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solid figure.-A proof of the truth of this statement is, that a painter, by carefully imitating the distribution of light and shade which he sees in objects, will make his work very naturally and exactly represent, not only the general outline of a body, but its prominences, depressions, and other irregularities. And yet his delineation, which, by the distribution of light and shade, gives such various representations, is on a smooth and plain surface.

§ 38. Illustration of the subject from the blind.

It was a problem submitted by Mr. Molyneux to Mr. Locke, whether a blind man, who has learned the difference between a cube and a sphere by the touch, can, on being suddenly restored to sight, distinguish between them, and tell which is the sphere and which is the cube, by the aid of what may be called his new sense merely? And the answer of Mr. Locke was, in agreement with the opinion of Molyneux himself, that he cannot. The blind man knows what impressions the cube and sphere make on the organ of touch, and by that sense is able to distinguish between them; but, as he is ignorant what impression they will make on the organ of sight, he is not able, by the latter sense alone, to tell which is the round body and which is the cubic.

It was remarked that solid bodies present to the eye nothing but a certain disposition of light and colours.-It seems to follow from this, that the first idea which will be conveyed to the mind on seeing a globe, will be that of a circle variously shadowed with different degrees of light. This imperfect idea is corrected in this way. Combining the suggestions of the sense of touch with those of sight, we learn by greater experience what kind of appearance solid, convex bodies will make to us. That appearance becomes to the mind the sign of the presence of a globe; so that we have an idea of a round body by a very rapid mental correction, whereas the notion first conveyed to the mind is truly that of a plane, circular surface, on which there is a variety in the dispositions of light and shade. It is an evidence of the correctness of this statement, that in paintings, plane surfaces, variously shaded, represent convex bodies, and with great truth and

exactness.

It appears, then, that extension and figure are originally perceived, not by sight, but by touch. We do not judge of them by sight until we have learned by our experience that certain visible appearances always accompany and signify the existence of extension and of figure. This knowledge we acquire at a very early period in life; so much so, that we lose, in a great measure, the memory both of its commencement and progress.

39. Measurements of magnitude by the eye.

What has been said naturally leads us to the consideration of MAGNITUDE. This is a general term for Extension, when we conceive of it not only as limited or bounded, but as related to, and compared with, other objects. Although we make use of the eye in judging of it, it is to be kept in mind that the knowledge of magnitude is not an original intimation of the sight, but is at first acquired by the aid of touch. So well known is this, that it has been common to consider Magnitude under the two heads of tangible or real, and visible or apparent; the tangible magnitude being always the same, but the visible varying with the distance of the object. A man of six feet stature is always that height, whether he be a mile distant, or half a mile, or near at hand; the change of place making no change in his real or tangible magnitude. But the visible or apparent magnitude of this man may be six feet or two feet, as we view him present with us and immediately in our neighbourhood, or at two miles' distance; for his magnitude appears to our eye greater or less, according as he is more or less removed.

In support of the doctrine that the knowledge of magnitude is not an original intimation of the sight, but is at first acquired by the aid of touch, we may remark, that, in judging of magnitude by the sight, we are much influenced, not merely by the visual perception, but particularly by comparison with other objects, the size of which is known or supposed to be known. "I remember once," says Dr. Abercrombie (Intellectual Powers, pt. ii., sect. i.), "having occasion to pass along Ludgate Hill when the great door of St. Paul's was open, and several persons were standing in it. They appeared to be very little

children; but, on coming up to them, were found to be full-grown persons. In the mental process which here took place, the door had been assumed as a known magnitude, and the other objects judged of by it. Had I attended to the door being much larger than any door that one is in the habit of seeing, the mind would have made allowance for the apparent size of the persons; and, on the other hand, had these been known to be full-grown persons, a judgment would have been formed of the size of the door."

