Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 40

_Mrs. B._ I do not, however, despair of convincing you of it. Look at that large sheet of water; can you tell why the sun appears to s.h.i.+ne on one part of it only?

_Caroline._ No, indeed; for the whole of it is equally exposed to the sun. This partial brilliancy of water, has often excited my wonder; but it has struck me more particularly by moonlight. I have frequently observed a vivid streak of moons.h.i.+ne on the sea, while the rest of the water remained in deep obscurity, and yet there was no apparent obstacle to prevent the moon from s.h.i.+ning equally on every part of the water.

_Mrs. B._ By moonlight the effect is more remarkable, on account of the deep obscurity of the other parts of the water; while by the sun's light, the effect is too strong for the eye to be able to observe it so distinctly.

_Caroline._ But, if the sun really s.h.i.+nes on every part of that sheet of water, why does not every part of it, reflect rays to my eyes?

_Mrs. B._ The reflected rays, are not attracted out of their natural course, by your eyes. The direction of a reflected ray, you know, depends on that of the incident ray; the sun's rays, therefore, which fall with various degrees of obliquity upon the water, are reflected in directions equally various; some of these will meet your eyes, and you will see them, but those which fall elsewhere, are invisible to you.

_Caroline._ The streak of suns.h.i.+ne, then, which we now see upon the water, is composed of those rays which by their reflection, happen to fall upon my eyes?

_Mrs. B._ Precisely.

_Emily._ But is that side of the house yonder, which appears to be in shadow, really illuminated by the sun, and its rays reflected another way?

_Mrs. B._ No; that is a different case, from the sheet of water. That side of the house, is really in shadow; it is the west side, which the sun cannot s.h.i.+ne upon, till the afternoon.

_Emily._ Those objects, then, which are illumined by reflected rays, and those which receive direct rays from the sun, but which do not reflect those rays towards us, appear equally in shadow?

_Mrs. B._ Certainly; for we see them both illumined by reflected rays.

That part of the sheet of water, over which the trees cast a shadow, by what light do you see it?

_Emily._ Since it is not by the sun's direct rays, it must be by those reflected on it from other objects, and which it again reflects to us.

_Caroline._ But if we see all terrestrial objects by reflected light, (as we do the moon,) why do they appear so bright and luminous? I should have supposed that reflected rays, would have been dull and faint, like those of the moon.

_Mrs. B._ The moon reflects the sun's light, with as much vividness as any terrestrial object. If you look at it on a clear night, it will appear as bright as a sheet of water, the walls of a house, or any object seen by daylight, and on which the sun s.h.i.+nes. The rays of the moon are doubtless feeble, when compared with those of the sun; but that would not be a fair comparison, for the former are incident, the latter, reflected rays.

_Caroline._ True; and when we see terrestrial objects by moonlight, the light has been twice reflected, and is consequently, proportionally fainter.

_Mrs. B._ In traversing the atmosphere, the rays, both of the sun, and moon, lose some of their light. For though the pure air, is a transparent medium, which transmits the rays of light freely, we have observed, that near the surface of the earth, it is loaded with vapours and exhalations, by which some portion of them are absorbed.

_Caroline._ I have often noticed, that an object on the summit of a hill, appears more distinct, than one at an equal distance in a valley, or a plain; which is owing, I suppose, to the air being more free from vapours in an elevated situation, and the reflected rays, being consequently brighter.

_Mrs. B._ That may have some sensible effect; but, when an object on the summit of a hill, has a back ground of light sky, the contrast with the object, makes its outline more distinct.

_Caroline._ I now feel well satisfied, that we see opaque objects, only by reflected rays; but I do not understand, how these rays, show us the objects from which they proceed.

_Mrs. B._ I shall hereafter describe the structure of the eye, very particularly, but will now observe, that the small round spot, which is generally called the sight of the eye, is properly denominated the _pupil_; and that the _retina_, is an expansion of the optic nerve on the back part of the ball of the eye, upon which, as upon a screen, the rays fall, which enter at the pupil. The rays of light, enter at the pupil of the eye, and proceed to the retina; and there they describe the figure, colour, and (excepting size) form a perfect representation of the object, from which they proceed. We shall again close the shutters, and admit the light, through the small hole made for that purpose, and you will see a picture, on the wall, opposite the aperture, similar to that which is delineated on the retina of the eye. The picture is somewhat confused, but by using a lens, to bring the rays to a focus, it will be rendered very distinct.

_Caroline._ Oh, how wonderful! There is an exact picture in miniature of the garden, the gardener at work, the trees blown about by the wind. The landscape, would be perfect, if it were not reversed; the ground, being above, and the sky beneath.

_Mrs. B._ It is not enough to admire, you must understand, this phenomenon, which is called a _camera obscura_, or dark chamber; from the necessity of darkening the room, in order to exhibit it. The camera obscura, sometimes consists of a small box, properly fitted up, to represent external objects.

This picture, you now see, is produced by the rays of light, reflected from the various objects in the garden, and which are admitted through the hole in the window shutter.

[Ill.u.s.tration: PLATE XVI.]

The rays from the glittering weatherc.o.c.k, at the top of the alcove, A, (plate 16.) represent it in this spot, _a_; for the weatherc.o.c.k, being much higher than the aperture in the shutter, only a few of the rays, which are reflected by it, in an obliquely descending direction, can find entrance there. The rays of light, you know, always move in straight lines; those, therefore, which enter the room, in a descending direction, will continue their course in the same direction, and will consequently fall upon the lower part of the wall opposite the aperture, and represent the weatherc.o.c.k, reversed in that spot, instead of erect, in the uppermost part of the landscape.

_Emily._ And the rays of light, from the steps, (B) of the alcove, in entering the aperture, ascend, and will describe those steps in the highest, instead of the lowest, part of the landscape.

_Mrs. B._ Observe, too, that the rays coming from the alcove, which is to our left, describe it on the wall, to the right; while those, which are reflected by the walnut tree, C D, to our right, delineate its figure in the picture, to the left, _c d_. Thus the rays, coming in different directions, and proceeding always in right lines, cross each other at their entrance through the aperture; those which come from 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 odour 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 const.i.tute 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-gla.s.s.

_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 s.p.a.ce, 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 b.u.t.terfly, 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.

_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 circ.u.mstance 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.