Mrs. B. It is the effect which transparent mediums

produce on light in its passage through them. Opaque bodies, you know, reflect the rays, and transparent bodies transmit them; but it is found, that if a ray, in passing from one medium into another of different density, fall obliquely, it is turned out of its course.

Caroline. It must then be acted on by some new power, otherwise it would not deviate from its first direction.

Mrs. B. The power which causes the deviation of the ray appears to be the attraction of the denser medium. Let us suppose the two mediums to be air and water; if a ray of light passes from air into water, it is more strongly attracted by the latter on account of its superior density.

Emily. In what direction does the water attract the ray? Mrs. B. It must attract it perpendicularly towards it, in the same manner as gravity acts on bodies.

If then a ray A B, (fig. 1, plate XIX.) fall perpendicularly on water, the attraction of the water acts in the same direction as the course of the ray: it will not therefore cause a deviation, and the ray will proceed straight on to E. But if it fall obliquely, as the ray C B, the water will attract it out of its course. Let us suppose the ray to have approached the surface of a denser medium, and that it there begins to be affected by its attraction; this attraction, if not counteracted by some other power, would draw it perpendicularly to the water, at B; but it is also impelled by its projectile force, which the attraction of the denser medium cannot overcome; the ray, therefore, acted on by both these powers, moves in a direction between them, and instead of pursuing its original course to D, or being implicitly guided by the water to E, proceeds towards F, so that the ray appears bent or broken.

Caroline. I understand that very well; and is not this the reason that oars appear bent in water ?

880. What is meant by the refraction of light-881. When does refraction in light take place?-882. What power causes the refraction of light?-883. How would you illustrate the refraction of light by an explanation of Fig. 1, plate XIX?-884. Why does the ray C. B. descend to F. instead of D. or E. in that figure?

Mrs. B. It is owing to the refraction of the rays reflected by the oar; but this is in passing from a dense to a rare medium, for you know that the rays, by means of which you see the oar, pass from water into air.

Emily. But I do not understand why a refraction takes place when a ray passes from a dense into a rare medium; I should suppose that it would be rather less, than more attracted by the latter.

Mrs. B. And it is precisely on that account that the ray is refracted. CB, fig. 2, represents a ray passing obliquely from glass into water: glass being the denser medium, the ray will be more strongly attracted by that which it leaves than by that which it enters. The attraction of the glass acts in the direction A B, while the impulse of projection would carry the ray to F; it moves therefore between these directions towards D.

Emily. So that a contrary refraction takes place when a ray passes from a dense into a rare medium.

Caroline. But does not the attraction of the denser medium affect the ray before it touches it?

Mrs. B. The distance at which the attraction of the denser medium acts upon a ray is so small as to be insensible; it appears therefore to be refracted only at the point at which it passes from one medium to the other.

Now that you understand the principle of refraction, I will show you the refraction of a real ray of light. Do you see the flower painted at the bottom of the inside of this tea-cup? (Fig. 3.)

Emily. Yes. But now you have moved it just out of sight, the rim of the cup hides it.

Mrs. B. Do not stir. I will fill the cup with water, and you will see the flower again.

Emily. I do indeed! Let me try to explain this : when you drew the cup from me so as to conceal the flower, the rays reflected by it no longer met my eyes, but were directed above them; but now that you have filled the cup with water, they are refracted by the attraction of the water, and bent downwards so as again to enter my eyes.

Mrs. B. You have explained it perfectly: Fig. 3.

885. Why does a straight stick appear crooked when one end of it is immersed obliquely in the water?-886. How would you explain Fig. 2, plate XIX. ?-887. Does the attraction of the denser medium affect the ray before it touches it?

You must ob

will help to imprint it on your memory. serve that when the flower becomes visible by the refraction of the ray, you do not see it in the situation which it really occupies, but an image of the flower higher in the cup; for as objects always appear to be situated in the direction of the rays which enter the eye, the flower will be seen in the direction of the reflected ray at B.

Emily. Then when we see the bottom of a clear stream of water, the rays which it reflects being refracted in their passage from the water into the air, will make the bottom appear higher than it really is.

Mrs. B. And the water will consequently appear more shallow. Accidents have frequently been occasioned by this circumstance; and boys who are in the habit of bathing should be cautioned not to trust to the apparent shallowness of water, as it will always prove deeper than it appears; unless indeed, they view it from a boat on the water, which will enable them to look perpendicularly upon it; when the rays from the bottom passing perpendicularly, no refraction will take place.

