CAROLINE.

I long to begin our lesson to-day, Mrs. B., for I expect that it will be very entertaining.

Mrs. B. Optics is that branch of philosophy, which treats of the nature and properties of light. It is certainly one of the most interesting branches of Natural Philosophy, but not one of the easiest to understand; I must, therefore, beg that you will give me your undivided attention.

I shall first inquire, whether you comprehend the meaning of a luminous body, an opaque body, and a transparent body.

Caroline. A luminous body is one that shines; an opaque....

Mrs. B. Do not proceed to the second, until we have agreed upon the definition of the first. All bodies that shine, are not luminous; for a luminous body is one that shines by its own light; as the sun, the fire, a candle, &c.

Emily. Polished metal then, when it shines with so much brilliancy, is not a luminous body?

Mrs. B. No, for it would be dark, if it did not receive light from a luminous body; it belongs, therefore, to the class of dark, as well as of opaque bodies, which comprehends all such as are neither luminous, nor will admit the light to pass through them.

Emily. And transparent bodies, are those which admit the light to pass through them, such as glass and water.

Mrs. B. You are right. Transparent, or pellucid bodies, are frequently called mediums, because they allow the rays of light to pass through them; and the rays which pass through, are said to be transmitted by them.

Light, when emanated from the sun, or any other luminous body, is projected forward in straight lines, in every possible direction; so that the luminous body, is not only the general centre, from whence all the rays proceed; but every point of it, may be considered as a centre, which radiates light in every direction. (Fig. 1, plate 15.)

Emily. But do not the rays which are projected in different directions, and cross each other, interfere, and impede each other's course?

Mrs. B. Not at all. The particles of light, are so extremely minute, that they are never known to interfere with each other. A ray of light, is a single line of light, projected from a luminous body; and a pencil of rays, is a collection of rays, proceeding from any one point of a luminous body, as fig. 2.

Caroline. Is light then a substance composed of particles, like other bodies?

Mrs. B. That is a disputed point, upon which I cannot pretend to decide. In some respects, light is obedient to the laws which govern bodies; in others, it appears to be independent of them: thus, though its course is guided by the laws of motion, it does not seem to be influenced by those of gravity. It has never been discovered to have weight, though a variety of interesting experiments have been made with a view of ascertaining that point; but we are so ignorant of the intimate nature of light, that an attempt to investigate it, would lead us into a labyrinth of perplexity, if not of error; we shall, therefore, confine our attention to those properties of light, which are well ascertained.

Let us return to the examination of the effects of the radiation of light, from a luminous body. Since the rays of light are projected in straight lines, when they meet with an opaque body through which they are unable to pass, they are stopped short in their course; for they cannot move in a curve line round the body.

Caroline. No, certainly; for it would require some other force besides that of projection, to produce motion in a curve line.

Mrs. B. The interruption of the rays of light, by the opaque body, produces, therefore, darkness on the opposite side of it: and if this darkness fall upon a wall, a sheet of paper, or any object whatever, it forms a shadow.

Emily. A shadow, then, is nothing more than darkness produced by the intervention of an opaque body, which prevents the rays of light from reaching an object behind it.

Caroline. Why then are shadows of different degrees of darkness; for I should have supposed, from your definition of a shadow, that it would have been perfectly black?

Mrs. B. It frequently happens that a shadow is produced by an opaque body, interrupting the course of the rays from one luminous body, while light from another, reaches the space where the shadow is formed; in which case, the shadow is proportionally fainter. This happens when the opaque body is lighted by two candles: if you extinguish one of them, the shadow will be both deeper, and more distinct.

Caroline. But yet it will not be perfectly dark.

Mrs. B. Because it is still slightly illuminated by light reflected from the walls of the room, and other surrounding objects.

You must observe, also, that when a shadow is produced by the interruption of rays from a single luminous body, the darkness is proportioned to the intensity of the light.

Emily. I should have supposed the contrary; for as the light reflected from surrounding objects on the shadow, must be in proportion to the intensity of the light, the stronger the light, the more the shadow will be illumined.

Mrs. B. Your remark is perfectly just; but as we have no means of estimating the degrees of light, and of darkness, but by comparison, the strongest light will appear to produce the deepest shadow. Hence a total eclipse of the sun, occasions a more sensible darkness than midnight, as it is immediately contrasted with the strong light of noonday.

Caroline. The reappearance of the sun, after an eclipse, must, by the same contrast, appear remarkably brilliant.

Mrs. B. Certainly. There are several things to be observed, in regard to the form, and extent, of shadows. If the luminous body A (fig. 3.) is larger than the opaque body B, the shadow will gradually diminish in size, till it terminates in a point.

Caroline. This is the case with the shadows of the earth, and the moon; as the sun, which illumines them, is larger than either of those bodies. And why is it not the case with the shadows of terrestrial objects? Their shadows, far from diminishing, are always larger than the object, and increase with the distance from it.

Mrs. B. In estimating the effect of shadows, we must consider the dimensions of the luminous body; when the luminous body is less, than the opaque body, the shadow will increase with the distance. This will be best exemplified, by observing the shadow of an object lighted by a candle.

