be involved in the operation of seeing.  We dwell chiefly upon the sense of sight, because it is mainly through its ministrations that a real objective universe is given to us.  Let the circle A be the whole circuit of vision.  We may begin by calling it the eye, the retina, or what we will.  Let it be provided with the ordinary complement of sensations—­the colours X Y Z. Now, we admit that these sensations cannot be extruded beyond the periphery of vision; and yet we maintain that, unless they be made to fall on the outside of that periphery, they cannot become real objects.  How is this difficulty—­this contradiction—­to be overcome?  Nature overcomes it, by a contrivance as simple as it is beautiful.  In the operation of seeing, admitting the canvass or background of our picture to be a retina, or what we will, with a multiplicity of colours depicted upon it, we maintain that we cannot stop here, and that we never do stop here.  We invariably go on (such is the inevitable law of our nature) to complete the picture—­that is to say, we fill in our own eye as a colour within the very picture which our eye contains—­we fill it in as a sensation within the other sensations which occupy the rest of the field; and in doing so, we of necessity, by the same law, turn these sensations out of the eye; and they thus, by the same necessity, assume the rank of independent objective existences.  We describe the circumference infinitely within the circumference; and hence all that lies on the outside of the intaken circle comes before us stamped with the impress of real objective truth.  We fill in the eye greatly within the sphere of light, (or within the eye itself; if we insist on calling the primary sphere by this name,) and the eye thus filled in is the only eye we know any thing at all about, either from the experience of sight or of touch. How this operation is accomplished, is a subject of but secondary moment; whether it be brought about by the touch, by the eye itself, or by the imagination, is a question which might admit of much discussion; but it is one of very subordinate interest.  The fact is the main thing—­the fact that the operation is accomplished in one way or another—­the fact that the sense comes before itself (if not directly, yet virtually) as one of its own sensations—­that is the principal point to be attended to; and we apprehend that this fact is now placed beyond the reach of controversy.

To put the case in another light.  The following considerations may serve to remove certain untoward difficulties in metaphysics and optics, which beset the path, not only of the uninitiated, but even of the professors of these sciences.