached respectively to the sides of the nearest tendons of the opposite surface. The anterior or posterior portion of the six, viewed together, exhibits the appearance of three penniform-radiated muscles. The anterior tendons of all the coats are situated in the same planes, and the posterior ones in the continuations of these planes beyond the axis. Such an arrangement of fibres can be accounted for on no other supposition than that of muscularity. This mass is inclosed in a strong membranous capsule, to which it is loosely connected by minute vessels and nerves; and the connection is more observable near its greatest circumference. Between the mass and its capsule is found a considerable quantity of an aqueous fluid, the liquid of the crystalline.

“When the will is exerted to view an object at a small distance, the influence of the mind is conveyed through the lenticular ganglion, formed from branches of the third and fifth pair of nerves by the filaments perforating the sclerotica, to the orbiculus ciliaris, which may be considered as an annular plexus of nerves and vessels; and thence by the ciliary processes to the muscle of the crystalline, which, by the contraction of its fibres, becomes more convex, and collects the diverging rays to a focus on the retina. The disposition of fibres in each coat is admirably adapted to produce this change; for, since the least surface that can contain a given bulk is that of a sphere (Simpson's Fluxions, pa. 486) the contraction of any surface must bring its contents nearer to a spherical form. The liquid of the crystalline seems to serve as a synovia in facilitating the motion, and to admit a sufficient change of the muscular part, with a smaller motion of the capsule.

Mr. Young proceeds to enquire whether these fibres can produce an alteration in the form of the lens sufficiently great to account for the known effects; and he finds, by calculation, that, supposing the crystalline to assume a spherical form, its diameter will be 642 thousandths of an inch, and its focal distance in the eye .926. Then, disregarding the thickness of the cornea, we find (by Smith, art. 370) that such an eye will collect those rays on the retina, which diverge from a point at the distance of 12 inches and 8 tenths. This is a greater change than is necessary for an ox's eye; for if it be supposed capable of distinct Vision at a distance somewhat less than 12 inches, yet it is probably far short of being able to collect parallel rays. The human crystalline is susceptible of a much greater change of form. The ciliary zone may admit of as much extension as this diminution of the diameter of the crystal- | line will require; and its elasticity will assist the cellular texture of the vitreous humour, and perhaps the gelatinous part of the crystalline, in restoring the indolent form.—Mr. Young apprehends that the sole office of the optic nerve is to convey sensation to the brain; and that the retina does not contribute to supply the lens with nerves.—As the human crystalline resembles that of the ox, it may reasonably be presumed that the action of both organs depends on the same general principles.

This theory of Mr. Young's however is strongly opposed by Dr. Hosack, (Philos. Trans. 1794, part 2, pa. 196). He contests the existence of the muscles, which Mr. Young has described, for several reasons. First, from the transparency they must possess; otherwise there would be some irregularity in the refraction of those rays which pass through the several parts, differing both in shape and density. Another circumstance is the number of these muscles. Mr. Young describes 6 in each lamina; and as Leuwenhoek makes 2000 laminæ in all, therefore the number of muscles must amount to 12 thousand, the action of which, Dr. Hosack apprehends, must exceed comprehension. But the existence of these muscles is still more doubtful, if the accuracy of Dr. Hosack's observations be admitted. With the assistance of the best glasses, and with the greatest attention, he could not discover the structure of the crystalline described by Mr. Young, but found it to be perfectly transparent. He first observed the lens in its viscid state, and then exposed different lenses to a moderate degree of heat, so that they became opaque and dry; and it was easy to separate the distinct layers described by Mr. Young. These were so numerous as not to admit of having, each of them, 6 muscles. Another consideration, which seems to prove that these layers possess no distinct muscles, is that, in this opaque state, they are not visible, but consist of an almost infinite number of concentric fibres, not divided into particular bundles, but similar to as many of the finest hairs of equal thickness, arranged in similar order. This regular structure of layers, composed of concentric sibres, Dr. Hosack thinks is much better adapted to the transmission of the rays of light than the irregular structure of muscles. Besides, it ought to be considered that the crystalline lens is not the most essential organ in viewing objects at different distances; and if this be the case, the power of the eye cannot be owing to any changes in this lens. It is a fact, says Dr. Hosack, that we can, in a great degree, do without it; as is the case after couching or extraction, by which operation all its parts must be destroyed. Dr. Porterfield, however, and Mr. Young, on his authority, maintain that patients, after the operation of couching, have not the power of accommodating the eye to different distances of objects. On the whole, Dr. Hosack concludes that no such muscles, as Mr. Young has described, exist, and that he must have been deceived by some other appearances that resembled muscles: neither will he allow the effects ascribed to the ciliary processes in changing the shape or situation of the lens.

