442 LECTURE XXIX. ANALYSIS OF THE FEELINGS ASCRIBED TO VISION, CONTINUED. THE chief part of my last Lecture was employed in considering the Phenomena of Vision, and particularly in proving, that vision, -simple and immediate, as it now seems to us, even in its most magnificent results,-is truly the application of an art, of long and tedious acquirement,—of that art with which we learn to measure forms and distances, with a single glance, by availing ourselves of the information, previously received from other sources;-the mixed product of innumerable observations, and calculations, and detections of former mistakes-which were the philosophy of our infancy, and each of which, separately, has been long forgotten, -recurring to the mind, in after-life, with the rapidity of an instinct. Of all the arts, which man can acquire, this is, without question, the richest, both in wonder and in value-so rich in value, that if the race of man had been incapable of acquiring it, the very possibility of their continued existence seems scarcely conceivable; and so rich in subjects of wonder, that to be most familiar with these, and to study them with most attention, is to find at every moment new miracles of nature, worthy of still increasing admiration. "Per te quicquid habet mundus, mirabile nobis, Das spectare loca, et Dias invisere sedes. Nativa hinc quamvis ferimur gravitate deorsum Exuimus terrenam, animosque æquamus Olympo."* On this subject the remarks of Dr Reid, which I am about to quote, are not less just than they are strikingly expressed. "If we shall suppose an order of beings, endued with every human faculty but that of sight, how incredible would it appear to such beings, accustomed only to the slow informations of touch, that, by the addition of an organ, consisting of a ball and socket of an inch diameter, they might be enabled in an instant of time, without changing their place, to perceive the disposition of a whole army, or the order of a battle, the figure of a magnificent palace, or all the variety of a landscape? If a man were by feeling to find out the figure of the peak of Teneriffe, or of even St Peter's Church at Rome, it would be the work of a lifetime. "It would appear still more incredible to such beings as we have supposed, if they were informed of the discoveries which may be made by this little organ in things far beyond the reach of any other sense: That by means of it we can find our way in the pathless ocean; that we can traverse the globe of the earth, determine its figure and dimensions, and delineate every region of it. Yea, that we can measure the planetary orbs, and make discoveries in the sphere of the fixed stars. "Would it not appear still more astonishing to such beings, if they should be further informed, That, by means of this same organ, we can perceive the tempers and dispositions, the passions and affections of our fellow-creatures, even when they want most to conceal them? That when the tongue is taught most artfully to lie and dissemble, the hypocrisy should appear in the countenance to a discerning eye; And that by this organ, we can often perceive what is straight and what is crooked in the mind as well as in the body?-How many mysterious things must a blind man believe, if he will give credit to the relations of those that see! Surely he needs as strong a faith as is required of a good Christian." Judicium Paridis, v. 146-158. Ap. Mus. Anglican, vol. II. p. 274. EDIT. + Inquiry into the Human Mind, &c. c. 6. sect. 1. 1741. The same observation has been put in a strong light, by the supposition, that it had been as uncommon, to be born with the power of sight, as it is now to be born incapable of it;-in which case it has been truly said, that "the few who had this rare gift would appear as prophets or inspired teachers to the many." The very easy predictions thus made, would be found, constantly, or almost constantly fulfilled, by those who could form no conception of the means by which the effects predicted were foreseen; and wonderful as the dreams and visions of prophetic inspiration may appear, they surely could not seem more wonderful, as a medium of communication, than that by which the very secrets of the mind, and events apparently the most distant, were made known, through the intervention of a small ball like the eye. In shewing the manner by which we learn to combine, with our visual sensations, the knowledge obtained by touch; or, as I am rather inclined to think, for reasons formerly stated, the knowledge falsely ascribed to mere touch; it will not be necessary to go over the different varieties of figure, magnitude, distance. The most striking of these is distance,-which, indeed, may be truly said to involve the other two; since the distance of an object is merely the extension of the long line that intervenes between the object and our eye, and the consequent magnitude of the intervening objects, and that which we consider, regarded as one extended whole. Of this one great whole, what we term the distant object, is nothing more than the boundary. The cottage, at the end of a field, is a part of that compound magnitude of which the field and the cottage are separately parts, exactly in the same manner as the wing of a house, is a part of the compound magnitude of the whole building. The line of field which connects