198 DISCOURSE VIII. ON THE PARADOXES OF VISION. AMONGST the difficulties connected with the physiology of vision, there are several of an interesting character; and, amongst these, there are two especially which have for a long period excited discussion and controversy, without having been hitherto settled or solved to general satisfaction. These, in the brief but inexact language usually employed, are described as erect vision with inverted images, and single vision with two eyes. It is well known to all who have the slightest knowledge of Optics, that the images or pictures painted on the retina are inverted, and that, when we see an object most perfectly, there is a picture of it in each eye, although the object is seen single. The difficulties presented by these facts, which had formerly engaged the curiosity of philosophers in no ordinary degree, seem to have been neglected for an interval of some years, till attention was not long ago recalled to them by Mr. Wheatstone, Mr. Whewell, Sir D. Brewster, and others. Mr. Wheatstone, in a very interesting and original paper in the Philosophical Transactions on Binocular Vision*, has brought forward a number of striking facts bearing on the second question, which have placed it in quite a new light. To these, and to some inferences flowing from them, which appear to me to have been overlooked, I shall hereafter more particularly call your at tention. Mr. Whewell, in his able work on the "Philosophy of the Inductive Sciences," has discussed both of the difficult questions before us. He terms them the Paradoxes of Vision, and has treated them and some kindred topics, if not with all the exactness desirable, at least with a free and powerful pen. Like every other writer of any celebrity, he soon found antagonists to contest some of his positions. His treatise had not been long published before the "Edinburgh Review" took him roundly to task, and went pretty largely into the discussion of these and collateral inquiries. I have no information at all as to the authorship of the article referred to, which appeared in the number for January last †; but, from internal evidence, there can be little hesitation in attributing it to one of the most distinguished men of science in Scotland. The two paradoxes of vision (to adopt Mr. Whewell's convenient phraseology) having been thus recently brought before the great philosophic world * Philosophical Transactions, 1838, p. 371. by writers of so much ability, it occurred to me that a paper on the subject might possibly be interesting to our smaller sphere, especially as I conceive some of their explanations and arguments to be unsuccessful or erroneous; and that there is room for a completer solution of some difficulties, and a more accurate statement of others, than any which has yet appeared. Although I cannot hope to make a discussion of so abstruse a nature intelligible to persons altogether unacquainted with physiological inquiries and metaphysical speculation, I flatter myself I shall be able to place the subject in a clearer light than that in which it has been hitherto exhibited auditors. to many of my Let us, in the first place, consider the paradox of erect vision with inverted images. The celebrated philosopher Kepler, although anticipated in some respects by Baptista Porta, has the credit of having first made the discovery, that inverted pictures of visible objects are painted upon the retina; and he showed from the principles of Optics how these pictures are necessarily formed in that position. The rays coming from any one point of an object, and falling divergently on the eye, are refracted by the cornea and crystalline, so as to unite again on some point of the retina, painting there the colour of that point of the object whence they come. As the rays from different points of the object cross each other before they arrive at the retina, the picture they form must be inverted. Kepler's own explanation of seeing objects erect by means of inverted images was this:-As the rays of light cross each other, we conclude that the impulse we feel upon the lower part of the retina comes from above, and the impulse upon the higher part comes from below. Descartes afterwards gave the same solution; but it is obviously unsatisfactory, for the simple reason that it assigns an imaginary or fictitious circumstance as the cause of that which is to be accounted for. In point of fact, we feel no impulse, either on the upper or the lower part of the retina, and can therefore form no such conclusion as he states.* It is astonishing to observe the answers which other distinguished philosophers have returned to this question. Buffon, for example, says, "The first great error in vision is the inverted representation of objects upon the retina. And till children learn the real position of bodies by the sense of feeling, they see every object inverted. . . . By the frequent handling of objects, they gradually learn they are neither double nor inverted; and custom soon makes them imagine they see objects in the order and position in which they are represented to the mind by the sense of touching."† * See Reid's Inquiry into the Human Mind, chap. vi. sect. 11. † Natural History, Smellie's Translation, vol. iii. p. 3. The same explanation has been adopted by Dr. Brown. That it is altogether incorrect, not to say absurd, will appear in the course of our remarks. At present it may suffice to observe, that the French philosopher is completely at variance with facts in asserting that children originally see objects inverted. In support of this assertion not a tittle of evidence has been ever adduced. But if they really did see objects in that position, all the handling in the world would never make them see a single thing erect. There is no such power in any one of our senses to change the perceptions of another sense. A straight stick, with one end placed in a basin of water, would still appear to the sight to be bent at that end after a thousand proofs by the touch that it was otherwise. I will not, however, detain you by a further account of the extraordinary explanations of eminent philosophers. All fallacies and absurdities on the subject will be best exploded by giving the true solution of the paradox before us, to which I will therefore now proceed. The fundamental position on which the true explanation rests is, that the picture of an external object formed on the retina is not seen. There can be no doubt, that, whenever we see the object, a picture of it must be painted on the retina; but that picture is itself invisible to the eye in which it is formed. We see the object, not the picture; |