If we examine the eyes of an infant, a few hours after its birth, we shall discern that it cannot make the smallest use of them; the organ not having acquired a sufficient consistency, the rays of light strike but confusedly upon the retina. Before the sixth week, children turn their eyes indiscriminately upon every object, without appearing to be affected by any, but at about this time they begin to fix them upon the most brilliant colours, and seem peculiarly desirous of turning them towards the light; this exercise does not give any exact notion of objects, but strengthens the eye, and qualifies it for future vision.
The first great error in the sense of seeing, is the inverted representation of objects upon the retina, and, till the sense of feeling has served to undeceive it, the child actually beholds every thing upside down. The second error in early vision is, that every object appears double; from the same object being formed distinctly upon each eye. This illusion like the other, can only be corrected by children from their being in the practice of handling different objects, and from which practice alone it is that they learn things are neither inverted nor double, and custom induces them to believe they see objects in the position the touch represents them to the mind; and therefore, were we denied the sense of feeling, that of seeing would not only deceive us as to the situation, but as to the number of every object around us.
We may easily be convinced that objects appear inverted, (which arises from the structure of the eye) by admitting the light to pass into a darkened room through a small aperture, when the images of the objects without will be represented upon the wall in an inverted position; for, as all the rays which issue from their different points cannot enter the hole in the same extent and position which they had in leaving the object, unless the aperture was as large as the object itself; as every part of the object sends forth its image on all sides; as the rays which from those images flow from all points of the object, as from so many centres, those only can pass through the small aperture which come in opposite directions. Thus the hole becomes a centre for the entire object, through which the rays from the upper, as well as from the lower parts of it, pass in converging directions; and, of consequence, they must cross each other at this centre, and thus represent the objects upon the wall, in an inverted position.
That we, in reality, see all objects double, is also evident; for example, if we hold up a finger, and look with the right eye at an object, it will appear against one particular part of the room, shutting that and looking with the left, it will seem to be on a different part, and if we open both eyes, the object will appear to be placed between the two extremes. But the truth is, the image of the object is formed in both eyes, one of which appears to the right, and the other to the left; and it is from the habit of touching that we suppose we see but one image placed between them both. From which it is clear that we see all objects double, although our imagination forms them single; and that, in fact, we see things where they are not, notwithstanding we have a pretty exact idea of their situation and position; and thus it is that till the sense of feeling has corrected the errors of sight, if instead of two eyes, we had an hundred, we should still fancy the objects single, although they were multiplied an hundred times.
In each eye, therefore, is formed a separate image of every object; and when the two images strike the correspondent parts of the retina, that is, the parts which are always affected at the same time, the object appears single: but, when the images strike the parts of the retina, which are not usually affected together, then it appears double, because we are not habitual to this unusual sensation, and are then somewhat in the situation of infants just beginning to exert the faculty of vision.
M. Chesselden relates the case of a man, who, in consequence of a blow on the head, became squint-eyed, and saw objects double for a long time: but who was at length enabled, by slow and gradual steps, to see them singly as he had formerly done, notwithstanding the squinting remained. Is not this a proof, still more evident, that in reality we see things double, and that it is by habit alone we conceive them to be single? Should it be asked why children require less time, in order to see things single, than persons more advanced in years, whose eyes may have been affected by accident? it might be answered, that the sensations of children, being unopposed by any contradictory habit, these errors are rectified with ease; but that persons who have for many years seen objects single, because they affected the two correspondent parts of the retina, and who now see them double, labour under the disadvantage of having a contrary habit to oppose, and must therefore be a considerable time before it is entirely obliterated.
By the sense of seeing we can form no idea of distances; without aided by the touch, every object would appear to be within our eyes; and an infant, that is as yet a stranger to the sense of feeling, must conceive that every thing it sees exists within itself. The objects only appear to be more or less bulky as they approach to, or recede from the eye; insomuch, that a a fly near the eye will appear larger than an ox at a distance. It is experience alone that can rectify this mistake; and it is by constantly measuring with the hand, and removing from one place to another, that children obtain ideas of distance and magnitude. They have no conception of size but from the extreme rays reflected from the object, of course every thing near appears large, and those at a distance small. The last man in a file of soldiers appears much more diminutive than the one who is nearest to us. We do not, however, perceive this difference, but continue to think him of equal stature; for the number of objects we have seen thus lessened by distance, and found by repeated experience to be of the natural size when we come closer, instantly correct the sense, and therefore we perceive every object nearly in its natural proportion, unless when we observe them in such situations as have not allowed us sufficient experience to correct the illusions of the eye. If, for example, we view men upon the ground, from a lofty tower, or look up to any object upon the top of a steeple, as we have not been in the habit of correcting the sense in that position, they appear to us exceedingly diminished, much more so than if we saw them at the same distance in an horizontal direction.
