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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 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 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 moun appear enlarged when other objects of considerable Jimensions, but so distant as to subtend a very small ongle at the eye,
are seen in the same direction or in the inoment 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.
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.
Ø 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
pect to find various intermediate objects, more or fewer in number, corresponding with the increase of the distance, showing themselves between the receding object and the spectator. And hence it is, that a certain visible appearance comes to be the sign of a certain distance.
Historical and landscape painters are enabled to turn these facts to great account in their delineations. By means of dimness of colour, indistinctness of outline, and the partial interposition of other objects, they are enabled apparently to throw back to a very considerable distance from the eye those objects which they wish to appear remote. While other objects, that are intended to appear near, are painted vivid in colour, large in size, distinct in outline, and are separated from the eye of the spectator by few or no intermediate objects. $ 44. Estimation of distance when unaided by intermediate objects.
(1.) As we depend, in no small degree, upon intermediate objects in forming our notions of distance, it results, that we are often much perplexed by the absence of such objects. Accordingly, we find that people frequently mistake, when they attempt to estimate by the eye the length or width of unoccupied plains and marshes, generally making the extent less than it really is. For the same reason they misjudge of the width of a river, estimating its width at half or three quarters of a mile at the most, when it is perhaps not less than double that distance. The same holds true of other bodies of water; and of all other things which are seen by us in a horizontal position and under similar circumstances.
(2.) We mistake in the same way also in estimating the height of steeples, and of other bodies that are perpendicular, and not on a level with the eye, provided the height be considerable. As the upper parts of the steeple out-top the surrounding buildings, and there are no contiguous objects with which to compare it, any measurement taken by the eye must be inaccurate, but is generally less than the truth.
(3) The fixed stars, when viewed by the eye, all appear to be alike indefinitely and equally distant. Being scattered over the whole sky, they make every part of it