Brown Thomas
Lectures on the Philosophy of the Human Mind (Vol. 1 of 3)
LECTURE XXIX
ANALYSIS OF THE FEELINGS ASCRIBED TO VISION, CONTINUED
The chief part of my last Lecture was employed in considering the Phenomena of Vision, and particularly in proving, that vision, – simple and immediate, as it now seems to us, even in its most magnificent results, – is truly the application of an art, of long and tedious acquirement, – of that art with which we learn to measure forms and distances, with a single glance, by availing ourselves of the information, previously received from other sources; – the mixed product of innumerable observations, and calculations, and detections of former mistakes – which were the philosophy of our infancy, and each of which, separately, has been long forgotten, – recurring to the mind, in after-life, with the rapidity of an instinct.
Of all the arts, which man can acquire, this is, without question, the richest, both in wonder and in value – so rich in value, that if the race of man had been incapable of acquiring it, the very possibility of their continued existence seems scarcely conceivable; and so rich in subjects of wonder, that to be most familiar with these, and to study them with most attention, is to find at every moment new miracles of nature, worthy of still increasing admiration.
“Per te quicquid habet mundus, mirabile nobis,
Panditur; acceptumque tibi decus omne refertur
Terrarum. Gentes nequicquam interluit æstu
Vicinas pelagus; tu das superare viarum
Ardua, et obtutu Seston conjungis Abydo.
Necmaris angusti tantum discrimina solers
Decipis, oceanique moras; Tu sidera Cœli
Subjicis humanis oculis, et dissita longe
Das spectare loca, et Dias invisere sedes.
Nativa hinc quamvis ferimur gravitate deorsum
Ad Stygias sedes, Ditisque inamabile regnum, —
Mente tamen sursum rapti ad sublimia; molem
Exuimus terrenam, animosque æquamus Olympo.”122
On this subject the remarks of Dr Reid, which I am about to quote, are not less just than they are strikingly expressed. “If we shall suppose an order of beings, endued with every human faculty but that of sight, how incredible would it appear to such beings, accustomed only to the slow informations of touch, that, by the addition of an organ, consisting of a ball and socket of an inch diameter, they might be enabled in an instant of time, without changing their place, to perceive the disposition of a whole army, or the order of a battle, the figure of a magnificent palace, or all the variety of a landscape? If a man were by feeling to find out the figure of the peak of Teneriffe, or of even St Peter's Church at Rome, it would be the work of a lifetime.
“It would appear still more incredible to such beings as we have supposed, if they were informed of the discoveries which may be made by this little organ in things far beyond the reach of any other sense: That by means of it we can find our way in the pathless ocean; that we can traverse the globe of the earth, determine its figure and dimensions, and delineate every region of it. Yea, that we can measure the planetary orbs, and make discoveries in the sphere of the fixed stars.
“Would it not appear still more astonishing to such beings, if they should be further informed, That, by means of this same organ, we can perceive the tempers and dispositions, the passions and affections of our fellow-creatures, even when they want most to conceal them? That when the tongue is taught most artfully to lie and dissemble, the hypocrisy should appear in the countenance to a discerning eye; And that by this organ, we can often perceive what is straight and what is crooked in the mind as well as in the body? – How many mysterious things must a blind man believe, if he will give credit to the relations of those that see! Surely he needs as strong a faith as is required of a good Christian.”123
The same observation has been put in a strong light, by the supposition, that it had been as uncommon, to be born with the power of sight, as it is now to be born incapable of it; – in which case it has been truly said, that “the few who had this rare gift would appear as prophets or inspired teachers to the many.”124 The very easy predictions thus made, would be found, constantly, or almost constantly fulfilled, by those who could form no conception of the means by which the effects predicted were foreseen; and wonderful as the dreams and visions of prophetic inspiration may appear, they surely could not seem more wonderful, as a medium of communication, than that by which the very secrets of the mind, and events apparently the most distant, were made known, through the intervention of a small ball like the eye.
In shewing the manner by which we learn to combine, with our visual sensations, the knowledge obtained by touch; or, as I am rather inclined to think, for reasons formerly stated, the knowledge falsely ascribed to mere touch; it will not be necessary to go over the different varieties of figure, magnitude, distance. The most striking of these is distance, – which, indeed, may be truly said to involve the other two; since the distance of an object is merely the extension of the long line that intervenes between the object and our eye, and the consequent magnitude of the intervening objects, and that which we consider, regarded as one extended whole. Of this one great whole, what we term the distant object, is nothing more than the boundary. The cottage, at the end of a field, is a part of that compound magnitude of which the field and the cottage are separately parts, exactly in the same manner as the wing of a house, is a part of the compound magnitude of the whole building. The line of field which connects our eye with the cottage, may, indeed, be a longer line, but it is a line of precisely the same sort as that which connects the wings of the house with our organ of sight, or with each other.
