VISIBLE

, something that is an object of vision or right, or the property of a thing seen.

The Cartesians say that light alone is the proper object of vision. But according to Newton, colour alone is the proper object of sight; colour being that property of light by which the light itself is Visible, and by which the images of opake bodies are painted on the retina.

Philosophers in general had formerly taken for granted, that the place to which the eye refers any Visible object, seen by reflection or refraction, is that in which the visual ray meets a perpendicular from the object upon the reflecting or the refracting plane. That this is the case with respect to plane mirrors is universally acknowledged; and some experiments with mirrors of other forms seem to favour the same conclusion, and thus afford reason for extending the analogy to all cases of vision. If a right line be held perpendicularly over a convex or concave mirror, its image seems to make one line with it. The same is the case with a right line held perpendicularly within water; for the part which is within the water seems to be a continuation of that which is without. But Dr. Barrow called in question this method of judging of the place of an object, and so opened a new field of inquiry and debate in this branch of science. This, with other optical investigations, he published in his Optical Lectures, first printed in 1674. According to him, we refer every point of an object to the place from which the pencils of light issue, or from which they would have issued, if no reflecting or refracting substance intervened. Pursuing this principle, Dr. Barrow proceeded to investigate the place in which the rays issuing from each of the points of an object, and that reach the eye after one reflection or refraction, meet; and he found that when the refracting surface was plane, and the refraction was made from a denser medium into a rarer, those rays would always meet in a place between the eye and a perpendicular to the point of incidence. If a convex mirror be used, the case will be the same; but if the mirror be plane, the rays will meet in the perpendicular, and beyond it if it be concave. He also determined, ac- cording to these principles, what form the image of a right line will take when it is presented in different manners to a spherical mirror, or when it is seen through a refracting medium.

Dr. Barrow however notices an objection against the maxim above mentioned, concerning the supposed place of visible objects, and candidly owns that he was not able to give a satisfactory solution of it. The objection is this: Let an object be placed beyond the focus of a convex lens, and if the eye be close to the lens, it will appear confused, but very near to its true place. If the eye be a little withdrawn, the confusion will increase, and the object will seem to come nearer; and when the eye is very near the focus, the confusion will be very great, and the object will seem to be close to the eye. But in this experiment the eye receives no rays but those that are converging; and the point from which they issue is so far from being nearer than the object, that it is beyond it; notwithstanding which the object is conceived to be much nearer than it is, though no very distinct idea can be formed of its precise distance.

The first person who took much notice of Dr. Barrow's hypothesis, and the difficulty attending it, was Dr. Berkeley, who (in his Essay on a New Theory of Vision, p. 30) observes, that the circle formed upon the retina, by the rays which do not come to a focus, produce the same confusion in the eye, whether they cross one another before they reach the retina, or tend to it afterwards; and therefore that the judgment concerning distance will be the same in both the cases, without any regard to the place from which the rays originally issued; so that in this case, by receding from the lens, as the confusion increases, which always accompanies the nearness of an object, the mind will judge that the object comes nearer. See Apparent Distance.

M. Bouguer (in his Traité d' Optique, p. 104) adopts Barrow's general maxim, in supposing that we refer objects to the place from which the pencils of rays seemingly converge at their entrance into the pupil. But when rays issue from below the surface of a vessel of water, or any other refracting medium, he finds that there are always two different places of this seeming convergence: one of them of the rays that issue from it in the same vertical circle, and therefore fall with different degrees of obliquity upon the surface of the refracting medium; and another of those that fall upon the surface with the same degree of obliquity, entering the eye laterally with respect to one another. He says, sometimes one of these images is attended to by the mind, and sometimes the other; and different images may be observed by different persons. And he adds, that an object plunged in water affords an example of this duplicity of images.

G. W. Krafft has ably supported Barrow's opinion, that the place of any point seen by reflection from the surface of any medium, is that in which rays issuing from it, infinitely near to one another, would meet; and considering the case of a distant object viewed in a concave mirror, by an eye very near it, when the image, according to Euclid and other writers, would be between the eye and the object, and Barrow's rule cannot be applied, he says that in this case the speculum may | be considered as a plane, the effect being the same, only that the image is more obscure. Com. Petrepol. vol. 12, p. 252, 256. See Priestley's Hist. of Light &c, p. 89, 688.

