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,