It is vain to think of ascribing the perception of distance to the measurement of the different angles subtended by objects, at different distances, or to an equally nice measurement of the different degrees of inclination of the axis of the eyes, necessary for distinct vision, in particular cases, – as if all men were instinctively geometers, and the peasant and the very idiot were 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 coexisted 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. A 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, coexisted with the tactual and muscular feeling, when he handled the cup; and that the one feeling was thus associated, forever 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, – the different affections of the muscles, employed in varying the refracting power of each eye, according to the distance of objects, and in producing that particular inclination of the axis 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 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.
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 object, 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 I 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 who contend for the immediate perception of visible figure, that it is now impossible for us to refer to our original feelings, and that we can speak, with absolute certainty, only of our present feelings, or, at least, of those which we remember, as belonging to a period long after our first sensations.
What may, or may not, have been originally separable, we cannot then, determine. But what, even now, is the species of extension, which it is impossible for us, in our visual perceptions, to separate from colour? Is there the slightest consciousness of a perception of visible figure, corresponding with the affected portion of the retina, – or is not the superficial magnitude, and the only magnitude, which we connect with colour, in any case, the very superficial magnitude which we term tangible, – a magnitude, that does not depend on the diameter of the retina, but is variously, greater or less, depending only on the magnitude and distance of the external object.
The mere length and breadth, then, which we cannot separate from colour, are not the length and breadth of the figure termed visible, – for of the perception of these limited dimensions, we have no consciousness, – but the length and breadth that are truly tangible; – and there is not a single moment of visual perception, in which the slightest evidence is afforded by our consciousness of that difficulty of separation, with respect to the affected portion of the expanse of the retina, on which the supposed argument, as to the perception of visible figure, is founded.
Even though the superficial dimensions of length and breadth, connected with colour in vision, were those of the figured retina affected, and were necessarily limited to its small expanse, there would still be no greater impossibility of separating the colour from mere length and breadth in vision, than of separating it from the triple dimensions of length, breadth, and thickness: and the argument, therefore, if it had any force, would be equally applicable to these.
I open my eyes, in the light of day, with a wide landscape around me. I have a sensation, or perception, of varieties of colour, and of all the dimensions of matter. I cannot separate the colour from the length and breadth of the trunk of a large oak before me; but equally impossible is it for me, to separate the colour from the convexity and the magnitude; and, from this equal impossibility, I might conclude, with equal force, that the perception of the convexity and the magnitude is immediate and original, as the perception of mere length and breadth. Where all things are equal, we cannot justly deny to one what we allow to another. He who affirms, that, in looking at a sphere, he can separate, as elements of his sensation, the colour and the convexity, may be allowed to use this argument of impossibility, as proof of original connexion, in the other case. But it is only a person so privileged by nature, – and where is such a person to be found? – who can fairly use it.
We are able, indeed, – not while we continue to look at the sphere, but with a sort of mental effort, afterwards to separate the colour from the convexity, and to imagine the same colour united with any other surface, plane or concave, – the reason of which is very evident. Our sensation of colour has not been uniformly associated with one species of extension, but with all its varieties; and may, therefore, be suggested in possible coexistence with all. In all these varieties, however, two dimensions have been constantly implied; and, therefore, the association of colour with these is complete and indissoluble. If every surface in nature had been convex, it is by no means improbable, that we should have found the same difficulty, in attempting to separate colour from convexity, which we now find, in attempting to separate it from mere length and breadth.
It is the same, in various other affections of the mind, as in our sensations. There are feelings, which we cannot separate from other feelings, and which, we yet know, must have been originally separate. I might refer to the silent growth and maturity of almost every passion, of which the mind is susceptible. But there is sufficient proof, even in affections, which seem instantaneous. The mother, when she looks at her babe, cannot behold it without feelings, very different from those, which the same form and colour, in another infant, would have excited; and yet, impossible as it is to separate, in this case, the mere visual sensation, from that emotion of happy and instant fondness which accompanies it, there is surely no natural connexion of the emotion, with the mere length, and breadth, and colour.
The impossibility of separating the sensation of colour from the notion of extension, it appears, then, is not a decisive proof of an original connexion of these; for, if it were decisive, it would prove still more; – and we might, from this alone, assert with equal confidence, the original visual perception of three dimensions, as that of two, and of the magnitude and figure, which we term tangible, as much as of those, which we have chosen to term visible. It is surely as little possible for us, when we open our eyes on some wide and magnificent landscape, to separate the colour, as a mere visual sensation, from the field, the mountain, the forest, the stream, the sky, as to separate it from the half inch, or inch of our retina, of the perception of which we have no consciousness in any case; and it is too much for those who deny the immediate perception of those greater magnitudes, to urge, in proof of the necessary original perception of this inch or half inch, what, if valid in any respect, must establish no less the proposition which they deny, than the proposition which they affirm.