_Mrs. B._ And I hope to convince you, that the sense of sight, is so likewise. The nerves, which constitute the sense of sight, are not different in their nature from those of the other organs; they are merely instruments which convey ideas to the mind, and can be affected only on contact. Now, since real objects cannot be brought to touch the optic nerve, the image of them is conveyed thither by the rays of light, proceeding from real objects, which actually strike upon the optic nerve, and form that image which the mind perceives. _Caroline._ While I listen to your reasoning, I feel convinced; but when I look upon the objects around, and think that I do not see them, but merely their image painted in my eyes, my belief is again staggered. I cannot reconcile to myself, the idea, that I do not really see this book which I hold in my hand, nor the words which I read in it. _Mrs. B._ Did it ever occur to you as extraordinary, that you never beheld your own face? _Caroline._ No; because I so frequently see an exact representation of it in the looking-glass. _Mrs. B._ You see a far more exact representation of objects on the retina of your eye: it is a much more perfect mirror, than any made by art. _Emily._ But is it possible, that the extensive landscape, which I now behold from the window, should be represented on so small a space, as the retina of the eye? _Mrs. B._ It would be impossible for art to paint so small and distinct a miniature; but nature works with a surer hand, and a more delicate pencil. That power alone, which forms the feathers of the butterfly, and the organs of the minutest insect, can pourtray so admirable and perfect a miniature, as that which is represented on the retina of the eye. _Caroline._ But, Mrs. B., if we see only the image of objects, why do we not see them reversed, as you showed us they were, in the camera obscura? Is not that a strong argument against your theory? _Mrs. B._ Not an unanswerable one, I hope. The image on the retina, it is true, is reversed, like that in the camera obscura; as the rays, from the different parts of the landscape, intersect each other on entering the pupil, in the same manner as they do, on entering the camera obscura. The scene, however, does not excite the idea of being inverted, because we always see an object in the direction of the rays which it sends to us. _Emily._ I confess I do not understand that. _Mrs. B._ It is, I think, a difficult point to explain clearly. A ray which comes from the upper part of an object, describes the image on the lower part of the retina; but, experience having taught us, that the direction of that ray is from above, we consider that part of the object it represents as uppermost. The rays proceeding from the lower part of an object, fall upon the upper part of the retina; but as we know their direction to be from below, we see that part of the object they describe as the lowest.