erect as soon as their power of vision is restored. At least, Cheselden and other observers have never stated anything to the contrary. This could hardly have been the case if so striking a phenomenon had passed under their notice. To this there seems but one exception. Sir W. Hamilton quotes a case from Professor Leidenfrost, of Duisburg, 1793, in which the fact was otherwise. A young man, blind from birth, had reached his seventeenth year, when his sight was restored after an attack of ophthalmia. When he first saw men, they seemed to him inverted; that is, their heads were towards his feet; and trees and other objects seemed to hold the same position. I am unable to account for this difference from ordinary experience. I would only remark, that we are always liable to err in reasoning from instances of this kind, because, when the condition of an organ is decidedly abnormal, it is impossible to say to what extent and in what direction the abnormal cause has been exerted. We know not how the image on the retina causes vision, section 12. Cheselden's case-Phil. Transactions, 1728, No. 402. Prof. Liedenfrost's case, Sir W. Hamilton-Reid, p. 158. SECTION IX. OF ACQUIRED PERCEPTIONS, OR THE INTERCHANGEABLE USE OF THE SENSES. IT has been already remarked that each of our senses furnishes us with a distinct species of knowledge. We cognize odors by smell, sounds by the ear, colors by the eye, and so of all the rest. Neither of the senses can be used in the place of the other. We can neither see with our ears, hear with our fingers, nor smell with our tongue. Such is manifestly the fact, if our senses be considered separately. But when the senses are considered collectively, we find that the above statement does not convey the whole truth. One sense seems to convey to us knowledge which could have been gained only by another. A single perception will frequently furnish us with knowledge, which we find, upon reflection, to have been originally given us by the action of another sense, or by the combined action of several of the senses. Considered in this light, our whole sensual organism seems to be one complicated system, designed in the most rapid and convenient manner to make us acquainted with the external world. We find ourselves, in a thousand cases, using one sense for another, whenever we can do it with advantage; and if by misfortune we are deprived of any particular sense, it is surprising to observe how readily the remaining senses come to our aid, and enable us to cognize objects in a manner which, at first view, would seem utterly impossible. The process by which this effect is produced is the following: We have already observed that the variety of impressions which may be received by several of our senses is beyond the power of computation. Who can estimate the infinite number of sounds which we are capable of hearing; capable of seeing, Now, we find that or of color and shading which we are and of distinguishing from each other? a quality cognized by one sense is, by the kind provision of our Creator, connected with some modification of a quality perceived by another sense. Observing this connection, we learn to associate the original with the secondary quality, and, from the observation of the one, infer the existence of the other. For example, if I wish to learn whether a body is hard or soft, I employ the sense of touch. This is the sense originally given to me for the purpose of gaining this knowledge. I see before me a piece of polished marble, and a piece of velvet, of the same color. I feel of them both, and ascertain that the one is hard, and the other soft. But I also observe that the visual appearance of these two substances is dissimilar. I carefully note this difference. When I see the same objects again, I shall not be obliged to feel them; I know, at a glance, not only the visual but the tactual character of each. I go farther; I generalize this difference. I know that one visual appearance, whereever it is seen, indicates hardness, and another softness. Hence, when we, for the first time, look upon a substance, we commonly form an opinion of its hardness or softness from its peculiarity of color. Hence, also, we frequently use the language of one sense for that of another. We say of a surface that it looks hard or it looks soft. So painters, having observed that warm weather in summer is accompanied by a particular appearance of the sky, associate the language of feeling with that of sight, and speak of a warm sky, of warm or of cold coloring, and of other distinctions. of a similar character. Illustrations of acquired perceptions are presenting themselves to us every day, in the ordinary experience of life. The apothecary learns how to distinguish medicines by their smell as accurately as by their taste. The mineralogist by breathing upon a mineral, and observing its smell, will know in an instant whether it is or is not argillaceous. Or, again, he will distinguish a calcareous from a magnesian mineral by the touch; or he will determine the character of another by its fracture. If a grocer wishes to know whether a cask is full or empty, he does not look into it, but merely strikes upon it, and ascertains the fact in an instant by sound. A man who wishes to know if a wall in a particular spot is solid, does not pull it down, but strikes it with his hammer. In the same way we determine whether an object before us is made of wood, or metal, or stone. When these indications are closely observed, the accuracy of the judgments to which they lead is frequently very remarkable. It is said that an Indian hunter, on the prairies, by placing his ear on the ground, will discover the approach of an enemy long before he can be recognized by the eye, and will distinguish a herd of buffaloes from a troop of dragoons with unerring certainty. We are told that the Arabs will tell the tribe to which a passer-by belongs, by the print of his foot in the sand, and by the track of a hare will know whether it be a male or a female. Inasmuch, however, as our visual perceptions are more varied and more rapid than those of our other senses, and as we, by the eye, cognize objects at great distances, the greater part of our acquired perceptions are referred to this sense. We judge of the qualities of almost all the substances in daily use by the eye alone. We continually determine distance and magnitude by the eye. The manner in which this is done is worthy of special notice. It is well known that, as an object recedes from us, its visual appearance presents several observable changes. First, its magnitude diminishes. Secondly, its color becomes dim and misty. Thirdly, its outline becomes indistinct; and, fourthly, as its distance increases, the number of intervening objects becomes greater. It is by the observation of these changes that we determine whether objects are receding from, or advancing towards us. In the same manner, by comparing these indications, we judge of the distance and magnitude of any object. In every case of this kind we go through a complicated act of judgment; yet, from habit, we do it so rapidly, that we should hardly be aware of it but from the mistakes which we occasionally commit. For instance; I see an object presenting a certain dimness of color, of a certain indistinctness of outline, and of a given visual magnitude, and observe various objects intervening between it and me. This is all that the sense of sight gives me. I immediately judge it to be a man of ordinary size, half a mile off; and my judgments are so generally accurate, that I am surprised if I find myself in error. When, however, any one of these conditions is changed, we are liable to be deceived. This is commonly the case when objects are seen through a mist. The deception here is not occasioned, as is generally supposed, by refraction of the rays of light, causing the object to seem larger. The object really seems to us of the proper size. The mist, however, renders the color and the outline indistinct, and we suppose the object to be at a much greater distance than it is. The body has the magnitude belonging to a quarter of a mile in distance, with the indistinctness of half a mile. With this magnitude at the latter distance, it would, of course, seem to us much larger than it actually was. An incident, illustrative of this fact, once occurred to the author. He was, early in the morning, in a dense fog, sailing through the harbor of Newport, and passed near the wharf of Fort Adams. He observed on the wharf some very tall men, and mentioned their remarkable size to the friends who accompanied him. Presently he was struck with their behavior. They were jumping and playing like children, |