Emily. I have often noticed, that the shadow of my figure, against the wall, grows larger, as it is more distant from me, which is owing, no doubt, to the candle that shines on me, being much smaller than myself.

Mrs. B. Yes. The shadow of a figure as A, (fig. 4.) varies in size, according to the distance of the several surfaces B C D E, on which it is described.

Caroline. I have observed, that two candles, produce two shadows from the same object; whilst it would appear, from what you said, that they should rather produce only half a shadow, that is to say, a very faint one.

Mrs. B. The number of lights (in different directions) while it decreases the intensity of the shadows, increases their number, which always corresponds with that of the lights; for each light, makes the opaque body cast a different shadow, as illustrated by fig. 5. which represents a ball A, lighted by three candles, B, C, D; and you observe the light B, produces the shadow b, the light C, the shadow c, and the light D, the shadow d; but neither of these shadows will be very dark, because the light of one candle only, is intercepted by the ball; and the spot is still illuminated by the other two.

Emily. I think we now understand the nature of shadows very well; but pray, what becomes of the rays of light, which opaque bodies arrest in their course, and the interruption of which, is the occasion of shadows?

Mrs. B. Your question leads to a very important property of light, Reflection. When rays of light encounter an opaque body, they cannot pass through it, and part of them are absorbed by it, and part are reflected, and rebound; just as an elastic ball rebounds, when struck against a wall.

By reflection, we mean that the light is turned back again, through the same medium which it had traversed in its first course.

Emily. And is light, in its reflection, governed by the same laws, as solid, elastic bodies?

Mrs. B. Exactly. If a ray of light fall perpendicularly on an opaque body, it is reflected back in the same line, towards the point whence it proceeded. If it fall obliquely, it is reflected obliquely, but in the opposite direction; the ray which falls upon the reflecting surface, is called the incident ray, and that which leaves it, the reflected ray; the angle of incidence, is always equal to the angle of reflection. You recollect that law in mechanics?

Emily. Oh yes, perfectly.

Mrs. B. If you will shut the shutters, we will admit a ray of the sun's light, through a very small aperture, and I can show you how it is reflected. I now hold this mirror, so that the ray shall fall perpendicularly upon it.

Caroline. I see the ray which falls upon the mirror, but not that which is reflected by it.

Mrs. B. Because it is turned directly back again; and the ray of incidence, and that of reflection, are confounded together, both being in the same line, though in opposite directions.

Emily. The ray then, which appears to us single, is really double, and is composed of the incident ray, proceeding to the mirror, and of the reflected ray, returning from the mirror.

Mrs. B. Exactly so. We will now separate them, by holding the mirror M, (fig. 6,) in such a manner, that the incident ray, A B, shall fall obliquely upon it—you see the reflected ray, B C, is marching off in another direction. If we draw a line from the point of incidence B, perpendicularly, to the mirror, it will divide the angle of incidence, from the angle of reflection, and you will see that they are equal.

Emily. Exactly; and now, that you hold the mirror, so that the ray falls more obliquely upon it, it is also reflected more obliquely, preserving the equality of the angles of incidence, and of reflection.

Mrs. B. It is by reflected rays only, that we see opaque objects. Luminous bodies, send rays of light immediately to our eyes, but the rays which they send to other bodies, are invisible to us, and are seen, only when they are reflected by those bodies, to our eyes.

Emily. But have we not just seen the ray of light, in its passage from the sun to the mirror, and its reflections? yet, in neither case, were those rays in a direction to enter our eyes.

Mrs. B. What you saw, was the light reflected to your eyes, by small particles of dust floating in the air, and on which the ray shone, in its passage to, and from, the mirror.

Caroline. Yet I see the sun, shining on that house yonder, as clearly as possible.

Mrs. B. Indeed you cannot see a single ray, which passes from the sun to the house; you see, by the aid of those rays, which enter your eyes; therefore, it is the rays which are reflected by the house, to you, and not those which proceed directly from the sun, to the house, that render the building visible to you.

Caroline. Why then does one side of the house appear to be in sunshine, and the other in shade? for, if I cannot see the sun shine upon it, the whole of the house should appear in the shade.

Mrs. B. That side of the house, which the sun shines upon, receives, and reflects more light, and therefore, appears more luminous and vivid, than the side which is in shadow; for the latter is illumined only, by rays reflected upon it by other objects; these rays are, therefore, twice reflected before they reach your sight; and as light is more, or less, absorbed by the bodies it strikes upon, every time a ray is reflected, its intensity is diminished.

Caroline. Still I cannot reconcile to myself, the idea that we do not see the sun's rays shining on objects, but only those which such objects reflect to us.

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 shine 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 moonshine 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 shining 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 shines 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 sunshine, 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 shine 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 shines. 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.

The rays from the glittering weathercock, at the top of the alcove, A, (plate 16.) represent it in this spot, a; for the weathercock, 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 weathercock, 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 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.

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 favour 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.