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.