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

by Thomas Brown

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.