incessantly occupied in measuring angles; for if this measurement were truly instinctive it would occur, in infancy, as in maturity, and be immediate, in those who have acquired the power of vision, by that surgical operation to which I alluded in my last Lecture. But the most decisive of all considerations, with respect to this supposed geometry, is, that the angles, subtended by the object at its different distances, and the inclination of the optic axis, in the spontaneous accommodation of the eyes to the distinct vision of the object at different distances, though truly existing, to the mere optical examiner of the object, and the light, and the eye, as one compound phenomenon, have no real existence, as feelings of the mind, of the individual who sees, and are known but to very few of the immense multitudes, who, without the slightest acquaintance with geometry, or the slightest knowledge of the very lines whose angles they are supposed to measure, are yet able to distinguish the distances of objects as accurately as the most expert mathematician. How is it possible that the angles, which remote objects make relatively to the eye, should be known originally, when the remote objects themselves are not known, but merely the points of light on the retina? In relation to the eye, as the organ, and to the mind as originally sentient in vision, these points of light were truly all that existed. The light, indeed, traversed a certain space, in passing from the object to the eye, and the lines of direction of the different rays, in arriving at one focal point at the retina, formed truly different angles. But the angles could not be known, unless the radiant lines themselves were known; and of these, the mind could have no knowledge. During the whole time of their convergence, till they reached the expansion of the optic nerve, the rays of light were as little capable of producing vision, as darkness itself; and, when they reached the retina, the lines, and consequently the angles, existed no more. Of whatever use, therefore, such angles may be to the optician, in laying down, and illustrating the principles of his science, they are of no use in the actual living measurements of sight. Man may reason, indeed, but he must reason from what he knows; and, therefore, if the determination of distance be the result of any judgment, it must be of a judgment formed from feelings which truly have, or have had existence.

Such feelings, the elements of our visual judgments, it is not very difficult to discover.

The great principle, in this case, is the principle of association, by which the notions derived from touch,-or, at least, the notions which are commonly supposed to be derived from that sense, are suggested immediately by the visual feelings which co-existed with the sensations of touch; in the same manner, as the words of a language, when a language has been fully learned, suggest whatever the words may have been used to denote. Å child, whose eye has already learned to distinguish objects, hears the word cup frequently repeated, when a cup is held before him; and the word afterwards suggests the thing. This process every one understands. But we are not equally aware, that, in the prior stage of learning to distinguish the cup by the eye, the child went through a process exactly similar,-that the visual feeling, which the rays of light from the cup excited, co-existed with the tactual and muscular feeling, when he handled the cup; and that the one feeling was thus associated, for ever after, with the other.

The means by which we acquire our knowledge of the distance of objects, may be reduced to three,- -the difference of the affections of the optic nerve, VOL. I.

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-the different affections of the muscles employed in varying the refracting power of each eye, according to the distance of the objects, and in producing that particular inclination of the axes of the two eyes, which directs them both equally on the particular object,-and, thirdly, the previous knowledge of the distance of other objects, which form, with that which we are considering, a part of one compound perception.

To begin, then, with the affections of the retina. These become signs of distance, in two ways, by the extent of the part of the retina affected, and by the more or less vivid affection of the part.

It is evident, from the laws of optics, that, according to the distance of the object from the eye, there must, when all other circumstances are the same, be a difference of the extent of the retina, on which the light falls. This illuminated portion of the nervous expanse, as supposed to be instantly perceived, is what is termed the visible figure of an object; and, though I am disposed to question the knowledge, which the mind is believed to acquire of this figure, from the mere sensation of colour, to which the affection of the retina gives rise,-I am far from denying, that the sensation itself, whatever it may originally be, will be different according to the extent of the retina affected, as the sensation of heat is different, according to the extent of the surface, which has grown warmer or colder,—or of fragrance, according as a small number of odorous particles have acted on a portion of the surface of the organ of smell, or a greater number of these on a greater portion of that surface. The different feelings, then, when more or less of the retina has been affected, are capable of being associated with other feelings, which may co-exist with them. An object, held at the distance of a foot from the eye, affects one part of the retina,-held at arm's length, it affects less of the retina; and this difference, not indeed as perceived in figure, but as perceived in the variety, whatever that may originally be, of the resulting sensation, being found constant and uniform, becomes of itself significant of the distance. Another mode, in which the affection of the retina becomes significant of distance, is by the brightness or dimness of the visible figure, and its distinctness or indistinctness of outline; or, as I would rather say, by the peculiar sensations, without regard to figure, which accompany those varieties of light. Since, at a distance, less light falls from objects on the eye, and their outline becomes less definite, a new measure is thus obtained, in addition to that which is derived from the mere difference in extent of the retina affected. In the illusion of this spontaneous measurement, consists the chief magic of the painter's art. By different shades of colour, he produces corresponding perceptions of distance; and thus, making one part of a plain surface seem more remote than another, converts it, as far as the mere eye can judge, into a cube or sphere, or any other solid, which he chooses to present to us. By the indistinct outline which he gives to his small figures, in the back-ground of a landscape, he leads us to consider them, not as diminutive in themselves, which we should conceive them to be, if, with equal smallness, their outline were clearer, but merely as less or more remote. He is thus able to vary his figures in three ways, to make them larger or smaller, more or less bright, and more or less precisely defined; and, by uniting these varieties, in various proportions, to distinguish not merely what is large from what is small, but the diminutive from the distant, and the gigantic from the