Dr. Hosack then proceeds to illustrate the structure and use of the external muscles of the eye; which are 6 in number, 4 called recti or straight, and 2 oblique, and by means of which he thinks the business is effect- ed. The common purposes to which these muscles are subservient are well known: but beside these, Dr. Hosack suggests that it is not inconsistent with the general laws of nature, nor even with the animal œconomy, to imagine that, from their combination, they should have a different action and an additional use. In describing the precise action of these muscles, he supposes an object to be seen distinctly first at the distance of 6 feet; in which case the picture of it falls exactly on the retina. He then directs his attention to another object at the distance of 6 inches, as nearly as possible in the same line. While he is viewing this, he loses sight of the first object, though the rays proceeding from it still fall on the eye; and hence he infers that the eye must have undergone some change; so that the rays meet either before or behind the retina. But, as rays from a more distant object concur sooner than those from a nearer one, the picture of the more remote object must fall before the retina, while the others form a distinct image upon it. But yet the eye continued in the same place; and therefore the retina must, by some means, have been removed to a greater distance from the forepart of the eye, so as to receive the picture of the nearer object. This object, he contends, could not be seen distinctly, unless the retina were removed to a greater distance, or the refracting power of the media through which the rays passed were augmented:—but as the lens is the chief refracting medium, if we admit that this has no power of changing itself, we are under the necessity of adopting the first of these two suppositions.

The next object of inquiry is, how the external muscles are capable of producing these changes. The recti are strong, broad, and flat, and arise from the back part of the orbit of the eye; and, passing over the ball as over a pulley, they are inserted by broad flat tendons at the anterior part of the eye. The oblique are inserted towards the posterior part by similar tendons. When these different muscles act jointly, the eye being in the horizontal position, and every muscle in action contracting itself, the four recti by their combination must compress the various parts of the eye and lengthen its axis, while the oblique muscles serve to keep the eye in its proper direction and situation. The convexity of the cornea, by means of its great elasticity, is also increased in proportion to the degree of pressure, and thus the rays of light passing through it are necessarily more converged. The elongation of the eye serves also to lengthen the media, in the aqueous, crystalline, and vitreous humours through which the rays pass, so that their powers of refraction are proportionably increased. This is the general effect of the contraction of the external muscles, according to Dr. Hosack's statement of it: to which it may be added, that we possess the same power of relaxing them in proportion to the greater distance of the object, till we arrive at the utmost extent of indolent Vision. Dr. Hosack also illustrates this hypothesis by some experiments.

The misrepresentations of Vision often depend upon the distance of the object. Thus, if an opake globe be placed at a moderate distance from the eye, the picture of it upon the retina will be a circle properly diversified with light and shade, so that it will excite in the mind the sensation of a sphere or globe; but, if | the globe be placed at a great distance from the eye, the distance between those lights and shades, which form the picture of a globe, will be imperceptible, and the globe will appear no otherwise than as a circular plane. In a luminous globe, distance is not necessary in order to take off the representation of prominent and flat; an iron bullet, heated very red hot, and held but a few yards distance from the eye, appears a plane, not a prominent body; it has not the look of a globe, but of a circular plane. It is owing to this misrepresentation of Vision that we see the sun and moon flat by the naked eye, and the planets also, through telescopes<*> flat. It is in this light that astronomers, when they speak of the sun, moon, and planets, as they appear to our view, call them the discs of the sun, moon, and planets, which we see.

The nearer a globe is to the eye, the smaller segment of it is visible, the farther off the greater, and at a due distance the half; and, on the same principle, the nearer the globe is to the eye, the greater is its apparent diameter, that is, under the greater angle it will appear, the farther off the globe is placed, the less is its apparent diameter. This is a proposition of importance, for, on this principle, we know that the same globe, when it appears larger, is nearer to our eye, and, when smaller, is farther off from it. Therefore, as we know that the globes of the sun and moon continue always of the same size, yet appear sometimes larger, and sometimes smaller, to us, it is evident, that they are sometimes nearer, and sometimes farther off from the place whence we view them. Two globes, of different magnitude, may be made to appear of exactly the same diameter, if they be placed at different distances, and those distances be exactly proportioned to their diameters. To this it is owing, that we see the sun and moon nearly of the same diameter; they are, indeed, vastly different in real bulk, but, as the moon is placed greatly nearer to our eyes, the apparent magnitude of that little globe is nearly the same with that of the greater.

