ginating from a resisting body, he would never possess a knowledge of either. Having first gained that knowlledge from the touch in the way just mentioned, he learns in time what appearance extended bodies (which are, of course, coloured bodies) make to the eye. At a very early period, having ascertained that all coloured bodies are spread out or extended, he invariably associates the idea of extension with that coloured appearance. Hence he virtually and practically transfers the knowledge obtained by one sense to another; and even, after a time, imagines extension to be a direct object of sight, when, in fact, what is seen is only a sign of it, and merely suggests it. An affection of the sense of touch is the true and original occasion of the origin of this notion; and it becomes an idea of sight only by acquisition or transference.

§ 80. Of the knowledge of the figure of bodies by the sight. Views similar to those which have been already advanced will evidently apply to the figure of bodies. We acquire a knowledge of the figure or form of bodies originally by the sense of touch. But it cannot be doubted that this knowledge is often confidently attributed to the sense of sight as well as the touch. Although there is reason to believe that men labour under a mistake in this, it is not strange, when we trace back our mental history to its earlier periods, that such a misapprehension should exist.

A solid body presents to the eye nothing but a certain disposition of colours and light. We may imagine ourselves to see the prominences or cavities in such bodies, when in truth we see only the light or the shade occasioned by them. This light and shade, however, we learn by experience to consider as the sign of a certain 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 rep resentations, is on a smooth and plain surface.

VOL. I.-K

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 see..g a globe, will be that of a circle on a plain surface, but 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 plain, 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, plain surfaces, variously shaded, represent convex bodies, and with great truth and exactness.

It appears, then, that extension and figure are originaly 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.

THE SENSE OF SIGHT.

§ 81. Measurements of magnitude by the eye.

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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.

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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 (Intellec. Powers, part ii., sect. 1), 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

Among the multitude of instances which might be adduced in illustration of the doctrine under notice, the following statement, to be found in the seventh number of the Edinburgh Journal of Science, is a somewhat striking

In examining a dioramic representation of the inside of Rochester Cathedral, which produced the finest effect, from the entire exclusion of all extraneous light and of all objects, excepting those on the picture itself, the writer of the statement referred to was struck with an appearance of distortion in the perspective, which he ascribed to the canvass not hanging vertically. Upon mentioning this to the gentleman who exhibited the picture, he offered to walk in front of it, and strike its surface with the palm of his hand, to show that the canvass was freely suspended. Upon doing this, a very remarkable deception, or illusion rather, took place. As his hand passed along, it gradually became larger and larger till it reached the middle, when it became enormously large. It then diminished till it reached the other end of the

canvass.

As the hand moved towards the middle of the picture, it touched the parts of the picture more and more remote from the eye of the observer; and, consequently, the mind referred the hand and the object in contact with it to the same remote distance; and, consequently, gave it a fictitious magnitude, corresponding with the visible figure it presented, combined with the supposition of its being placed at a distance. (See Edin. Journ. of Science, No. vii., p. 90, and Art. Science, Edin. Encyc.)

82. Of objects seen in the mist, and of the sun and moon in the horizon. 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.

These remarks naturally remind us of the well-known fact, that the sun and moon seem larger in the horizon than in the meridian. A number of reasons may be given for this appearance. (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 rays of light form precisely the same image in the eye, 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, which is sometimes assigned, 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.--When the atmosphere is not clear, but unusual masses of vapour are accumulated in it, whether immediately around us or anywhere else in the direction of the rays, the refraction is increased, and the object is proportionally enlarged. This circumstance helps to explain the fact of the enlargement not being uniform, but sometimes greater and at others less.—(3.) Our estimate of the size of the sun and moon is also affected by the simultaneous perception of other objects of known magnitude, which happen to be in the same direction. The setting sun, for instance, when it is seen through distant woods, appears much enlarged. The woods, in