or two hundred feet distant; because, beyond that, there is no sensible change, either of the conformation of the eyes, or of the inclination of their axes: the third mean, is but a vague and indeterminate sign, when applied to distances above two or three hundred feet, unless we know the real colour and figure of the object and the fifth mean, to be afterward mentioned, can only be applicable to objects which are familiar, or whose real magnitude is known. Hence it follows, that when unknown objects, upon, or near the surface of the earth, are perceived to be at the distance of some miles, it is always by this fourth mean that we are led to that conclusion.

Dr. Smith hath observed, very justly, that the known distance of the terrestrial objects which terminate our view, makes that part of the sky which is toward the horizon, appear more distant than that which is toward the zenith. Hence it comes to pass, that the apparent figure of the sky is not that of a hemisphere, but rather a less segment of a sphere. And hence likewise it comes to pass, that the diameter of the sun or moon, or the distance between two fixed stars, seen contiguous to a hill, or to any distant terrestrial object, appears much greater than when no such object strikes the eye at the same time.

These observations have been sufficiently explained and confirmed by Dr. Smith. I beg leave to add, that when the visible horizon is terminated by very distant objects, the celestial vault seems to be enlarged in all dimensions. When I view it from a confined street or lane, it bears some proportion to the buildings that surround me: but when I view it from a large plain, terminated on all hands by hills which rise one above another, to the distance of twenty miles from the eye, methinks I see a new heaven, whose magnificence declares the greatness of its Author, and puts every human edifice out of countenance; for now the lofty spires and the gorgeous palaces shrink into nothing before it, and bear no more proportion to the celestial dome, than their makers bear to its Maker.

5. There remains another mean by which we perceive the distance of visible objects, and that is the diminution of their visible or apparent magnitude. By experience I know what figure a man, or any other known object, makes to my eye, at the distance of ten feet: I perceive the gradual and proportional diminution of this visible figure, at the distance of twenty, forty, a hundred feet, and at greater distances, until it vanish altogether. Hence a certain visible magnitude of a known object, becomes the sign of a certain determinate distance, and carries along with it the conception and belief of that distance.

In this process of the mind, the sign is not a sensation; it is an original perception. We perceive the visible figure and visible magnitude of the object, by the original powers of vision;

but the visible figure is used only as a sign of the real figure; and the visible magnitude is used only as a sign either of the distance, or of the real magnitude, of the object; and therefore these original perceptions like other mere signs, pass through the mind, without any attention or reflection.

This last mean of perceiving the distance of known objects, serves to explain some very remarkable phenomena in optics, which would otherwise appear very mysterious. When we view objects of known dimensions through optical glasses, there is no other mean left of determining their distance, but this fifth. Hence it follows, that known objects seen through glasses, must seem to be brought nearer, in proportion to the magnifying power of the glass, or to be removed to a greater distance, in proportion to the diminishing power of the glass.

If a man who had never before seen objects through a telescope, were told, that the telescope, which he is about to use, magnifies the diameter of the object ten times; when he looks through this telescope at a man six feet high, what would he expect to see? Surely he would very naturally expect to see a giant sixty feet high. But he sees no such thing. The man appears no more than six feet high, and consequently no bigger than he really is; but he appears ten times nearer than he is. The telescope indeed magnifies the image of this man upon the retina ten times in diameter, and must therefore magnify his visible figure in the same proportion; and as we have been accustomed to see him of this visible magnitude, when he was ten times nearer than he is presently, and in no other case; this visible magnitude, therefore, suggests the conception and belief of that distance of the object with which it hath been always connected. We have been accustomed to conceive this amplification of the visible figure of a known object, only as the effect or sign of its being brought nearer and we have annexed a certain determinate distance to every degree of visible magnitude of the object; and therefore, any particular degree of visible magnitude, whether seen by the naked eye or by glasses, brings along with it the conception and belief of the distance which corresponds to it. This is the reason why a telescope scems not to magnify known objecta, but to bring them nearer to the eye.

When we look through a pinhole, or a single microscope, at an object which is half an inch from the eye, the picture of the object upon the retina is not enlarged, but only rendered distinct; neither is the visible figure enlarged: yet the object appears to the eye twelve or fourteen times more distant, and as many times larger in diameter, than it really is. Such a telescope as we have mentioned amplifies the image on the retina, and the visible figure of the object, ten times in diameter, and yet makes it seem no bigger, but only ten times nearer. These

appearances had been long observed by the writers on optics; they tortured their invention to find the causes of them from optical principles; but in vain: they must be resolved into habits of perception, which are acquired by custom, but are apt to be mistaken for original perceptions. The bishop of Cloyne first furnished the world with the proper key for opening up these mysterious appearances; but he made considerable mistakes in the application of it. Dr. Smith, in his elaborate and judicious treatise of Optics, hath applied it to the apparent distance of objects seen with glasses, and to the apparent figure of the heavens, with such happy success, that there can be no more doubt about the causes of these phenomena.

SECTION XXII.

OF THE SIGNS USED IN OTHER ACQUIRED PERCEPTIONS.