The refraction of light prevents our seeing the heavenly bodies in their real situation; the light they send to us being refracted in passing into the atmosphere, we see the sun and stars in the direction of the refracted ray; as described in fig. 4, plate XIX., the dotted line represents the extent of the atmosphere, above a portion of the earth, EBE: a ray of light coming from the sun S falls obliquely on it, at A, and is refracted to B: then, since we see the object in the direction of the refracted ray, a spectator at B will see an image of the sun at C, instead of the real object as S.

Emily. But if the sun were immediately over our heads, its rays falling perpendicularly on the atmosphere would not be refracted, and we should then see the real sun in its true situation.

Mrs. B.

You must recollect that the sun is vertical only to the inhabitants of the torrid zone; its rays, there

888. How would you describe the experiment represented in Fig. 3, plate XIX. ?-889. Why does water appear more shallow than it really is?-890. In what situation may the bottom of water be viewed so as to appear of its real depth? - 891. Do we see the heavenly bodies in their real situation? -892. Why do we not?-893. By which Figure is this illustrated, and how would you describe the illustration given?-894. In what situation may the sun be seen in its true place?

fore, are always refracted in these climates. There is also another obstacle to our seeing the heavenly bodies in their real situations; light, though it moves with extreme velocity, is about eight minutes and an half in its passage from the sun to the earth; therefore, when the rays reach us, the sun must have quitted the spot he occupied on their departure; yet we see him in the direction of those rays, and consequently in a situation which he had abandoned eight minutes and a half before.

Emily. When you speak of the sun's motion, you mean, I suppose, his apparent motion, produced by the diurnal motion of the earth ?

Mrs. B. No doubt; the effect being the same, whether it is our earth, or the heavenly bodies which move: it is more easy to represent things as they appear to be, than as they really are.

Caroline. During the morning, then, when the sun is rising towards the meridian, we must (from the length of time the light is in reaching us) see an image of the sun below that spot which it really occupies.

Emily. But the refraction of the atmosphere counteracting this effect, we may perhaps, between the two, see the sun in its real situation.

Caroline. . And in the afternoon, when the sun is sinking in the west, refraction and the length of time which the light is in reaching the earth, will conspire to render the image of the sun higher than it really is.

Mrs. B. The refraction of the sun's rays by the atmosphere prolongs our days, as it occasions our seeing an image of the sun both before he rises and after he sets; for below the horizon, he still shines upon the atmosphere, and his rays are thence refracted to the earth. So likewise we see an image of the sun before he rises, the rays that previously fall upon the atmosphere being reflected to the earth. *

* It is entirely owing to the reflection of the atmosphere that the heavens appear bright in the day time. For without it, only that part would be luminous in which the sun is placed; and if

395. How long is light in coming from the sun to the earth? -896. How would you explain the effect this has on the apparent situation of that luminary? -897. What effect does the refraction of light from the atmosphere have on the length of our days?-898. What would be the appearance of the heavens vere it not for the atmosphere?

Caroline. On the other hand we must recollect that light is eight minutes and an half on its journey; so that, by the time it reaches the earth, the sun may perhaps be risen above the horizon.

Emily. Pray, do not glass windows refract the light? Mrs. B. They do; but this refraction is not perceptible, because, in passing through a pane of glass, the rays suffer two refractions, which being in contrary directions, produce the same effect, as if no refraction had taken place.

Emily. I do not understand that.

is

Mrs. B. Fig. 5. plate plate XIX. will make it clear to you: A A represents a thick pane of glass seen edgeways. When the ray B approaches the glass at C, it is refracted by it; and instead of continuing its course in the same direction, as the dotted line describes, it passes through the pane to D; at that point returning into the air, it i again refracted by the glass, but in a contrary direction to the first refraction, and in consequence proceeds to E. Now you must observe that the ray B C and the ray D E. being parallel, the light does not appear to have suffered any refraction.

Emily. So that the effect which takes place on the ray entering the glass, is undone on its quitting it. Or, to express my self more scientifically, when a ray of light passes from one medium into another, and through that into the first again, the two refractions being equal and in opposite directions, no sensible effect is produced.

Mrs. B. This is the case when the two surfaces of the refracting medium are parallel to each other; if they are not, the two refractions may be made in the same direction, as I shall show you.

we could live without air, and should turn our backs to the sun, the whole heavens would appear as dark as in the night. In this case, also, we should have no twilight, but a sudden transition from the brightest sunshine to dark, immediately upon the setting of the sun.

899. In what manner would the changes of day and night then take place?-900. Is light refracted in passing through common window glass? - 901. Why then is not the refraction perceptible?-902. Which figure illustrates this?

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