40. Of objects seen in a mist.

In accordance with the above-mentioned principle, it happens that objects seen by a person in a mist ̄`seem larger than life. Their faint appearance rapidly conveys to the mind the idea of being considerably removed, although they are actually near to us. And the mind immediately draws the conclusion, (so rapidly as to seem a simple and original perception,) that the object having the same visible or apparent magnitude, and yet supposed to be at a considerable distance, is greater than other objects of the same class. So that it is chiefly the view of the mind, a law or habit of the intellect, which, in this particular case, gives a fictitious expansion to bodies; although it is possible that the result may in part be attributed to a difference in the refraction of the rays of light, caused by their passing through a denser and less uniform medium than usual.

41. Of the sun and moon when seen in the horizon.

These remarks naturally remind us of the well-known fact, that the sun and moon seem larger in the horizon than in the meridian. Three reasons may be given for this appearance; and perhaps ordinarily they are combined together. (1.) The horizon may seem more distant than the zenith, in consequence of intervening objects. We measure the distance of objects in part by means of those that are scattered along between, and any expanse of surface, where there are no such intervening objects, appears to us of less extent than it actually is. Now if the of light form precisely the same image in the eye,

rays

but the source of them is supposed to be further off in the horizon than in the zenith, such have been our mental habits, that the object in the horizon will probably appear the largest. (2.) Another reason of the enlarged appearance of the sun and moon in the horizon is, that the rays from them fall on the body of the atmosphere obliquely, and, of course, are reflected downward towards the beholder, and subtend a larger angle at his eye. Hence, as we always see objects in the direction of the ray just before it enters the eye, if we follow the rays back in the precise direction of their approach, they will present to the eye the outlines of a larger object as their source than they would if they had not been refracted.— Also, when the atmosphere is not clear, but masses of vapour exist in it, the refraction is increased and the object proportionally enlarged.-(3.) The sun and moon appear enlarged when other objects of considerable dimensions, but so distant as to subtend a very small angle at the eye, are seen in the same direction or in the moment of passing their disk, such as distant trees in the horizon, or ships far off at sea. These objects, though small in the eye or in their visual appearance, are yet, in consequence of our previous knowledge, enlarged in our conceptions of them. And this conceptive enlargement communicates itself, by a sort of mental illusion, to other objects with which they seem to come in contact.

§ 42. Of the estimation of distances by sight.

We are next led to the consideration of distances as made known and ascertained by the sight. By the distance of objects, when we use the term in reference to ourselves, we mean the space which is interposed between those objects and our own position. It might be objected, that space interposed is only a synonymous expression for the thing to be defined. Nevertheless, no one can be supposed to be ignorant of what is meant. Even blind men have a notion of distance, and can measure it by the touch, or by walking forward until they meet the distant object.

The perception of distance by the sight is an acquired and not an original perception; although the latter was universally supposed to be the fact until comparatively a recent period,

All objects in the first instance appear to touch the eye; but our experience has corrected so many of the representations of the senses, before the period which we are yet able to retrace by the memory, that we cannot prove this by a reference to our own childhood and infancy. It appears, however, from the statement of the cases of persons born blind on the sudden restoration of their sight." When he first saw," says Cheselden, the anatomist, when giving an account of a young man whom he had restored to sight by couching for the cataract," he was so far from making any judgment about distance, that he thought all objects touched his eye, as he expressed it, as what he felt did his skin; and thought no objects so agreeable as those which were smooth and regular, although he could form no judgment of their shape, or guess what it was in any object that was pleasing to him."

This anatomist has further informed us, that he has brought to sight several others who had no remembrance of ever having seen; and that they all gave the same account of their learning to see, as they called it, as the young man already mentioned, although not in so many particulars; and that they all had this in common, that, having never had occasion to move their eyes, they knew not how to do it, and, at first, could not at all direct them to a particular object; but in time they acquired that faculty, though by slow degrees.

§ 43. Signs by means of which we estimate distance by sight.

Blind persons, when at first restored to sight, are unable to estimate the distance of objects by that sense, but soon observing that certain changes in the visible appearance of bodies always accompany a change of distance, they fall upon a method of estimating distance by the visible appearance. And it would no doubt be found, if it could be particularly examined into, that all mankind come to possess the power of estimating the distances of objects by sight in the same way. When a body is removed from us and placed at a considerable distance, it becomes smaller in its visible appearance, its colours are less lively, and its outlines less distinct; and we may ex

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