able to retrace by the memory, that we cannot prove this by a reference to our own childhood and infancy. It appears, however, from the statement of the cases of persons born blind on the sudden restoration of their sight.

"When he first saw, (says Cheselden, the anatomist, when giving an account of a young man, whom he had restored to sight by couching for the cataract,) he was so far from making any judgment about distances, that he thought all objects touched his eyes, as he expressed it, as what he felt, did his skin; and thought no objects so agreeable as those, which were smooth and regular, although he could form no judgment of their shape, or guess what it was in any object, that was pleasing to him."

This anatomist has further informed us, that he has brought to sight several others, who had no remembrance of ever having seen; and that they all gave the same account of their learning to see, as they called it, as the young man already mentioned, although not in so many particulars; and that they all had this in common, that having never had occasion to move their eyes, they knew not how to do it, and, at first, could not at all direct them to a particular object; but in time they acquired that faculty, though by slow degrees.

Blind persons, when at first restored to sight, are unable to estimate the distance of objects by that sense, but soon observing, that certain changes in the visible appearance of bodies always accompany a change of distance, they fall upon a method of estimating distances by the visble appearance. And it would no doubt be found, if it could be particularly examined into, that all mankind come to possess the power of estimating the distances of objects by sight in the same way. When a body is removed from us and placed at a considerable distance, it becomes smaller in its visible appearance, its colours are less lively, and its outlines less distinct; and we may expect to find a number of intermediate objects, more or fewer, as the distance may happen to be, showing themselves between the receding object and the spectator. And hence it is, that a certain

visible appearance comes to be the sign of a certain dis

tance.

Historical and landscape painters are enabled to turn these facts to great account in their delineations. By means of dimness of colour, indistinctness of outline, and the partial interposition of other objects, they are enabled apparently to throw back at a very considerable distance from the eye those objects, which they wish to appear remote. While other objects, that are intended to appear near, are painted vivid in colour, large in size, distinct in outline, and separated from the eye of the spectator by few or no intermediate objects.

§. 37. Further illustrations of this subject.

A vessel seen at sea by one, who is not accustomed to the ocean, appears much nearer, than it actually is. In his previous observations of objects at a distance he has commonly noticed a number of intermediate objects, interposed between the distant body and himself. The absence of those intermediate objects causes the deception, under which he labours in the present instance; or is, at least, a prominent cause of his erroneous supposition, that the vessel is nearer than it truly is.

For the same reason people misjudge of the width of a river, estimating its width at a half or three quarters of a mile at the most, when it is perhaps not less than double that distance.

The same in estimating by the eye the width and length of plains and marshes.

We mistake in the same way also in estimating the height of steeples and other similar elevated bodies. As the upper parts of the steeple out-top the surrounding buildings and there are no contiguous objects with which to compare it, any measurement taken by the eye must be inaccurate, but is generally less than the truth.

A man on the top of a steeple seems smaller to those below, than the same man would seem to the same persons, and at the same distance on level ground. As we have

been in the habit of measuring distances on the ground by the eye, we can give a pretty near guess, whether a person be at an hundred feet distance, or more or less; and the mind immediately makes an allowance and corrects, so rapidly that we do not remember it, the first visual representation. But having never been in the habit of measuring perpendicular distances, the mind is at a loss, and fails to make that correction, which it would very readily, and, as it were, intuitively make in the case of any objects on level ground. So that a man an hundred feet in the air appears to us smaller, than at the same removal from us on the earth.

The fixed stars when viewed by the eye, all appear to be alike indefinitely and equally distant. Being scattered over the whole sky, they make every part of it seem like themselves at an indefinite and equal distance, and, therefore, give the whole sky the appearance of the inside of a sphere. Moreover, the horizon seems to the eye to be further off than the zenith; because between us and the former there lie many things, as fields, hills, waters, which we know to occupy a great space; whereas between us and the zenith there are no considerable things of known dimensions. And, therefore, the heavens appear like the segment of a sphere, and less than a hemisphere, in the centre of which we seem to stand. And the wider our prospect is, the greater will the sphere appear to be and the less the segment.

In connection with what has been said we are led to make this further remark, that a change in the purity of the air will perplex in some measure those ideas of distance, which we receive from sight. Bishop Berkely remarks while travelling in Italy and Sicily, he noticed, that cities and palaces, seen at a great distance, appeared nearer to him by several miles than they actually were. The cause of this he very correctly supposed to be the purity of the Italian and Sicilian air, which gave to objects at a distance a degree of brightness and distinctness, which in the less clear and pure atmosphere of his native country, could be

observed only in those towns and separate edifices, which were near. At home he had learnt to estimate the distance of objects by their appearance; but his conclusions failed him, when they came to be applied to objects in countries, where the air was so much clearer.

§. 38. Idea of extension not originally from sight.

Our

We have seen, that our idea of distance is not derived originally from the sight, but from the touch. idea of extension has the same origin; for, as distance is the space interposed between one object and another, extension is the distance between the parts of the same object where in the intermediate parts there is a continuity of the same substance.

If a man, endued with sight, were to be fixed all his days in one place immoveably, and were deprived of the means of gaining any experience by the touch, that man could never, from the information of his own senses, receive any accurate knowledge of extension. But having learnt in time what appearance coloured and extended bodies make to the eye, he comes to learn from that appearance the extension of bodies, much the same as he estimates their distance from their appearance.

And this statement leads us to the consideration of magnitude or limited extension, which is also estimated by the eye, although the power of thus measuring it, like that of measuring distances and extension, is not an original perception, but is acquired by the aid of the touch.

§. 39. Measurements of magnitude by the eye.

Magnitude is divided into two kinds, tangible and visible; the tangible magnitude being always the same, but the visible, varying with the distance of the object. A man of six feet stature is always that height, whether he be a mile distant, or half a mile, or near at hand; the change of place making no change in his real or tangible magnitude. But the visible magnitude of this man may be six feet or not one foot, as we view him present with us, or at two

miles distance; for his magnitude appears to our eye greater or less, according as he is more or less removed.

Of two objects equally distant or supposed to be equally distant, that, which has the greatest visible magnitude, is supposed to have the greatest tangible magnitude.

To a man bewildered in a mist, objects seem larger than the life, because their faint appearance conveys the idea of great distance, and an object at a considerable distance, which has the same visible magnitude with one near, the mind immediately concludes to be larger.

The sun and moon seem larger in the horizon than in the meridian, appearing then to be at the greatest distance, either because the horizon for a reason already given seems more remote than the zenith, or because the atmosphere, being more full of vapour towards the horizon, makes the heavenly bodies appear fainter, and consequently more distant.

§. 40. Of the knowledge of the figure of bodies by the sight.

A solid body presents to the eye nothing but a certain disposition of colours and light. We may imagine ourselves to see the prominencies or cavities in such bodies, when in truth we see only the light or the shade, occasioned by them. This light and shade, however, we learn by experience to consider as the sign of a certain, solid figure.

A proof of the truth of this statement is, that a painter by carefully imitating the distribution of light and shade, which he sees in objects, will make his work very naturally and exactly represent, not only the general outline of a body, but its prominencies, depressions, and other irregularities. And yet his delineation, which by the distribution of light and shade gives such various representations, is on a smooth and plain surface.

It was a problem submitted by Mr. Molyneux to Mr. Locke, whether a blind man, who has learnt the difference between a cube and a sphere by the touch, can, on being suddenly restored to sight, distinguish between them, and