Though a small degree of reflection may serve to convince us of the truth of these positions, yet it may not be amiss to corroborate them by facts which cannot be disputed. M. Chesselden, having couched for a cataract a lad of thirteen years of age, who had from his birth been blind, and thus communicated to him the sense of seeing, was at great pains to mark the progress of his visual powers; his observations were afterwards published in the Philosophical Transactions. This youth was not absolutely and entirely blind: Like every other person, whose vision is obstructed by a cataract, he could distinguish day from night and even black from white, but of the figure of bodies he had no idea. At first the operation was performed only upon one of his eyes, and when he saw for the first time, so far was he from judging of distances, that he supposed (as he himself expressed it) every thing he saw touched his eyes, in the same manner as every thing he felt touched his skin. The objects that pleased him most were those whose surfaces were plain and the figures regular, though he could in no degree judge of their different forms, or assign why some were more agreeable to him than others. His ideas of colours during his former dark state were so imperfect, that when he saw them in reality he could hardly be persuaded they were the same. When such objects were shewn him as he had been formerly familiar with by the touch, he beheld them with earnestness, in order to know them again, but as he had too many to retain at once the greatest number were forgotten, and for one thing which he knew, after seeing it, there were a thousand, according to his own declaration, of which he no longer possessed the smallest remembrance. He was very much surprised to find that those persons, and those things, which he had loved best, were not the most pleasing to his sight; nor could he help testifying his disappointment in finding his parents less handsome than he had conceived them to be. Before he could distinguish that pictures resembled solid bodies above two months elapsed; till then he only considered them as surfaces diversified by a variety of colours; but when he began to perceive that these shadings actually represented human beings, he expected also to find their inequalities; and great was his surprise to find smooth and even what he had supposed a very unequal surface; and he inquired whether the deception existed in feeling or seeing. He was then shewn a miniature portrait of his father, which was contained in his mother’s watch-case, and though he readily perceived the resemblance, yet he expressed his amazement how so large a face could be comprised in so small a compass; to him it appeared as strange as that a pint vessel should contain a bushel. At first he could bear but a very small quantity of light, and every object appeared larger than the life; but in proportion as he observed objects that were in reality large, he conceived the others to be equally diminished. Beyond the limits of what he saw he had no conception of any thing. He knew that the apartment he occupied was only a part of the house, and yet he could not imagine how the latter should be larger than the former. Before the operation he formed no great expectations of the pleasure he should receive from the new sense he was promised, excepting that thereby he might be enabled to read and write. He said among other things, that he could enjoy no greater delight from walking in the garden, because there he already walked at his ease, and was acquainted with every part of it. He also remarked that his blindness gave him one advantage over the rest of mankind, namely, that of being able to walk in the night with more confidence and security. No sooner, however, had he began to enjoy his new sense than he was transported beyond measure; he declared that every new object was a new source of delight, and that his pleasure was so great he had not language to express it. About a year after, he was carried to Epsom, where there is a beautiful and extensive prospect; with this he seemed greatly charmed; and the landscape before him he called a new method of seeing. He was couched in the other eye a year after the former, and the success was equally great. Every object appeared larger when he looked at it with the second eye to what it did with the other; and when he looked at any thing with both eyes it appeared twice as large as when he saw but with one, though he did not see double, or at least he shewed no marks from which any such conclusion might be drawn.
Mr. Chesselden instances several other persons who were in the same situation with this lad, and on whom he performed the same operation; and he assures us, that on first obtaining the use of their eyes they expressed their perceptions in the same manner, though less minutely; and that he particularly observed of them all, that as they had never had any occasion to move their eyes while deprived of sight, they were exceedingly embarrassed in learning how to direct them to the objects they wished to observe.