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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.
36. 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 applica tion 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 immoveably 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, 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 transfer
37. 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 only see 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, depres sions, 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 dis tinguish 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 experi ence 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 tha one is in the habit of seeing, the mind would have made allowance for the apparent size of the persons; and, cn 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 cbjects 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, ap. pears to us of less extent than it actually is. Now if the rays of light form precisely the same image in the eye,