observe the truth of this law in our daily experience. When we look upon a well-executed painting, every figure, when viewed from a proper position, appears to stand out from the canvas. It seems to us impossible that it should be a plane surface. But if we draw near, the illusion vanishes. When we arrive at the position at which the figures, if solid, would form different images on the two eyes, and no such 'difference exists, we know at once that the surface is a plane. If it be objected that persons with one eye are able to distinguish solidity, it is replied that they do it less perfectly than others; that they are obliged to do it by observing the shading of the surface, and that they are frequently seen to move the head in a horizontal direction rapidly, in order to form the different images on the same eye.*

In consequence of this discovery, a very beautiful optical instrument has been invented, by which the effect of daguerreotype pictures has been much improved. A picture is taken separately for each eye. When these are looked at together, through glasses adapted to the purpose, we perceive only one figure; but it has all the appearance of solidity. Daguerreotypes of statuary have thus all the effect of the original marble.

The question has frequently been asked, How do we see objects single with two eyes? To this question I do not know that any more satisfactory answer has been given than the plain statement of the fact that so we were created. It seems to me not half so strange as the fact that we see at all. But I would inquire, is it more remarkable that we receive a single impression from two organs of sight, than from any of our other senses? All our nerves of sense are double. Every other sense has a right and a left nerve; yet all the impressions made upon us from a single object are

*Transactions of the Royal Society, vol. 56, p. 371. June 21, 1838.

single. Each ear receives an auditory impulse, yet we hear but one sound. When we feel of an object, each hand receives a distinct impression, yet we perceive but one. object. It does not seem strange to us that we do not hear two sounds with two ears, or that we do not feel two cubes when we hold one with our two hands. The case, however, seems to me precisely similar to that in which we look upon one object with our two eyes. The sense of sight, then, merely conforms to the general law by which all our senses are governed. It would seem, then, unnecessary to proceed farther than to refer the case of sight to the general law of the senses. The question thus resolves itself into the general one, How are single impressions made with double organs? To this I do not know that any answer has been either given or attempted.

Again, it has been asked, How do we see objects erect, when the image on the retina is inverted? Dr. Reid answers this question by stating it as a general law that we see every object in the direction of the right line that passes from the picture of the object on the retina to the centre of the eye, "as the rays from the upper part of the object form the lower part of the image, and, vice versa, we see the upper part of the object with the lower part of the retina, and the contrary; and thus we see the object as it is, that is, we see it erect." In how far this relieves the difficulty, or carries us back to a more general law, I will not pretend to determine. To me it does not seem to throw that light on the subject which seems obvious to others. I have thought that, possibly, this effect was in some way connected with the decussation of the optic nerve. No nerves, except those of sight, unite before entering the brain, and in no other case is this peculiarity observed. May there not be some connection between the facts?

Persons who have been couched for cataract see objects

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

Carpenter's Physiology, article sight.

Cheselden's case- Phil. Transactions, 1728, No. 402.

Wheatstone's paper, Phil. Trans., vol. 56, p. 371.

Prof. Liedenfrost's case, Sir W. Hamilton-Reid, p. 158.

SECTION IX.

OF ACQUIRED PERCEPTIONS, OR THE INTERCHANGEABLE USE OF THE SENSES.

Ir 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;

or of color and shading which we are capable of seeing, and of distinguishing from each other? Now, we find that 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 warın 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