Mrs. B. That is, doubtless, the reason of the opinion he formed, before the sense of touch had corrected his judgment.
Caroline. But since an image must be formed on the retina of each of our eyes, why do we not see objects double?
Mrs. B. The action of the rays, on the optic nerve of each eye, is so perfectly similar, that they produce but a single sensation; the mind, therefore, receives the same idea, from the retina of both eyes, and conceives the object to be single.
Caroline. This is difficult to comprehend, and I should think, can be but conjectural.
Mrs. B. I can easily convince you, that you have a distinct image of an object formed on the retina of each eye. Look through the window, with both eyes open, at some object exactly opposite to one of the upright bars of the sash.
Caroline. I now see a tree, the body of which, appears to be in a line exactly opposite to one of the bars.
Mrs. B. If you now shut your right eye, and look with the left, it will appear to the left of the bar; then by closing the left eye, and looking with the other, it will appear to the right of the bar.
Caroline. That is true, indeed!
Mrs. B. There are, evidently, two representations of the tree in different situations, which must be owing to an image of it being formed on each eye; if the action of the rays, therefore, on each retina, were not so perfectly similar as to produce but one sensation, we should see double; and we find that to be the case with some persons, who are afflicted with a disease in one eye, which prevents the rays of light from affecting it in the same manner as the other.
Emily. Pray, Mrs. B., when we see the image of an object in a looking-glass, why is it not inverted, as in the camera obscura, and on the retina of the eye?