From the principle above illustrated several remarkable phenomena of vision may be accounted for: as— That if the distance between two Visible objects be an angle that is insensible, the distant bodies will appear as if contiguous: whence, a continuous body being the result of several contiguous ones, if the distances between several Visibles subtend insensible angles, they will appear one continuous body; which gives a pretty illustration of the notion of a continuum.—Hence also parallel lines, and long vistas, consisting of parallel rows of trees, seem to converge more and more the farther they are extended from the eye; and the roofs and floors of long extended alleys seen, the former to descend, and the latter to ascend, and approach each other; because the apparent magnitudes of their perpendicular intervals are perpetually diminishing, while at the same time we mistake their distance.

The mind perceives the distance of Visible objects, 1st, From the different configurations of the eye, and the manner in which the rays strike the eye, and in which the image is impressed upon it. For the eye disposes itself differently, according to the different distances it is to see; viz, for remote objects the pupil is dilated, and the crystalline brought nearer the retina, and the whole eye is made more globous; on the contrary, for near objects, the pupil is contracted, the crystalline thrust forwards, and the eye lengthened. The mode of performing this however, has greatly divided the opinions of philosophers. See Priestley's Hist. of Light &c, p. 638—652, where the several opinions of Descartes, Kepler, La Hire, are Le Roi, Porterfield, Jurin, Musschenbroek, &c, stated and examined.

Again, the distance of Visible objects is judged of by the angle the object makes; from the distinct or confused representation of the objects; and from the briskness or feebleness, or the rarity or density of the rays.

To this it is owing, 1st, That objects which appear obscure or confused, are judged to be more remote; a principle which the painters make use of to cause some of their figures to appear farther distant than others on the same plane. 2d, To this it is likewise owing, that rooms whose walls are whitened, appear the smaller; that fields covered with snow, or white flowers, shew less than when clothed with grass; that mountains covered with snow, in the night time, appear the nearer, and that opake bodies appear the more remote in the twilight.

The quantity or magnitude of Visible objects, is known chiefly by the angle contained between two rays drawn from the two extremes of the object to the centre of the eye. An object appears so large as is the angle it subtends; or bodies seen under a greater angle appear greater; and those under a less angle less, &c. Hence the same thing appears greater or less as it is nearer the eye or farther off. And this is called the apparent magnitude.

But to judge of the real magnitude of an object, we must consider the distance; for since a near and a remote object may appear under equal angles, though the magnitudes be different, the distance must necessarily be estimated, because the magnitude is great or small according as the distance is. So that the real magnitude is in the compound ratio of the distance and the apparent magnitude; at least when the subtended angle, or apparent magnitude, is very small; otherwise, the real magnitude will be in a ratio compounded of the distance and the fine of the apparent magnitude, nearly, or nearer still its tangent.

Hence, objects seen under the same angle, have their magnitudes in the same ratio as their distances. The chord of an arc of a circle appears of equal magnitude from every point in the circumference, though one point be vastly nearer than another. Or if the eye be fixed in any point in the circumference, and a right line be moved round so as its extremes be always in the periphery, it will appear of the same magnitude in every position. And the reason is, because the angle it subtends is always of the same magnitude. And hence also, the eye being placed in any angle of a regular polygon, the sides of it will all appear of equal magnitude; being all equal chords of a circle described about it.

If the magnitude of an object directly opposite to the eye be equal to its distance from the eye, the whole object will be distinctly seen, or taken in by the eye, but nothing more. And the nearer you approach an object, the less part you see of it.

The least angle under which an ordinary object becomes visible, is about one minute of a degree.

This is estimated chiefly from our opinion of the situation of the several parts of the object. This opinion of the situation, &c, enables the mind to apprehend an external object under this or that figure, more justly than any similitude of the images in the retina, with the object can; the images being often elliptical, oblong, &c, when the objects they exhibit to the mind are circles, or squares, &c.

The laws of vision, with regard to the figures of Visible objects, are,

1. That if the centre of the eye be exactly in the direction of a right line, the line will appear only as a point.

2. If the eye be placed in the direction of a surface, it will appear only as a line.

3. If a body be opposed directly towards the eye, so as only one plane of the surface can radiate on it, the body will appear as a surface.

4. A remote arch, viewed by an eye in the same plane with it, will appear as a right line.

5. A sphere, viewed at a distance, appears a circle.

6. Angular figures, at a distance, appear round.

7. If the eye look obliquely on the centre of a regular figure, or a circle, the true figure will not be seen; but the circle will appear oval, &c. |

Visible Horizon, Place, &c. See the substantives.