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

by Thomas Brown

VISION.

Our inquiry into the nature of the sensations of touch,—or at least of those sensations, which are truly, and of others which are commonly, though, I think, falsely, ascribed to this organ, has led us into speculations, in the course of which I have been obliged to anticipate many remarks, that more peculiarly belong to the sense which still remains to be considered by us,—the sense of sight, that to which we owe so much of our most valuable information, with respect to nature, and so many of those pleasures, which the bounty of Him, who, has formed us to be happy, as well as to be wise, has so graciously intermingled, with all the primary means of our instruction.

The anticipations, into which I have been led, were necessary for throwing light on the subjects before considered, particularly on the complex feelings ascribed to touch,—the knowledge of which feelings, however, was still more necessary, for understanding fully the complex perceptions of this sense. It is thus scarcely possible, in science, to treat of one subject, without considering it in relation to some other subject, and often to subjects between which, on first view, it would be difficult to trace any relation. Every thing throws light upon every thing,—though the reflection,—which is, in many cases, so bright, as to force itself upon common eyes—may, in other cases, be so faint, as to be perceptible only to eyes of the nicest discernment. It may almost be said, that there is an universal affinity in truths,—like that universal attraction, which unites to each other, as one common system, the whole masses which are scattered through the infinity of space, and by which, as I have before remarked, the annihilation of a single particle of matter, in any one of these orbs,—however inconceivably slight its elementary modification might be of the general sum of attraction,—would in that very instant be productive of change throughout the universe. It is not easy to say, what any one science would have been, if any other science had not existed. How different did Astronomy become, in consequence of the accidental burning of a few sea-weeds upon the sand, to which the origin of glass has been ascribed; and, when we think of the universal accessions, which navigation has made to every department of knowledge, what an infinity of truths may be considered as almost starting into existence, at the moment, when the polarity of the magnet was first observed!

“True to the pole, by thee the pilot guides

His steady helm, amid the struggling tides,

Braves with broad sail the unmeasurable sea,

Cleaves the dark air, and asks no star but thee.”[115]

The anticipations, which have been made, in the present instance, will be of advantage, in abridging much of the labour, which would have been necessary in treating of vision simply. I may now safely leave you, to make, for yourselves, the application of many arguments, on which I have dwelt at length, in treating of the other senses.

The organ of sight, as you well know, is the eye,—a machine of such exquisite and obvious adaptation to the effects produced by it, as to be, of itself, in demonstrating the existence of the Divine Being who contrived it, equal in force to many volumes of theology. The atheist, who has seen, and studied, its internal structure, and yet continues an atheist, may be fairly considered as beyond the power of mere argument to reclaim. The minute details of its structure, however, belong to the anatomist. It is enough for our purpose to know, that, by an apparatus of great simplicity, all the light, which, from every quarter strikes on the pellucid part of the ball of the eye,—and which if it continued to pass in the same direction, would thus produce one mingled and indistinct expanse of colour,—is so refracted, as it is termed, or bent from its former direction to certain focal points, as to be distributed again on the retina, in distinct portions, agreeing with the portions which come from each separate object, so exactly, as to form on it a miniature landscape of the scenery without. Nor is this all. That we may vary, at our pleasure, the field of this landscape, the ball of the eye is furnished with certain muscles, which enable us to direct it more particularly toward the objects which we wish to view; and according as the light which falls from these may be more or less intense, there are parts which minister to the sensibility of the eye, by increasing or diminishing in proportion the transparent aperture at which the light is admitted. There are, then, in this truly wonderful and beautiful process, in the first place, as determining what objects, in the wide scene around us, are to be visible at the moment, the contraction of certain muscles, on which the particular field of our vision depends, and which may almost be said to enable us to increase the extent of our field of vision, by enabling us to vary it at will;—in the second place, the external light, emitted from all the objects within this radiant field, which, on its arrival at the retina, is itself the direct object of vision; in the third place, the provision for increasing or diminishing the diameter of the pupil, in proportion to the quantity of that incident light;—in the fourth place, the apparatus, by which the dispersed rays of light are made to assume within the eye, the focal convergence necessary for distinct vision;—and lastly, the expansion of the optic nerve, as a part of the great sensorial organ, essential to sensation. The difference of the phenomena, produced by the varieties of the external light itself, is exhibited in almost every moment of our waking existence; and the diversities, arising from other parts of the process, are not less striking. There are peculiar diseases which affect the optic nerve, or other parts of the sensorial organ immediately connected with it,—there are other diseases which affect the refracting apparatus,—others which affect the iris, so as to prevent the enlargement or diminution of the pupil, when different quantities of light are poured on it,—others, which affect the muscles that vary the position of the ball,—and, in all these cases, we find, as might be expected, a corresponding difference of the phenomena.