near.

Accordingly we find, that, in circumstances, in which the medium of

transmission of light from objects is much altered, our perception of distance and magnitude becomes less accurate. In a fog, objects appear to us greatly magnified; because, the effect produced on the retina, in the extent of the visible figure and its dimness and indefinite outline, is truly the same, as when a larger object, in the common state of the atmosphere, is seen by us at a distance. From the same principle, objects seen under a brighter sky, and in purer air, seem nearer than they really are, to those, whose notions of distance have been acquired in a less happy climate. This has been remarked, by travellers in Italy, and particularly by one of the most illustrious of those who have visited that beautiful country,-a traveller, whose attention had been particularly turned to observations of this sort. The very acute observer, of whom I speak, is Berkeley, in whose Theory of Vision there is to be found a very interesting section, in which he at once describes this impression, and accounts for it.

Our affections of the retina, then, both in the extent of the nervous expansion affected, and in the species of affection, afford one set of feelings, with which the notion of distance may be associated in the same manner as the sounds or visual characters of a language may be associated with the conceptions which they denote, or any other feelings with any other feelings.

The next set of feelings which we have to consider, in relation to our perception of distance, belong to a class, of the importance of which I have had frequent occasion to speak, the muscular feelings, in the contraction of those muscles, which adapt the nice refracting apparatus in each eye, to the degree of refraction, necessary for distinct vision in the particular case, and produce that inclination of the axis of vision to each other, which is necessary for directing both eyes equally on the object. The muscular feeling may be slight indeed, but still it is sufficient to modify, in some degree, the whole compound sensation of the moment. One degree of contraction is attended with a particular feeling; another degree with a different feeling; and, as there are various muscles, subservient to the motions of the eyes, some of which are exerted, while others are quiescent,-the feeling, it is evident, must vary, not with the degree of contraction merely, but also with the muscles contracted. A certain muscular feeling, however simple or complex, accompanies the mere visual sensation, and blends with it; and it is with this compound feeling, muscular and visual, that the notion of distance is associated.

The muscular adaptation, however, it may be remarked, seems, in a great measure, to imply the very knowledge which it is supposed to give; since we cannot, instantly and voluntarily, adapt our eyes to the state necessary for distinct vision, at a particular distance, unless we have previously known that particular distance. The necessary adaptation, however, if it be not the result of a rapid change of various degrees of contraction in each particular case, may depend, not on our knowledge and will, but on an instinctive connexion of certain motions with certain feelings, in which there is as little consciousness of design, as in that very analogous instinct, or connexion of motions with feelings, which increases or diminishes the diameter of the pupil, according to the quantity of light which is poured upon the eye, when the individual, far from willing the contraction, does not know even that such a contraction has taken place.

A third element, in the calculation of the distance of an object, is the previous knowledge of the distance of other objects, which form together with it

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one compound perception. Thus, when we look along a road, and observe a man on horseback, who has nearly approached a house which we know, we have of course little difficulty in determining the distance of the rider. Every one must have felt how much easier his judgments of the distance of moving objects are, in scenes with which he is in some degree acquainted, than in a country which is new to him; and what aid the interposition of a variety of objects gives, even though we may not be well acquainted with the exact extent and distance of each. To an inexperienced eye, therefore, in a first voyage, a ship at a distance seems far nearer than it truly is, from the absence of varied intervening objects in the line between. Even in the case of a river, which is not so broad as to prevent us from distinguishing objects on the opposite side, it is with great difficulty that we attempt to guess the distance, with any approach to exactness. There is a constant tendency to suppose the breadth of the river less than it is, and consequently the objects on the opposite bank nearer than they are. For the same reason, the horizontal line, in which innumerable objects intervene between the eye and the horizon, appears so much longer than the line of altitude of the meridian, that the vault of the sky does not seem a hemisphere, but a far smaller segment of a great sphere. On this subject, however, rich as it is in illustration, my time will not allow me to dwell longer. But I regret this the less, as the subject is one of those, which in the department of optics, come under the consideration of one of my colleagues, whose happy genius has the art of describing fully what the narrow compass of his lectures, may have obliged him to state briefly; and who leaves little for others to add, even on subjects to which he alludes only for incidental illustration.

