Effect of Size, Brightness, and Colour. "The mind judges of external objects by the relative size, brightness and colour of the minute but perfect images or pictures of them formed at the back of the eye on the expansion of nerve called the retina; and the art of the painter is successful in proportion as it produces on a larger scale such a picture, which, when afterwards held before the eye to reproduce itself in miniature upon the retina, may excite nearly the same impression as the original object." 605 835. We now understand how an exact miniature resemblance of the objects before us is produced upon the rctina of the eye, by the iight from them refracted in passing through the different parts of the eye; but after all, this is only a picture, and the inquiry remains —which many persons would suppose so simple as to be trivial, but which is in reality very curious and important-how are we thereby enabled to judge of the magnitudes, distances, and other particulars respecting the things examined. Here it will be found, to the surprise of persons first entering upon the study, that we learn the meaning of a scene or of pictorial signs only gradually, as we do of any other system of signs, and that a person whose eyes, although perfect, had been kept covered from infancy up to maturity, would no more see" and understand any scene on which he first opened his eyes, and so had a perfect picture of it on his retina, than a child understands or can read a printed page, when he first looks into a book. Highly interesting information has been obtained on this subject, by observing the facts where an obstruction from birth has, by a surgical operation, been suddenly removed in persons arrived at maturity. 66 If a man were placed from infancy in an apartment fitted up as a camera obscura, and had no means of becoming acquainted with the external world, but by watching the images appearing from time to time upon the screen, he could learn scarcely anything of objects around him; but if after a time he were allowed to walk out, and to examine by the touch and by measurement, the different objects whose images he had been in the habit of viewing, and to ascertain what size, shape, and distance of an object corresponded with a certain magnitude, form, position, and brightness of image, the transient imagery might at last be to him a tolerably clear indication of the real particulars; making him in imagination present to the objects, nearly as if he went out and examined them with his hands боб Principles of Perspective. Thus, in a degree, the mind may be considered as stationed in or near the little camera obscura of the eye, from whence it cannot itself escape to examine external nature, but must learn the meaning of the images formed on the retina, through the services of the bodily members, and the other organs of sense, examining the realities. The judging of things by sight, then, is merely the interpreting one set of signs, as judging by sounds or language is interpreting another, and judging by hieroglyphics or any written characters is interpreting a third. The common visual signs on the retina, however, are among signs the most easily learned or understood, from having certain fixed relations in form, magnitude, and position to the things signified: while words, hieroglyphics, and written characters are quite arbitrary, and have no such relations. Pictorial Representation and Perspective. 836. BODIES, as visible objects, differ and are distinguished among themselves chiefly by their comparative dimensions, that is, their form and magnitude, or shape and size; and to ascertain these and the relative distances and positions, are the great objects which, by means of the eyes, the mind seeks to accomplish. It effects its ends by considering collectively, Ist. The space and place occupied by objects in the field of view, measured by what is called the visual angle. 2nd. The intensity of light, shade, and colour. 3rd. The divergence of the rays of light entering the eye. 4th. The convergence of the axes of the eyes viewing an object. We shall treat of these particulars separately in the order now stated. Ist. The space and place occupied in the field of view, measured by the visual angle. 837. The term field of view is used to designate that open or visible space before the eyes, in which objects are seen; and it may mean either the smaller field visible in one position of the person's head, or that which is commanded on directing them all round. This is called the sphere of vision. If a man, as at e (fig. 201), were surrounded by a globe or sphere of glass, as a, through which his eye, placed at the centre, might view the several objects around occupying certain situations and certain proportions of the circumference; and if the globe had any equal divisions or degrees marked upon it all around like the lines marking, on a library globe, the degrees of longitude Size determined by Visual Angle. 