our eye with the cottage, may, indeed, be a longer line, but it is a line of precisely the same sort as that which connects the wings of the house with our organ of sight, or with each other. It is vain to think of ascribing the perception of distance to the measurement of the different angles subtended by objects, at different distances, or to an equally nice measurement of the dif * Reid's Inquiry into the Human Mind, &c. c. 6. sect. 2. ferent degrees of inclination of the axis of the eyes, necessary for distinct vision, in particular cases,- -as if all men were instinctively geometers, and the peasant and the very idiot were incessantly occupied in measuring angles; for if this measurement were truly instinctive, it would occur, in infancy, as in maturity, and be immediate, in those who have acquired the power of vision, by that surgical operation to which I alluded in my last Lecture. But the most decisive of all considerations, with respect to this supposed geometry, is, that the angles, subtended by the object at its different distances, and the inclination of the optic axis, in the spontaneous accommodation of the eyes to the distinct vision of the object at different distances, though truly existing, to the mere optical examiner of the object, and the light, and the eye, as one compound phenomenon, have no real existence, as feelings of the mind, of the individual who sees, and are known but to very few of the immense multitudes, who without the slightest acquaintance with geometry, or the slightest knowledge of the very lines, whose angles they are supposed to measure, are yet able to distinguish the distances of objects as accurately as the most expert mathematician. How is it possible that the angles, which remote objects make relatively to the eye, should be known originally, when the remote objects themselves are not known, but merely the points of light on the retina? In relation to the eye, as the organ, and to the mind, as originally sentient in vision, these points of light were truly all that existed. The light, indeed, traversed a certain space, in passing from the object to the eye, and the lines of direction of the different rays, in arriving at one focal point at the retina, formed truly different angles. But the angles could not be known, unless the radiant lines themselves were known; and of these, the mind could have no knowledge. During the whole time of their convergence, till they reached the expansion of the optic nerve, the rays of light were as little capable of producing vision, as darkness itself; and, when they reached the retina, the lines, and consequently the angles, existed no more. Of whatever use, therefore, such angles may be to the optician, in laying down, and illustrating the principles of his science, they are of no use in the actual living measurements of sight. Man may reason, indeed, but he must reason from what he knows; and, therefore, if the determination of distance be the result of any judgment, it must be of a judgment formed from feelings which truly have, or have had existence. Such feelings, the elements of our visual judgments, it is not very difficult to discover. The great principle, in this case, is the principle of association, by which the notions derived from touch,-or, at least, the notions which are commonly supposed to be derived from that sense, are suggested immediately by the visual feelings which coexisted with the sensations of touch; in the same manner, as the words of a language, when a language has been fully learned, suggest whatever the words may have been used to denote. A child, whose eye has already learned to distinguish objects, hears the word cup frequently repeated, when a cup is held before him; and the word afterwards suggests the thing. This process every one understands. But we are not equally aware, that, in the prior stage of learning to distinguish the cup by the eye, the child went through a process exactly similar,-that the visual feeling, which the rays of light from the cup excited, coexisted with the tactual and muscular feeling, when he handled the cup; and that the one feeling was thus associated, forever after with the other. The means by which we acquire our knowledge of the distance of objects, may be reduced to three,-the difference of the affections of the optic nerve,-the different affections of the muscles, employed in varying the refracting power of each eye, according to the distance of objects, and in producing that particular inclination of the axis of the two eyes, which directs them both equally on the particular object,--and, thirdly, the previous knowledge of the distance of other objects, which form, with that which we are considering, a part of one compound perception. To begin, then, with the affections of the retina. These become signs of distance, in two ways, by the extent of the part of the retina affected, and by the more or less vivid affection of the part. It is evident, from the laws of optics, that, according to the distance of the object from the eye, there must, when all other circumstances are the same, be a difference of the extent of the retina, on which the light falls. This illuminated portion of the nervous expanse, as supposed to be instantly perceived, is what is termed the visible figure of an object; and, though I am dispos |