horrible crashes, smooth surfaces and lacerating inequalities, pleasant tastes and caustic bitterness, refreshing colour and sable gloom.

In mere sensation, the mind appears to be very nearly passive: when the organ is in a free and healthy state, it is impressed by outward objects without any choice of ours. Whoever walks out into the country, cannot avoid seeing the colour of the grass and the shape of the trees to which his eyes are directed. He has not sensations because he chooses to have them, but they come upon him till he removes the organ, and for a time deprives it of its powers. †

One of the most important branches of this subject of sensation is, the distinction between those sensations which are really derived from the sense itself, and those which are connected with them by mere association. We say we hear a bell ring when in fact it is utterly impossible we should do so, for a bell is an object of sight and touch; and we might as well say that we heard a colour, or heard a thick substance. The fact is, we hear only a sound, which constant experience has led us to refer to a bell as its cause. We smell that something is burning, in the same manner. Burning is an object of sight, and cannot be smelt; but that odour can be smelt which experience has taught us to connect with the phenomena of burning. So that what we are at first apt to consider and to call simple sensations, are in fact accompanied by, and involved with, numberless other sensations, which experience has combined together. Our senses would be comparatively of small importance to us but for these rapid, compound, and indissoluble associations; so that a man becomes to have a sort of sixth sense, compounded of all the others, and exercising in a single act their aggregate perfections.

† [Four pages of manuscript are here wanting.]

A child can hear, and see, and feel, as well as a man; but he exercises these senses without connecting them. with all that their intelligences imply. The case is precisely the same with men skilled in any art or profession, and others ignorant of it; the difference between them is in those intimate associations of sensation which one has formed and the other not. I can see out at sea as well as a sailor; but he pronounces that object to be a three-decked ship in which I can neither distinguish mast, or deck, or anything else. We both see precisely the same thing,—a brown mass of a certain magnitude. It was to him, when first he went to sea, a brown lump also; long experience has taught him, that this is the appearance of a man-of-war. I have had no experience, and it is to me only a simple sensation. I see only the object; he sees the thing signified. There are, in the case of vision, a prodigious variety of sensations which we suppose ourselves to derive from the eye, and which are, in fact, derived from the touch. It will appear very singular to those who have never reflected on these subjects, when I say, that we can neither see the distance. of any objects, nor their size, nor their figure; and yet there is nothing which science has more clearly proved. The eye originally sees nothing but colour and surface. A man born blind and suddenly restored to sight would not have the least conception of the distance of objects; all objects, whether far or near, would appear to be near to his eye. This was long imagined to be the fact, and was afterwards proved to be so, in the memorable case of the young man who was couched by Cheselden. He actually made this mistake, and conceived the pictures on the opposite wall to be quite close to his eye. If the eye can see nothing but colour and surface, why should the alteration of colour and surface give the idea of distance? A colour half as bright, and a surface half as great, do not necessarily imply a distance proportionally greater. We might have been so constituted

F

as that an object should have become fainter the nearer it approached. The fact is, we have determined by experience that these signs to the eye, of fainter colour and diminished surface, are inseparably connected with distance, and that bodies are nearer to the touch when they are brighter to the eye: therefore the moment we see brightness we think of proximity, and so imagine we see that a thing is near; and the moment the colour becomes confused we think of remoteness, and so imagine we see that a thing is remote. It is by rendering colour more languid and confused, that painters can represent objects at a very different distance upon the same flat canvas. The mere diminution of the magnitude of an object would not have the effect of making it appear at a greater distance. For if, in a cattle piece, the artist were to make one cow ten times as little as all the rest, the animal would by no means appear ten times as distant from the eye, but would be taken for a calf in the foreground instead of a cow in the distant scenery.

Dr. Reid quotes a very curious observation made by Bishop Berkeley in his travels through Italy and Sicily, which, by the by, I rather believe he performed on foot. He observed that, in those countries, cities and palaces seen at a great distance appeared to him nearer by several miles than they really were; and he very judiciously imputed it to this cause,-that the purity of that air gave to very distant objects a degree of brightness and distinctness which, in the grosser air of his own country, belonged only to those which are near. It would be curious to know whether Italians are apt to make the reverse of the Bishop's observation in this country, and to ascertain what the apparent distance is, according to their estimation, from London to Kensington, during a thick fog in this pleasant month of December. This mode of discovering distance by the distinctness or indistinctness of colour, is the reason why we mistake the size of objects in a fog. A little gentleman who

understands optics may always be sure to enjoy a temporary elevation in a fog; and by walking out in that state of the weather, will be quite certain of being taken for a man six feet high; for the indistinctness of colour first makes us consider him to be at a much greater distance than he really is, and then a man who appears so big at the supposed distance of 300 yards we cannot but judge to be one of the tallest and most robust of men. Secondly, another mode in which we determine. the distance of objects is by changing the form of the eye. Nature has given us the power of adapting this organ to certain distances by contracting one set of muscles, and to other distances by contracting another set. As to the manner in which this is done, anatomists are not agreed; but whatever be the manner, it is certain that young people have commonly the power of adapting their eyes to all distances of the object, from six or seven inches to fifteen or sixteen feet, so as to have perfect and distinct vision at any distance within these limits. Now, place an object at the distance of six inches from the eye, and gradually remove it to sixteen or seventeen feet, you will find that all the muscles of the eye are employed all that time in altering the shape of the eye, and accommodating it to different distances; so that, by long experience, the efforts I am compelled to make in order to see at these different distances become themselves the signs of these distances; and if any person were wounded in these muscles about the eye, so as to disturb his usual offorts to obtain distinct vision, he would lose his guide of distance, and become unable to see as well as before, though precisely the same appearances would be presented to his eye.

A third mode by which we acquire the notion of distance is, the inclination of the eyes towards each other. A line drawn through the centre of the eye to the retina, and produced beyond it, is called the axis of the eye; and it is plain that the inclination of these lines.

towards each other must vary as the distance of the objects varies towards which they are directed. Of this inclination we are not conscious; but we are conscious of the effort employed in making it; and this effort, as well as the others of which I have been last speaking, becomes the sign of the distance of objects. It is for this reason that those who have lost the sight of one eye are apt, even within arm's length, to make mistakes in the distance of objects which are easily avoided by those who see with two eyes; though, after some time, in persons blind of one eye, this inclination of the axes ceases to be a criterion of distance, and these mistakes. are avoided. This inclination of the optic axes is the principal obstacle to complete deception in the art of painting. The colouring (one mode by which we determine distance) may be perfect, and may give us the notion of an object being at the distance of many miles; but, unfortunately, the figure of the eye, and the inclination of the axes, are set for the distance of two or three yards (the real space between the eye and the picture), so that the mind, wanting one of its signs of distance, is far from being completely deceived. In order to remove this defect, connoisseurs in painting, look at a picture with one eye, through a tube, which excludes the view of all other objects. By this means, the inclination of the eyes towards each other (one method by which we judge of the deception) is prevented. Dr. Reid proposes, as an improvement, this method, -that the aperture of the tube next the eye should be as small as a pin-hole; because then the other mode of judging of distances, the conformation of the eye, is avoided, and we have no means left of judging of the distances but the light and the colour, which are in the power of the painter. When the optic axes are, on account of the great distance of objects, nearly parallel, so that to look at an object still more distant requires no fresh effort, our power of judging of distances entirely ceases.