607 and latitude, he would be able at once to say exactly what portion of his sphere or field of view was shadowed or occupied by any single object, as the cross here shown at i, and thus to describe very intelligibly, its relative magnitude and situation as then appearing to him. For example, he might say, on looking at a tree in the garden through a common window (which is a portion of the field of view really divided by the cross-bars), whether he saw the whole tree through one pane or through several, and through which pare or panes he saw it. It may be remarked farther, that whether the supposed sphere of glass were large or small, viz., were as indicated at a, or b, or c, the part of its surface apparently occupied by any object either beyond it or within it, would bear the same proportion a Fig. 201. to the whole surface; if a d were a tenth of the small circle or globe, cg would be a tenth of a larger. Now as it has beer. found convenient to consider a circle (and every circle) as divisible into 360 equal parts, to be called degrees (which are smaller therefore in a small circle than in a large circle, although in each having the same relation to the whole), the ready mode of comparing the apparent magnitude of objects is to say how many of these degrees of the field of view, in length or breadth, each object occupies; and this is what is meant by the apparent size of an object. Then, because the 608 Judgment of Size and Distance. most convenient way of measuring a portion of a circle, of which the whole is not seen, is to measure by a fit instrument the angle or corner formed at its centre by lines drawn from the extremities of the portion to the centre, as in fig. 201, the angle at e, formed by the lines ce and g e, the object is said either to occupy a certain number of degrees of the circumference of the circle, or to subtend an angle of the same number of degrees at its centre, and this angle is called the visual angle. 838. It is important to advert here to the difference between the length of line which measures the height or breadth of an object, and the amount of surface or space occupied by it in the field of view, the latter being always as the square of the former (see Art. 26). A single pane of glass, one foot high and one foot broad, forms a small window, but a window two feet high and broad has four such panes, and a window of ten feet borders has one hundred such. The full moon in the sky has breadth, or visual angle of nearly half a degree of the celestial vault; a moon twice as broad would have four times the surface. In the diagram of the field of view (fig. 201) the figures of the cross span about thirty degrees of the circles, and the cross itself is made curved to coincide with the circle; but for small angles the portions of the circle included between the bounding lines, being so short, are regarded as straight lines without leading to error. The adjoining figure illustrates several of the matters here referred to. The visual angle, in regard to any object, being that included between the two lines or rays, as a u and d i (fig. 202), which pass from the extreme points of the object, a d, for instance, to form the extremes of the corresponding image on the retina at i u; it is evident, as shown also in Art. 820, that the same angle is formed by the rays on each side of the lens, and that the image on the retina is less than the external object in exact proportion as its distance from the centre of the lens is less than that of the object. It follows also, therefore, that the small cross, & d, produces the same-sized image Visual Estimate of Size. 609 on the retina as the cross, c e, which is twice as large, but twice as distant; and that an image only half as large as that from the cross, a d, when near, is produced by a similar cross, ce, when twice as far removed. The visual angle then becomes an exact indication of the size of the object when the distance is known, or of the distance when the size is known. Many familiar facts receive their explanation from the law of the visual angle or apparent size being less always in pro- 839. A man (or the cross here substituted for simplicity) at d (fig. 203), standing near the outside of a window, b c (here supposed to be seen edgeways), may, to the eye of a spectator within the window at h, subtend the same visual angle, or appear as tall as the window, the light from the man's head passing through the top of the window, and that from his feet passing through the bottom; but if the man then move away from the window, the eye of the spectator will be able to see his whole body through a smaller and a smaller extent of the window, as his distance increases; through half bad Fig. 203. h its height, or a c (fig. 203), when he is twice as distant, or at ƒ, and through the third, or o c, when he shall be three times as distant, or at g, and so forth, for any other distance; so that soon a small figure of a man cut in paper, if applied upon the glass, would exactly cover the part of it through which the light from his body entered to the spectator's eye, and would then, by completely hiding him from view, be an exact measure of his apparent size; at last a fly passing over the pane might equally hide him, and the fly then would subtend a larger visual angle than he does, that is, would be forming on the retina a larger image than the man. Thus it may happen, that a person sitting near a window, and intent upon some subject of study or of conversation, may for an instant mistake a fly on the glass for a man at a distance; or, on the contrary, a man for a fly. In accordance with the principle now explained, a telescope has been constructed in which the field of view is divided by fine cross wires, or otherwise, so that the person using it can say at once how |