To open our eyes at present, is not to have a single simple feeling; it is, as it were, to have innumerable feelings. The colour, the magnitude, the figure, the relative position of bodies, are seen by us at once. It is not a small expanse of light, which we perceive, equal merely to the surface of the narrow expansion of the optic nerve. It is the universe itself. We are present with stars, which beam upon us, at a distance that converts to nothing the whole wide diameter of our planetary system. It is as if the tie, which binds us down to the globe on which we dwell, belonged only to our other senses, and had no influence over this, which, even in its union with the body, seems still to retain all the power, and unbounded freedom, of its celestial origin.

It is of importance, however, to remember, that, even in the perception of the most distant body, the true object of vision is not the distant body itself, but the light that has reached the expansive termination of the optic nerve; and the sense of vision, therefore, which seems so independent of the tie that binds us to our small spot of earth, is as truly limited to it, as any of our other senses. If the light could exist in the same manner, moving in the same varieties of direction, as at present,—though no other bodies were in existence, than the light itself, and our sensorial organ,—all the sensations belonging to mere sight would be exactly the same as now; and accordingly we find, as light is, in a great measure, manageable by us, that we have it in our power to vary at pleasure, the visual notions, which any one would otherwise have formed of bodies,—without altering the bodies themselves, or even their position with respect to the eye,—by merely interposing substances, to modify the light reflected or emitted from them. The same paper, which we term white, when we observe it with our naked eye, seems blue or red, when we look at it through glass, of such a kind, as absorbs all the light which enters it, but the rays of those particular colours; and it seems larger or smaller, as we look at it through a concave or convex lens, which leaves the object precisely as it was, and affects only the direction of the rays that come from it:—the reason of all which diversities of perception is, that, though what we are accustomed to term the object continues the same,—whatever substance may be interposed between it and the eye,—that, which is really the object of vision, is different; and our perceptions, therefore, correspond with the diversity of their real objects.

In treating of the distinction which has been made, of those objects of sense which act directly on our organs, and of those which act through a medium, as it has been termed, I before remarked to you the confusion, into which we might be led, by this distinction, which forgets that the supposed medium is itself the real object, as truly, as any of the objects, which in their relations to other senses, are termed direct. In no instance, however, has it led to so much confusion, as in the case of vision. It is the more important, therefore, for you, to have precise notions on this subject, and to have constantly in mind, that, though indirectly, we may be said to perceive by sight distant objects, as truly as we perceive colour, still the direct object of vision is not the object, existing permanently at a distance, but those rays of light, whose existence is independent of the object, and which have received, from the object that reflects them, nothing more than a change of their direction, in consequence of which they have come within the boundary of that small pellucid circle of the eye, which, insignificant as it may seem, comprehends in itself what is truly the whole sphere of our vision.

Sight, then, which comprehends all the varieties of colour, is the object, and the only object, of the sense which we are considering. But, simple as it is, of what instruction, and joy, and beauty, and ever-varying magnificence, is it the source!

“Carmine quo Dea te dicam, gratissima coeli

Progenies, ortumque tuum; gemmantia rore

Ut per prata levi lustras, et floribus halans

Purpureum Veris gremium, scenamque virentem

Pingis, et umbriferos colles, et cærula regna?

Gratia te Venerisque lepos, et mille colorum,

Formarumque chorus sequitur, motusque decentes.

At caput invisum Stygiis Nox atra tenebris.

Abdidit, horrendæque simul Formidinis ora

Pervigilesque æstus Curarum, atque anxius Angor;

Undique Lætitiâ florent mortalia corda,

Purus et arridet largis fulgoribus Æther.”[116]

“Hail, holy light, offspring of heaven first born!

Or of the Eternal, coeternal beam,

May I express thee unblam'd? since God is light,

And never but in unapproached light

Dwelt from eternity; dwelt then in Thee,

Bright Effluence of bright Essence increate!

—Or hear'st thou rather, pure ethereal Stream!