THE distance of objects from the eye, is the most important lesson in vision. Many others are easily learned in consequence of it. The distance of the object, joined with its visible magnitude, is a sign of its real magnitude; and the distance of the several parts of an object, joined with its visible figure, becomes a sign of its real figure. Thus, when I look at a globe, which stands before me, by the original powers of sight I perceive only something of a circular form, variously coloured. The visible figure hath no distance from the eye, no convexity, nor hath it three dimensions; even its length and breadth are incapable of being measured by inches, feet, or other linear measures. But when I have learned to perceive the distance of every part of this object from the eye, this perception gives it convexity and a spherical figure; and adds a third dimension to that which had but two before. The distance of the whole object makes me likewise perceive the real magnitude; for being accustomed to observe how an inch or a foot of length affects. the eye at that distance, I plainly perceive by my eye the linear dimensions of the globe, and can affirm with certainty that its diameter is about one foot and three inches.

It was shown in the seventh section of this chapter, that the visible figure of a body may, by mathematical reasoning, be inferred from its real figure, distance, and position, with regard to the eye: in like manner, we may, by mathematical reasoning, from the visible figure, together with the distance of the several parts of it, from the eye, infer the real figure and position. But this last inference is not commonly made by mathematical reasoning, nor indeed by reasoning of any kind, but by custom.

The original appearance which the colour of an object makes

to the eye, is a sensation for which we have no name, because it is used merely as a sign, and is never made an object of attention in common life: but this appearance, according to the different circumstances, signifies various things. If a piece of cloth, of one uniform colour, is laid so that part of it is in the sun, and part in the shade; the appearance of colour, in these different parts, is very different: yet we perceive the colour to be the same; we interpret the variety of appearance as a sign of light and shade, and not as a sign of real difference in colour. But if the eye could be so far deceived, as not to perceive the difference of light in the two parts of the cloth, we should, in that case, interpret the variety of appearance to signify a variety of colour in the parts of the cloth.

Again, if we suppose a piece of cloth placed as before, but having the shaded part so much brighter in the colour, that it gives the same appearance to the eye as the more enlightened part; the sameness of appearance will here be interpreted to signify a variety of colour, because we shall make allowance for the effect of light and shade.

When the real colour of an object is known, the appearance of it indicates, in some circumstances, the degree of light or shade; in others, the colour of the circumambient bodies, whose rays are reflected by it; and in other circumstances, it indicates the distance or proximity of the object, as was observed in the last section; and by means of these, many other things are suggested to the mind. Thus, an unusual appearance in the colour of familiar objects, may be the diagnostic of a disease in the spectator. The appearance of things in my room, may indicate sunshine or cloudy weather, the earth covered with snow, or blackened with rain. It hath been observed, that the colour of the sky, in a piece of painting, may indicate the country of the painter, because the Italian sky is really of a different colour from the Flemish.

It was already observed, that the original and acquired perceptions which we have by our senses, are the language of nature to man, which in many respects, hath a great affinity to human languages. The instances which we have given of acquired perceptions, suggest this affinity, that as, in human languages, ambiguities are often found, so this language of nature in our acquired perceptions is not exempted from them. We have seen, in vision particularly, that the same appearance to the eye, may, in different circumstances, indicate different things. Therefore, when the circumstances are unknown upon which the interpretation of the signs depends, their meaning must be ambiguous; and when the circumstances are mistaken, the meaning of the signs must also be mistaken.

This is the case in all the phenomena which we call fallacies of the senses; and particularly in those which are called fal

lacies in vision. The appearance of things to the eye, always corresponds to the fixed laws of nature; therefore, if we speak properly, there is no fallacy in the senses. Nature always speaketh the same language, and useth the same signs in the same circumstances: but we sometimes mistake the meaning of the signs, either through ignorance of the laws of nature, or through ignorance of the circumstances which attend the signs.

To a man unacquainted with the principles of optics, almost every experiment that is made with the prism, with the magic lantern, with the telescope, with the microscope, seems to produce some fallacy in vision. Even the appearance of a common mirror, to one altogether unacquainted with the effects of it, would seem most remarkably fallacious. For how can a man be more imposed upon, than in seeing that before him which is really behind him? How can he be more imposed upon, than in being made to see himself several yards removed from himself? Yet children, even before they can speak their mother tongue, learn not to be deceived by these appearances. These, as well as all other surprising appearances produced by optical glasses, are a part of the visual language; and, to those who understand the laws of nature concerning light and colours, are in no ways fallacious, but have a distinct and true meaning.

SECTION XXIV.

OF THE ANALOGY BETWEEN PERCEPTION, AND THE CREDIT WE GIVE TO HUMAN TESTIMONY.

THE objects of human knowledge are innumerable, but the channels by which it is conveyed to the mind are few. Among these, the perception of external things by our senses, and the informations which we receive upon human testimony, are not the least considerable and so remarkable is the analogy between these two, and the analogy between the principles of the mind, which are subservient to the one, and those which are subservient to the other, without further apology we shall consider them together.

In the testimony of nature given by the senses, as well as in human testimony given by language, things are signified to us by signs and in one, as well as the other, the mind, either by original principles or by custom, passes from the sign to the conception and belief of the things signified.

We have distinguished our perceptions into original and acquired; and language, into natural and artificial. Between acquired perception, and artificial language, there is a great ana