directions, and proceeding always in right lines, cross each other at their entrance through the aperture: those which come above proceed below, those from the right go to the left, those from the left towards the right; thus every object is represented in the picture, as occupying a situation the very reverse of that which it does in nature.

Caroline. Excepting the flower-pot E F, which, though its position is reversed, has not changed its situation in the landscape.

Mrs. B. The flower-pot is directly in front of the aperture; so that its rays fall perpendicularly upon it, and, consequently, proceed perpendicularly to the wall, where they delineate the object directly behind the aperture.

Emily. And is it thus that the picture of objects is painted on the retina of the eye?

Mrs. B. Precisely. The pupil of the eye, through which the rays of light enter, represents the aperture in the window-shutter; and the image delineated on the retina, is exactly similar to the picture on the wall.

Caroline. You do not mean to say, that we see only the representation of the object which is painted on the retina, and not the object itself?

Mrs. B. If, by sight, you understand that sense by which the presence of objects is perceived by the mind, through the means of the eyes, we certainly see only the image of those objects painted on the retina.

Caroline. This appears to me quite incredible. Mrs. B. The nerves are the only part of our frame capable of sensation: they appear, therefore, to be the instruments which the mind employs in its perceptions; for a sensation always conveys an idea to the mind.

Now it is known, that our nerves can be affected only by contact; and for this reason the organs of sense cannot act at a distance: for instance, we are capable of smelling only particles which are actually in contact with the nerves of the nose. We have already observed, that the odour of a flower consists in effluvia, composed of very minute particles, which penetrate the nostrils, and strike upon the olfactory nerves, which instantly convey the idea of smell to the mind.

Emily. And sound, though it is said to be heard at a distance, is, in fact, heard only when the vibrations of the air, which convey it to our ears, strike upon the auditory nerve.

Caroline. There is no explanation required, to prove that the senses of feeling and of tasting are excited only by contact.

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 myself to 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, which forms the feathers of the butterfly, and the flowerets of the daisy, can alone portray 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, unless from a very small object, 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.

Caroline. When I want to see an object above me, I look up; when an object below me, I look down. Does not this prove that I see the objects themselves? for if I beheld only the image, there would be no necessity for looking up or down, according as the object was higher or lower than myself.

Mrs. B. I beg your pardon. When you look up to an elevated object, it is in order that the rays reflected from it should fall upon the retina of your eyes; but the very circumstance of directing your eyes upwards convinces you that the object is elevated, and teaches you to consider as uppermost the image it forms on the retina, though it is, in fact, represented in the lowest part of it. When you look down upon an object, you draw your conclusion from a similar reasoning; it is thus that we see all objects in the direction of the rays which reach our eyes.

But I have a further proof in favor of what I have advanced, which I hope will remove your remaining doubts; I shall, however, defer it till our next meeting, as the lesson has been sufficiently long to-day.

CONVERSATION XV.

OPTICS-CONTINUED.

ON THE ANGLE OF VISION, AND THE REFLECTION OF MIRRORS.

Angle of Vision.-Reflection of Plain Mirrors.-Reflection of Convex Mirrors.-Reflection of Concave Mirrors.

CAROLINE.

WELL, Mrs. B., I am very impatient to hear what further proofs you have to offer in support of your theory. You must allow that it was rather provoking to dismiss us as you did at our last meeting.

Mrs. B. You press so hard upon me with your objections, that you must give me time to recruit my forces.

Can you tell me, Caroline, why objects at a distance appear smaller than they really are?

Caroline. I know no other reason than their distance. Mrs. B. I do not think I have more cause to be satisfied with your reasons, than you appear to be with mine.