Whose fountain who shall tell? Before the Sun,

Before the heavens, Thou wert, and at the voice

Of God, as with a mantle didst invest

The rising world of waters dark and deep,

Won from the void and formless infinite.”[117]

How pathetic is the very beauty of this invocation, when we consider the feelings with which it must have been written by him, who,

“Like the wakeful bird,

Sung darkling,”[118]

and who seems to have looked back on that loveliness of nature, from which he was separated, with the melancholy readiness, with which the thoughts of the unfortunate and the sorrowful still revert to past enjoyments; as the prisoner, even when fettered to his dungeon-floor, still turns his eye, almost involuntarily, to that single gleam of light, which reminds him only of scenes that exist no longer to him.

“Thus with the year

Seasons return; but not to me returns

Day, or the sweet approach of even or morn,

Or sight of vernal bloom, or summer's rose,

Or flocks, or herds, or human face divine;

But cloud instead, and ever-during dark

Surround me.”[119]

How often must he have felt,—and how deeply must such a mind have felt,—the force of that complaint, which he puts into the mouth of Samson,—a complaint, which may surely be forgiven, or almost forgiven, to the blind:—

“O why was sight

To such a tender ball as the eye confined,

So obvious, and so easy to be quench'd;

And not, as feeling, through all parts diffused,

That she might look at will through every pore?”[120]

The immediate object of vision, we have seen, then, is light, which gives rise to all the various sensations of colour; and, since the days of Berkeley, philosophers have, with scarcely any exception, admitted, that the knowledge of the distance, magnitude, and real figure of objects, which seems at present to be immediately received by sight, is the result of knowledge acquired by the other senses:—though they have,—I think without sufficient reason,—as universally supposed, that the superficial extension, of length and breadth, becomes known to us by sight originally;—that there is, in short, a visible figure of objects, corresponding with the picture which they form on the retina, and changing, therefore, with their change of position relatively to the eye,—and a tangible figure of objects, permanent and independent of their change of place; the latter being the real figure suggested by the former, nearly in the same manner as the conception of objects is suggested, by the arbitrary sounds, or written character, which denote them. The inquiry, with respect to the truth of this visible figure, as a sensation, may, however, be omitted, till we have considered the former opinion, which respects the visual perception of distance, and of the figure and magnitude which are termed tangible.

If it had been duly considered, that it is light which is the true object of vision, and not the luminous body, the question, as far as it depends on reasoning a priori, exclusively of any instinctive connexions that might be supposed, could not have admitted of very long discussion. From whatever distance light may come, it is but the point of the long line which terminates at the retina, of which we are sensible; and this terminating point must be the same, whether the ray has come from a few feet of distance, or from many miles. The rays, that beam from the adjacent meadow, or the grove, are not nearer to my eye, at the instant of vision, than those which have been reflected from the mountain, on the very verge of the horizon, or from the cloud that hangs at an immeasurable distance above my head. The light, that converges on our eye, from all the stars of heaven, within what we term the field of our vision, is collected, in a space, that cannot be larger than the retina on which it falls. A cube or a sphere is represented to us, by the two dimensions of a coloured plane, variously shaded, as truly, as by the object itself with its triple dimensions; and, in the determination of the exact correspondence of these double and triple dimensions, in all their varieties of relation to the eye, the whole art of perspective consists. A coin of a single inch in diameter, when placed before the eye, and, of course, intercepting only an extent of light equal to the extent of its own surface, is sufficient to hide from us, by actual eclipse, the fields, and villages, and woods, that seemed stretched in almost endless continuity before us.

Unless, therefore, there be some instinctive and immediate suggestion, of certain distances, magnitudes, and figures, by certain varieties of the sensation of colour, there is nothing in the mere light itself, or in its relation to the eye at the moment of vision, which seems fit to communicate the knowledge of these. Not of distance; for the rays from distant objects, when they produce vision, are as near to the retina, as the rays from objects that are contiguous to the eye. Not of real magnitude; for an object, with which we are familiar, appears to us of the same size, at distances, at which every thing merely visual is so completely changed, that its magnitude, as far as it depends on mere radiation, may be demonstrated, from the laws of optics, to be equal only to a half, or a tenth part of its apparent magnitude, when nearer. Not of figure; for, without the knowledge of longitudinal distance, we could not distinguish a sphere or a cube from a plane surface of two dimensions; and an object, with the shape of which we are familiar, appears to us of the same form, in all directions; though it may be demonstrated optically, that the visual figure, as far as it depends on mere radiation, must vary with every variety of position.

