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INAL, and those which are ACQUIRED. Nothing is properly original with the sense of sight but the sensations of color, such as red, blue. yellow. These sensations, (or perceptions, as they are otherwise called, when the internal feeling is combined with a reference to the external cause,) are exceedingly numerous. In this respect the intimations of the sense of sight stand on the same footing with those of taste and hearing; although distinctive names, in consequence of the difficulty of accurately separating and drawing the line between each, are given only in a few cases. All the sensations of color are original with the sight; and are not to be ascribed to any other sense.

A part however of that knowledge, which we attribute to the sight, and which has the appearance of being immediate and original in that sense, is not so. Some of its alleged perceptions are properly the results of sensations, combined not only with the usual reference to an external cause, but with various other acts of the judgment. In some cases the combination of the acts of the judgment with the visual sensation is carried so far, that there is a sort of transfer to the sight of the knowledge, which has been obtained from some other source. And not unfrequently, in consequence of a long and tenacious association, we are apt to look upon the knowledge thus acquired, as truly original in the seeing power. This will suffice perhaps as a statement of the general fact, while the brief examination of a few instances will help to the more thorough understanding of those acquired perceptions of the sight, which are here referred to.

§. 78 The idea of extension not originally from sight.

It is well known that there is nothing more common than for a person to say, that he sees the length or breadth of any external object; that he sees its extent, &c. These expressions appear to imply, (and undoubtedly are so understood,) that extension is a direct object of sight. There is no question that such is the common sentiment, viz, that the outlines and surface, which bodies permanently expand and present to the view, are truly seen. An opinion different from this might even incur the charge of great absurdity.

But properly the notion of extension, as we have already

seen, has its origin in the sense of touch. Being a simple and elementary thought, it is not susceptible of definition; nor, when we consider extension as existing outwardly and materially, can we make it a matter of description without running into the confusion of using synonymous words. But whatever it is, (and certainly there can be neither ignorance nor disagreement on that point, however much language may fail of conveying our ideas,) the knowledge of it is not to be ascribed originally to the sight.

The notion of extension is closely connected with externality. It is not possible to form the idea of extension from mere consciousness, or a reflection on what takes place within us. But making a muscular effort, and thus applying the touch to some resisting body, we first have the notion of outness; and either from the same application of that sense, or when we have repeated it continuously on the same surface, we have the additional notion of its being extended or spread out. If a man were fixed immovably in one place, capable of smelling, tasting, hearing, and seeing, but without tactual impressions originating from a resisting body, he would never possess a knowledge of either. Having first gained that knowledge from the touch in the way just mentioned, he learns in time what appearance extended bodies, (which are of course colored bodies,) make to the eye. At a very early period, having ascertained that all colored bodies are spread out or extended, he invariably associates the idea of extension with that colored 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.

§. 79, 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 knowl

edge is often confidently attributed to the sense of sight as well as the touch. Although there is reason to believe, that men labor 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 colors and light. We may imagine ourselves to see the prominencies 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 prominencies, 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.

It was a problem submitted by Mr. Molyneux to Mr. Locke, whether a blind man, who has learnt 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 colors, 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 sug gestions 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 learnt 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.

§. 80. 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 heighborhood, 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 judg

ing 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 one. 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 exibited 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 mag

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