CHAPTER X.

EFFECT OF

EXPERIENCE-AUXILIARIES

IN JUDGING OF

OBJECTS-THE EYE THE MIRROR OF THE SOUL.

It appears that the first notions conveyed by our senses, are very limited and imperfect, and that our real knowledge is acquired only after considerable observation and experience. The impressions of one sense are thus aided and corrected by others, and by the mind acting on the whole. The primary objects of vision seem to be simply light, or colour and expansion. Yet our judgments on vision are much more extensive, embracing also distance, magnitude, and what is called tangible figure, as that of a cube, or a sphere. The last is obviously the result of experience, derived from the sense of touch. Distance and magnitude, also, are not primary objects of vision; for persons who have suffered from blindness, and been restored to sight, have had no idea of either, but could only perceive expansion of surface with colour. Our judgment of these is therefore an acquired habit, founded on our knowledge of the

properties of objects received by other means. Accordingly, we have no idea of the distance of an object, unless we have some notion of its magnitude; nor of its magnitude without some knowledge of its distance. In perspective drawing, the idea of distance is conveyed by the diminished size of known objects. On the same principle, known objects seen through a telescope do not appear to be magnified, but to be brought nearer.

An interesting proof of this occurred in the case of a boy who was blind from his birth, and who at the age of twelve, by the removal of what is called a cataract from his eyes, acquired the power of seeing. At first, he imagined that as what he felt touched his skin, so all the objects he saw did his eyes; nor could he distinguish one object from another, or judge of distances aright by the sight alone; the sense of touch was therefore called in to the aid of the power of vision.

Being shown a small miniature of his father, and told what it was, he acknowledged a likeness, but was greatly surprised; asking how it could be that a large face could be exhibited in such little room, and remarking, that it would have seemed as impossible to him as to put a bushel into a pint. At first he could bear only a little light, and the things he saw he considered extremely large; but on seeing things larger, those first seen he

supposed less, never being able to imagine any lines beyond the bounds he saw.

He said he knew the room he was in was but part of the house, yet he could not conceive that the whole house could look larger. Before he was couched he expected little advantage from seeing, worth the endurance of an operation, except reading and writing; for he said he thought he could have no more pleasure in walking abroad than he had in the garden, which he could do very safely and readily. He observed, that even blindness had this advantage, that he could go any where in the dark much better than those who could see; and after he had seen, he did not soon lose this ability, nor did he desire a candle to go about the house in the night.

He said that every new object was a new delight, and the pleasure was so great, that he wanted ways in which to express it; but he could not conceal his gratitude to Mr. Cheselden, the oculist, and for some time he never saw him without tears of joy, and other marks of affection. About a year after the operation, he was taken to Epsom Downs, and on the view of an extensive prospect, he was exceedingly delighted, and called it a new kind of seeing. The other eye being afterwards couched, he remarked that objects at first appeared

large to this eye, but not so large as they did at first to the other; and looking on the same object with both eyes, he thought it appeared about twice as large as with the first couched eye only, but not double, that he could in any way discover.

The picture formed on the retina by the humours of the eye, causes, therefore, all the perceptions belonging to the sense of vision; yet the visible appearance which such pictures suggest, when taken by themselves, could have conveyed no notion of the size, distance, or situation of the object represented. It is consequently from the experience acquired by the use of other senses, that we learn the connexion of these appearances with such objects, so that in the course of time the former become the signs of the latter.

On the eye being directed, however, to any point of a landscape, it sees with perfect distinctness only that point of it which is directly in the axis of the eye, or the image of which falls on the central hole of the retina, but the other parts are discerned with sufficient distinctness for the enjoyment of its general effect. The apparent defect is supplied by the extreme mobility of the eye, and the duration of the impressions made on the retina; and thus the landscape is beheld as perfectly as if every part were seen with equal distinctness.

RANGE OF SIGHT.

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We are greatly influenced in our judgments by the degree in which objects are illuminated, and that of the distinctness of their outline and minute parts. Thus in a picture, distant objects are pourtrayed as having a faint light, and, also, with indistinctness; while those which are near are represented as clearly defined, and in a strong light. In the same way, objects seen through a fog, or in obscure light, appear, as has already been intimated, much larger than they really are; because we first assume them to be distant, from their defective outline and faint light, and then, judging from this supposed distance, we conclude they are of great size.