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 distance by the visible 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 distance.

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 are separated from the eye of the spectator by few or no intermediate objects.

§. 49. Of the estimation of distance when unaided by intermediate

objects.

As we depend in no small degree upon intermediate objects in forming our notions of distance, it results, that we are often much perplexed by the absence of such objects. Accordingly we find, that people frequently mis

take, when they attempt to estimate by the eye the length or width of unoccupied plains and marshes, generally making the extent less than it really is. For the same reason they misjudge of the width of a river, estimating its width at half or three quarters of a mile at the most, when it is perhaps not less than double that distance. The same holds true of other bodies of water; and of all other things, which are seen by us in a horizontal position, and under similar circumstances.

We mistake in the same way also in estimating the height of steeples, and of other bodies, that are perpendicular, and not on a level with the eye, provided the height be considerable. As the upper parts of the steeple outtop 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.

But it has been noticed, that a man on the top of a steeple appears smaller to those below, than the same man would seem to the same persons, and at the same distance on level ground. This does not disagree with the preceding statement; but then it is in part to be explained in this way.As we have been in the habit of measur ing distances 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 the first visual representation so rapidly that we do not remember it. 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, contribute to 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, and 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 Berkeley 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. And the same thing has been noticed by other travellers, who have been placed in the like circum

stances.

§. 50. Of objects seen on the ocean, &c.

A vessel seen at sea by a person, who is not accustomed to the ocean, appears much nearer than it actually is; and on the same principles as already illustrated. 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. It is probably the absence of such objects, that chiefly causes the de ception, under which he labours in the present instance.

And this naturally leads to a remark on the power, which those who follow the sea are said to possess, of distinguishing objects at a distance. They not only judge of the distance better, but they are thought to see much further than others. Frequently the experienced sailor will discover a vessel approaching in the edge of the horizon, and even be able to state the direction of her course and the number of her masts, while the land-man sees nothing of the kind; At least he thinks so. Perhaps, however, they both in many cases see the same small speck, but it speaks to them a very different language. To the sailor that mere speck is a vessel; the small breaks, which he discerns in it, distinctly convey to his mind the idea of sails and masts; he interprets the dim and diminished signs before him as certainly and as gladly, as one reads his mother tongue. But to the land-man, who has never been practised in this sort of interpretation, these signs are without signification; they are a foreign language, which conveys no meaning.

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

We have seen, that our idea of distance is not derived originally from the sight, but from the touch. Our idea of extension has the same origin. As distance is the space interposed between one object and another, extension may be described as the distance between the parts of the same object, where in the intermediate parts there is a continuity of the same substance. And yet the true notion of extension, like that of distance, is better learnt from personal experience than from any form of words, which may be employed to convey it.

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 es

timates their distance from their appearance. Therefore, we rightly say, that this idea is originally from the sense of touch. An affection of the sense of touch is the occasion, on which the mind possesses it; and it is an idea of the sense of sight only by acquisition or experience.

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.

§. 52. 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, both because the horizon for a reason already given may seem more remote than the zenith, and particularly because there is necessarily a greater mass of atmosphere between the eye and those bodies when they are thus sit