I have said, that the knowledge of the real magnitude, figure, and position of bodies, could not be obtained immediately from the diversities of the mere surfaces of light at the retina; unless it were the suggestion of some instinctive principle, by which the one feeling was, originally and inseparably, connected with the other: and I have made this exception, to prevent you from being misled, by the works on this subject, so as to think, that the original perception of distance implies, in the very notion of it, a physical impossibility. Some diversity there evidently must be of the immediate sensation of sight, or of other feelings coexisting with it, when a difference of magnitude or figure is suggested: the visual affection, which is followed by the notion of a mile, cannot be the same as that which is attended with the notion of half a foot; nor that which is attended with the perception of a sphere, be the same as that which suggests a plane circular surface. Whatever the number of the varied suggestions of this kind may be, there must be, at least, an equal variety of the immediate sensations that give rise to them; and these corresponding series of sensations and suggestions, may originally be associated together by an instinctive principle, as much as any other pairs of phenomena, the connexion of which we ascribe to instinct; or, in other words, suppose an adaptation of them to each other, by the gracious provision of the Power which formed us, for a purpose unforeseen by us, and unwilled at the moment. It is not more wonderful, a priori, that a sensation of colour should be immediately followed by the notion of a mile of distance, than that the irritation of the nostril, by any very stimulant odour, should be immediately and involuntarily, followed by the sudden contraction of a distant muscular organ, like the diaphragm, which produces, in sneezing, the violent expiration necessary for expelling the acrid matter;—or that an increase of the quantity of light poured on the eye, should be instantly, and without our consciousness, followed by a contraction of the transparent aperture. I am far from saying, that there truly is such an instinctive association of our original visual feelings, with corresponding notions of distance and magnitude, in the present case; for, at least in man, I believe the contrary. I mean only, that the question has, a priori, only greater probability on one side, not absolute certainty; and that experience is necessary, before we can decide it with perfect confidence.

In the case of the other animals, there seems to be little reason to doubt, that the tedious process, by which man may be truly said to learn to see, is not necessary for their visual perceptions. The calf, and the lamb, newly dropt into the world, seem to measure forms and distances with their eyes, as distinctly, or at least almost as distinctly, as the human reasoner measures them, after all the acquisitions of his long and helpless infancy. Of these races of our fellow animals, Nature is as once the Teacher and the great Protectress,—supplying to them, immediately, the powers which are necessary for their preservation,—as, in the long continued affection of the human parent, she far more than compensates to man, the early instincts which she has denied to him. If the other animals had to learn to see, in the same manner with ourselves, it would be scarcely possible, that their existence should be preserved to the period, at which the acquisitions necessary for accurate perception could be made; even though the hoof had been an instrument of touch and measurement, as convenient as the hand. For this difference in the relative circumstances of their situation, the Almighty Being,—to whose universal benevolence, nothing which he has created is too humble for his care,—has made sufficient provision, in giving them that early maturity, which makes them, for many months, the superiors of him, who is afterwards to rule them with a sway, that is scarcely conscious of effort.

“Hale are their young, from human frailties freed,

Walk unsustained, and, unsupported, feed.

They live at once,—forsake the dam's warm side,—

Take the wide world, with nature for their guide,—

Bound o'er the lawn, or seek the distant glade,

And find a home in each delightful shade.”[121]

This instinctive suggestion, which, however subsequent it may be to the primary visual sensation, seems like immediate perception in the young of other races of animals, is a very strong additional proof, if any such were necessary, that there is no physical impossibility, in the supposition that a similar original suggestion may take place in man. The question, as I before said, becomes truly a question of observation and experiment.

But, in man, there is not that necessity for the instinct, which exists in the peculiar situation of the other animals; and we find accordingly, that there is no trace of the instinct in him. It is long before the little nurseling shews, that his eye has distinguished objects from each other, so as to fix their place. We are able almost to trace in his efforts the progress which he is gradually making;—and, in those striking cases, which are sometimes presented to us, of the acquisition of sight, in mature life, in consequence of a surgical operation,—after vision had been obstructed from infancy,—it has been found, that the actual magnitude and figure, and position, of bodies, were to be learned like a new language,—that all objects seemed equally close to the eye,—and that a sphere and a cube, of each of which the tangible figure was previously known, were not so distinguishable in the mere sensation of vision, that the one could be said, with certainty, to be the cube, and the other the sphere. In short, what had been supposed, with every appearance of probability, was demonstrated by experiment,—that we learn to see,—and that vision is truly, what Swift has paradoxically defined it to be, the art of seeing things that are invisible.