Lectures on the Philosophy of the Human Mind (Vol. 1 of 3)

by Thomas Brown

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 coexist 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 painters art. By different shades of colour, he produces corresponding perceptions of distance; and thus, making one part of a plane 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 chuses 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 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 few very 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 mole hill or of a mountain.