LITMIR.BIZ

Скачать книгу

Is it by vision that we learn primarily the distance of objects and their locality? These are points requiring investigation.

      Does Sight give Extension and Figure.—As to the first of these questions, whether extension and figure are objects of direct visual perception. No doubt they are associated in our minds with the act of vision, so that the moment we see an object we obtain an idea of it as extended, and of such and such dimensions and figure. The question is, whether it is really through the sense of sight that we obtain this idea, or in some other way. Had we no other means of information, would sight alone give us this? When we first open our eyes on external objects, do we receive the idea of extension and figure, or only of color? The fact that as matters are, we cannot in our experience separate the notion of some surface extension from the sensation of color, is not decisive of these questions. We cannot, as Dr. Brown observes, separate the color from the convexity and magnitude of an oak before us, but this does not prove that convexity and magnitude are objects of immediate and original perception. If every surface in nature had been convex, suggests the same writer, we should probably have found the same difficulty in attempting to conceive of color as separate from convexity, that we now find in attempting to conceive of it as separate from length and breadth. As it is, however, our sensation of color has not always been associated with convexity, while it has been always associated with surface extension. Hence it is, he maintains, that we seem to perceive, by the eye, the length, and breadth, and objects along with their color.

      Argument from the Affection of a Portion of the Retina.—The fact that in vision a certain portion of the retina in length and breadth is actually affected by the light falling on it, has been supposed by some to be conclusive of the fact that we perceive the length and breadth of the external object by the eye. This does not necessarily follow. As Dr. Brown contends, it is equally true that a certain part of the organ of smell is affected by odors, and a certain part of the auditory nerve is affected by sounds, yet we are not conscious of any perception of extension by either of these organs; we neither smell nor hear the length, and breadth, and magnitude of objects; nor is there any reason to suppose that the particular portion of the retina affected has any thing to do with the original sensation of sight.

      Amount of the preceding Arguments.—These arguments however, do not strike me as conclusive. They merely show the possibility that extension and figure may be acquired rather than original perceptions. They do not amount to positive evidence that they are so.

      An Argument to the Contrary.—On the other hand, there is one consideration of a positive character, which to most minds will be likely to outweigh the merely negative arguments already adduced. Color is a property of light, and light comes to us reflected from objects occupying space; we perceive it only as we perceive it spread over and reflected from some surface. Extension, then, surface expansion of the reflecting object, is the indispensable condition of the visibility of light itself, and so of color, as reflected from the object. Now it is difficult to persuade ourselves that what we know to be an essential condition of the perception of color, and what we seem to perceive along with the color, and cannot, even in thought, wholly separate from it, is not, after all, really perceived by the eye.

      Argument from recent Discoveries.—Indeed, recent discoveries in science seem to vindicate that not only surface extension, but trinal extension, or solidity, may be an object of direct perception by the eye. I refer to the researches of Wheatstone, in binocular vision, which go to show, that in consequence of the difference of the images formed upon the right and the left eye, as occupying different positions with reference to the object seen, we are enabled by the eye to cognize the solidity as well as the extension of objects. The difference of figure in the two images gives us this. That such is the case is shown by an instrument, the stereoscope, so constructed as to present separately the image as formed on each eye, which, when separately viewed, appear as mere plane surfaces, but when viewed together, the right image with the right eye, and the left one with the left eye, at the same time, present no longer the appearance of plane surfaces, but the two images combine to form one distinct figure, and that a solid, having length, breadth, thickness, and standing out with all the semblance of the real object.

      It is hardly necessary to say that if extension is an object of perception by the eye, so also is figure, which is merely the limitation of extension in different directions.

      Second Question—Does Sight give Distance?—Is it also by vision that we obtain the idea of the distance of objects and their externality? Does vision alone give the idea that what we see is numerically distinct from ourselves, and that it occupies this or that particular locality? So it would seem, judging from the impression left upon the mind in the act of vision. We seem to see the object as here or there, external, more or less distant in space. We distinguish it from ourselves.

      The negative View.—This is denied by some. All that we see, they contend, is merely the light coming from the object, and from the variations and modifications which this exhibits we learn to judge by experience of the distance and locality of the object. It is a matter of judgment and not of perception. We have learned to associate the two things, the visual appearance and the distance.

      Argument in the Negative.—In proof of this they adduce the fact that we are frequently mistaken in our estimate of the distance of objects. If there be more or fewer intervening objects than usual, if the atmosphere be more or less clear than usual, or any like circumstance affords a variation from our ordinary experience, we are misled as to the distance of the object. Hence we mistake the distance of ships at sea, or of objects on a prairie or a desert, the width of rivers, the height of steeples, towers, etc.

      Further Argument in the Negative.—It is further contended that facts show that the impressions of sight alone, uncorrected by experience, do not convey the idea of distance at all, but that what we see seems to be in connection with the eye itself, until we learn the contrary by the aid of other senses. This, it is said, is the experience of persons who have been operated upon for cataract, particularly of a patient whose case is described by Cheselden, and who thought every thing which he saw, touched his eyes. It is said also to have been the same with Caspar Hauser, when first liberated from the long confinement of his dungeon, and permitted to look out upon the external world. The goodly landscape seemed to him to be a group of figures, drawn upon the window.

      Force of this Argument.—This, however, is not inconsistent with the perception of externality by vision, since even what seems to be in contact with the eye, nay, what is known to be so, may still be known as external. Contact implies externality. It is very much to be doubted, moreover, whether the cases now referred to, coincide with the usual experience of those who are learning to see. The little child seems to recognize the externality and remoteness from his own person of the objects which attract his attention, as soon as he learns to observe surrounding objects at all, and, though he may not judge correctly of their relative distance from himself, never seems by his movements to suppose that they are in contact with his eye or with any part of his person. The young of animals, also, as soon as they are born, seem to perceive by the eye, the externality, the direction, and the distance of objects, and govern their movements accordingly. It is not, in these cases, a matter of experience, but of direct perception. These facts render it doubtful, to say the least, whether the common impression—that which in spite of all arguments to the contrary, is, and always will be made upon the mind in the act of vision, viz., that we see objects as external, as having locality, and as more or less remote from us—is not, after all, the correct impression.

      Learning to judge of Distance not inconsistent with this View.—- Nor does it conflict with this view that we learn to judge of the true distance of objects, and are often deceived in regard to it. The measurement of distance, the more or less of it, is of course a matter of experience, a thing to be learned by practice. It does not follow, however, that we may not by the eye directly, and at first, perceive an object to be external, and removed from us, in other words distant, though we may not know at first how distant. The rays of light that come to us from this external object, may give us direct perception of the object as external, as extended, and as occupying apparently a given locality in space more or less remote, while at the same time it may be left to other senses and to experience

Скачать книгу