consequence of their distance, subtend but a small angle at the eye; but, being objects of known magnitude, they appear enlarged and nearly of their natural size in our conception of them. And as the sun fills a larger space in our eye than the separate trees which fall within its disk, it experiences in our conception an enlargement, precisely corresponding with the imagined or conceptive enlargement of the objects which are encircled by its rays. Just as in the case of a balloon, which, at a great elevation, crosses the disk of the sun or moon. The balloon is an object of known size and of great size; but, in fact, when seen at a great elevation, it is materially and visually a mere speck, although much enlarged mentally. Accordingly, when it passes over the disk of the sun or moon, those bodies will appear greatly enlarged, so as to correspond with our previous conceptions of the size of the body which their rays at that time encircle.-(The reader will find this subject more fully explained in Dr Arnott's Elements of Physics, vol. i.) 83. 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 per appears, however, from the statement of the cases of sons 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.* * 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 * Some doubts have been raised from time to time of the correctness of Cheselden's experiments and inquiries here referred to. Fortunately, Mr. Stewart has taken up the subject with his accustomed caution and candour in his Account of James Mitchell, a boy born deaf and blind. He shows to ample satisfaction, in a note near the commencement of that Narration, that the facts which have been brought forward in opposition to Cheselden may be satisfactorily explained, without any impeachment of the correctness of his statements or the justness of his conclu sions from them.-See additional confirmations of this subject in the life of Caspar Hauser rich nor a (2.) Of the mo jury must not b (3.) Of the be worshipped 44. The doc It will not this subject, There seems idence, that whatever a argument b trine was t om the e aintance as and wled rou e, G AZON des Wanner great entarge. 521 CorrespOL WIth our previous concentions & the siz the bocs whct the rays at the time encirclereader win this subiect more fully explame i Arnott's Elements o: Physics, vo.. 1. 8. the estimation, o: distances sunt. We are next led to the consideration o: distances as made known and ascertained to the sight. By the detance of objects, when we use the term in reference t ourselves, we mean the space which is interposed betweer those objects and our own position. It might be objectes. that space interposed is only a synonymous expression for the thing to be defined. supposed to be ignorant of what is meant. men have a notion of distance, and can measure it by the Nevertheless, no one can n touch, or by walking forward until they meet the distanc object. Even bimc The perception of distance by the s and not an original percent universally suppos 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.* 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 * Some doubts have been raised from time to time of the correctness of Cheselden's experiments and inquiries here referred to. Fortunately, Mr. Stewart has taken up the subject with his accustomed caution and candour in his Account of James Mitchell, a boy born deaf and blind. He shows to ample satisfaction, in a note near the commencement of that Narration, that the facts which have been brought forward in opposition to Cheselden may be satisfactorily explained, without any impeachment of the correctness of his statements or the justness of his conclu sions from them.-See additional confirmations of this subject in the life of Caspar Hauser 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, at 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. 84. Estimation of distance when unaided by intermediate objects. 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 reasc. 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. 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 continuous objects with which to compare it, any measurement taken by the eye must be inaccurate, but is generally less than the truth. Hence perhaps it is that a man on the top of a steeple appears smaller to those below than the same man would seem to the same person and at the same distance |