In this instance of the sun and moon (for there cannot be a more striking one) we see the misrepresentation of Vision in two or three several ways. The apparent diameters of these globes are so nearly equal, that, in their several changes of place, they do, at times, appear to us absolutely equal, or mutually greater than one another. This is often to be seen, but it is at no time so obvious, and so perfectly evinced, as in eclipses of the sun, which are total. In these we see the apparent magnitudes of the two globes vary so much according to their distances, that sometimes the moon is large enough exactly to cover the disc of the sun, sometimes it is larger, and a part of it every where extends beyond the disc of the sun; and, on the contrary, sometimes it is smaller, and, though the eclipse be absolutely central, yet it is annular, or a part of the sun's disc is seen in the middle of the eclipsed part, enlightened, and surrounding the opake body of the moon in form of a lucid ring.

When au object, which is seen above, without other objects of comparison, is of a known magnitude, we judge of its distance by its apparent magnitude; and custom teaches us to do this with tolerable accuracy. This is a practical use of the misrepresentation of Vision, and in the same manner, knowing that we see things, which are near us, distinctly, and those which are distant, confusedly, we judge of the distance of an object by the clearness, or confusion, in which we see it. We also judge yet more easily and truly of the distance of an object by comparing it to another seen at the same time, the distance of which is better known, and yet more by comparing it with several others, the distances of which are more or less known, or more or less easily judged of. These are the circumstances which assist us, even by the misrepresentation of Vision, to judge of distance; but, without one or more of these, the eye does not, in reality, enable us to judge concerning the distance of objects.

This misrepresentation, although it serves us on some occasions, yet is very limited in its effects. Thus, though it helps us greatly in distinguishing the distance of objects that are about us, both with respect to ourselves and them, and with respect to themselves with one another, yet it can do nothing with the very remote. We see that immense concave circle, in which we suppose the fixed stars to be placed, at all this vast remove from us, and no change of place that we could make to get nearer to it, would be of any avail for determining the distance of the stars from one another. If we look at three or four churches from a distance of as many miles, we see them stand in a certain position with regard to one another. If we advance a great deal nearer to them, we see that position differ, but, if we move forward only eight or ten feet, the difference is not seen.

, in Optics. The laws of Vision, brought under mathematical demonstrations, make the subject of Optics, taken in the greatest latitude of that word: for, among mathematical writers, optics is generally taken, in a more restricted signification, for the doctrine of direct Vision; catoptrics, for the doctrine of reflected Vision; and dioptrics, for that of refracted Vision.

Direct or Simple Vision, is that which is performed by means of direct rays; that is, of rays passing directly, or in right lines, from the radiant point to the eye. Such is that explained in the preceding article Vision.

Reflected Vision, is that which is performed by rays reflected from speculums, or mirrors. The laws of which, see under Reflection, and Mirror.

Refracted Vision, is that which is performed by means of rays refracted, or turned out of their way, by passing through mediums of different density; chiefly through glasses and lenses. The laws of this, see under the article Refraction.

Arch of Vision. See Arch.

Distinct Vision, is that by which an object is seen distinctly. An object is said to be seen distinctly, when its outlines appear clear and well defined, and the several parts of it, if not too small, are plainly distinguishable, so that they can easily be compared one with another, in respect to their figure, size, and colour.

In order to such Distinct Vision, it had commonly been thought that all the rays of a pencil, flowing from a physical point of an object, must be exactly united in a physical, or at least in a sensible point of the retina. But Dr. Jurin has made it appear from experiments, that such an exact union of rays is not always necessary | to Distinct Vision. He shews that objects may be seen with sufficient distinctness, though the pencils of rays issuing from the points of them do not unite precisely in the same point on the retina; but that since, in this case, pencils from every point either meet before they reach the retina, or tend to meet beyond it, the light that comes from them must cover a circular spot upon it, and will therefore paint the image larger than perfect Vision would represent it. Whence it follows, that every object placed either too near or too remote for perfect Vision, will appear larger than it is by a penumbra of light, caused by the circular spaces, which are illuminated by pencils of rays proceeding from the extremities of the object.

The smallest distance of perfect Vision, or that in which the rays of a single pencil are collected into a physical point on the retina in the generality of eyes, Dr. Jurin, from a number of observations, states at 5, 6, or 7 inches. The greatest distance of distinct and perfect Vision he found was more difficult to determine; but by considering the proportion of all the parts of the eye, and the refractive power of each, with the interval that may be discerned between two stars, the distance of which is known, he fixes it, in some cases, at 14 feet 5 inches; though Dr. Porterfield had restricted it to 27 inches only, with respect to his own eye.

For other observations on this subject, see Jurin's Essay on Distinct and Indistinct Vision, at the end of Smith's Optics; and Robins's Remarks on the same, in his Math. Tracts, vol. 2, pa. 278 &c. See also an ingenious paper on Vision in the Philos. Trans. 1793, pa. 169, by Mr. Thomas Young.

Field of Vision. See Field.