Page:Popular Science Monthly Volume 1.djvu/486

472 forms the frame of the field of view, and what is within this frame is itself the field of view.

This space, immovable as regards the head, becomes the arena to which both those excitations produced by the senses, and those belonging to subjective sight, are transposed; and this transposition always takes place in the same direction as that in which a regular act of sight would in any case lie. Although long experience has taught us it is a delusion, still we always place the image projected by a mirror behind it, because the reflected light falls into our eye just as if the object were behind it. But the sparks produced by pressing the eye from the side of the temples, we seem to see in the opposite side of the field of view; but we do so because, in the normal act of sight, the retina is irritated (from the side of the temples) by light falling from the opposite side. By the action of projection, the reversed image naturally regains its upright position in the field of view.

A general irritation of the retina unaccompanied by a perception of objects will give us a light field of view, and, on the other hand, a perfect repose of those parts will give us a dark field of view. The former represents the sense of the repose of a mechanism endowed with the power of action; the latter corresponds to the absence of all mechanism whatever. The feeling of darkness, therefore, results merely from the expansion of your field of view as opposed to your retina, if I may so say; while behind your back you have the feeling neither of light nor of darkness; you simply miss all sensation of light.

Touching the size of the images on the retina, as compared with the objects themselves, I need merely remind you of the rules of perspective. The images on the retina stand in reversed proportion to the distance of the objects. The image of a pencil, held a foot from your eye, covers the trunk of the tree before your window; that of a pea, at a like distance, covers the moon in the sky. If, notwithstanding, we think the moon bigger than the pea, and the tree than the pencil, the reason is that, apart from our being well acquainted with the tree, our judgment is a combination of the size of the image on the retina and the distance of the object. Now, as consciousness is for the most part founded on experience, so just and correct perspective sight is in the main something we acquire. A child will assuredly not appreciate the difference between the pencil and the trunk of the tree in the same degree as an individual who by experience has learned to know the value of his impressions. What the child first knows of the moon is, that he cannot reach it with his hand; "But," as I once heard a child say, "mother can reach it down." Other inferences have helped him to this conclusion. We are so accustomed to merely play with children, that we are easily blinded to the full seriousness of such requests.

We cannot break off these reflections on the image of the retina without making mention of a remarkable spot in the background of