These very few slight remarks, however, will be sufficient to show, in what manner the notion of distance may be associated with mere visual feelings, that in themselves originally involve no notion of distance, as the words of a language, which, in themselves, either as sounds or characters, involve no relation to one object more than to another, become instantly significant of particular objects, and excite emotions of love or joy, or hate, or indignation, like the very presence of some living friend or foe.

It has been very justly remarked, that, if all men had uniformly spoken the same language, in every part of the world, it would be difficult for us not to think that there is a natural connexion of our ideas and the words which we use to denote them; and it is not wonderful, therefore, that a similar illusion should take place with respect to what may be termed the universal language of vision; since, in the case of visual perception, all men may be truly said to have the same language; the same sensations of sight, being to all significant of magnitude and distance. And it is well that the judgments which we form, on these important points, are thus prompt and spontaneous; for, if we had to wait till we had calculated the distance and magnitude of every thing around us, by a measurement of angles, we should be cut off, in our optical career, before we could with all our geometry, determine, with precision, whether the things which we needed most, or the objects of greatest peril to us, were ten or a thousand paces distant, and whether they were of the bulk of a molehill or of a mountain.

A miniature image of the objects which we see, is pictured on the retina, in an inverted position; and though an image is pictured in each eye, we see not two objects but one. To philosophers, who are even more expert in finding mysteries than in solving them, this single vision of the erect ob

ject, from a double image of the object inverted, has usually seemed very mysterious; and yet there is really nothing in it at all mysterious, to any one who has learned to consider how much of the visual perception is referable to association. If the light, reflected from a single object touched by us, had produced not two merely, but two thousand separate images in our eyes, erect or inverted, or in any intermediate degree of inclination, the visual feeling, thus excited, however complex, would still have accompanied the touch of a single object; and if only it had accompanied it uniformly, the single object would have been suggested by it, precisely in the same manner as it is now suggested by the particular visual feeling that attends the present double inverted image. To this supposed anomaly in the language of vision, a perfect analogy is to be found in the most obvious cases of common language. The two words he conquered excite exactly the same notion as the single Latin word vicit; and if any language were so paraphrastic as to employ ten words for the same purpose, there would be no great reason for philosophic wonder at the unity of the notion suggested by so many words. The two images of the single object, in the arbitrary language of visual perception, are, as it were, two words significant of one notion.

Whatever the simple original sensation of vision may be, then, it is capable of being associated with other notions, so as to become significant of them. But to what does the simple original sensation itself amount? Is it mere colour, or is it something more?

The universal opinion of philosophers is, that it is not colour merely which it involves, but extension also,-that there is a visible figure, as well as a tangible figure-and that the visible figure involves, in our instant original perception, superficial length and breadth, as the tangible figure which we learn to see, involves length, breadth, and thickness.

That it is impossible for us, at present, to separate, in the sensation of vision, the colour from the extension, I admit though not more completely impossible, than it is for us to look on the thousand feet of a meadow, and to perceive only the small inch of greenness on our retina; and the one impossibility, as much as the other, I conceive to arise only from intimate association, subsequent to the original sensations of sight. Nor do I deny, that a certain part of the retina,-which, being limited, must therefore have figure, -is affected by the rays of light that fall on it, as a certain breadth of nervous expanse is affected in all the other organs. I contend only, that the perception of this limited figure of the portion of the retina affected, does not enter into the sensation itself, more than in our sensations of any other species, there is a perception of the nervous breadth affected.

The immediate perception of visible figure has been assumed as indisputable, rather than attempted to be proved,-as, before the time of Berkeley, the immediate visual perception of distance, and of the three dimensions of matter, was supposed, in like manner, to be without any need of proof;and it is, therefore, impossible to refer to arguments on the subject. I presume, however, that the reasons, which have led to this belief, of the immediate perception of a figure termed visible, as distinguished from that tangible figure, which we learn to see, are the following two, the only reasons which can even imagine,-that it is absolutely impossible, in our present sensations of sight, to separate colour from extension,-and that there are, in fact, a certain length and breadth of the retina, on which the light falls.

With respect to the first of these arguments, it must be admitted, by those