Chapter XII: Introduction: General Properties of Bodies. Impenetrability. Extension (10)

_Caroline._ That must be occasioned by the rays approaching closer together. I think you hold the paper just in the focus now, the light is so small and dazzling--Oh, Mrs. B., the paper has taken fire!

_Mrs. B._ The rays of light cannot be concentrated, without, at the same time, accumulating a proportional quantity of heat: hence concave mirrors have obtained the name of burning mirrors.

_Emily._ I have often heard of the surprising effects of burning mirrors, and I am quite delighted to understand their nature.

_Caroline._ It cannot be the true focus of the mirror, at which the rays of the sun unite, for as they proceed from so large a body, they cannot fall upon the mirror parallel to each other.

_Mrs. B._ Strictly speaking, they certainly do not. But when rays, come from such an immense distance as the sun, they may be considered as parallel: their point of union is, therefore, the true focus of the mirror, and there the image of the object is represented.

Now that I have removed the mirror out of the influence of the sun's rays, if I place a burning taper in the focus, how will its light be reflected? (Fig. 6.)

_Caroline._ That, I confess, I cannot say.

_Mrs. B._ The ray which falls in the direction of the axis of the mirror, is reflected back in the same line; but let us draw two other rays from the focus, falling on the mirror at B and F; the dotted lines are perpendicular to those points, and the two rays will, therefore, be reflected to A and E.

_Caroline._ Oh, now I understand it clearly. The rays which proceed from a light placed in the focus of a concave mirror fall divergent upon it, and are reflected, parallel. It is exactly the reverse of the former experiment, in which the sun's rays fell parallel on the mirror, and were reflected to a focus.

_Mrs. B._ Yes: when the incident rays are parallel, the reflected rays converge to a focus; when, on the contrary, the incident rays proceed from the focus, they are reflected parallel. This is an important law of optics, and since you are now acquainted with the principles on which it is founded, I hope that you will not forget it.

_Caroline._ I am sure that we shall not. But, Mrs. B., you said that the image was formed in the focus of a concave mirror; yet I have frequently seen glass concave mirrors, where the object has been represented within the mirror, in the same manner as in a convex mirror.

_Mrs. B._ That is the case only, when the object is placed between the mirror and its focus; the image then appears magnified behind the mirror, or, as you would say, within it.

_Caroline._ I do not understand why the image should be larger than the object.

_Mrs. B._ This results from the convergent property of the concave mirror. If an object, A B, (fig. 7.) be placed between the mirror and its focus, the rays from its extremities fall divergent on the mirror, and on being reflected, become less divergent, as if they proceeded from C: to an eye placed in that situation, the image will appear magnified behind the mirror at _a b_, since it is seen under a larger angle than the object.

You now, I hope, understand the reflection of light by opaque bodies. At our next meeting, we shall enter upon another property of light, no less interesting, and which is called _refraction_.

Questions

1. (Pg. 168) What is meant by the angle of vision, or the visual angle?

2. (Pg. 169) Why do objects of the same size appear smaller when distant, than when near?

3. (Pg. 169) Why do not two objects, known to be equal in size, appear to differ, when at different distances from the eye?

4. (Pg. 169) How is this exemplified, by a house seen through a window?

5. (Pg. 170) Why do rows of trees, forming an avenue, appear to approach as they recede from the eye, until they eventually seem to meet?

6. (Pg. 170) In drawing a view from nature, what do we copy?

7. (Pg. 170) What is the difference in sculpture, in this respect?

8. (Pg. 170) Excepting the rays from an object enter the eye, under a certain angle, they cannot be seen; what must this angle exceed?

9. (Pg. 170) What two circumstances may cause the angle to be so small, as not to produce vision?

10. (Pg. 170) Motion may be so slow as to become imperceptible, what is said on this point?

11. (Pg. 170) Under what circumstances may a body, moving with great rapidity, appear to be at rest?

12. (Pg. 170) Upon what does the real velocity of a body, depend?

13. (Pg. 171) What must be known, to enable us to ascertain the real space contained in a degree?

14. (Pg. 171) What is explained by fig. 2, plate 17?

15. (Pg. 171) What is said respecting the evidence afforded by our senses, and how do we correct the errors into which they would lead us?

16. (Pg. 171) An image of a visible object is formed upon the retina of each eye, why, therefore, are not objects seen double?

17. (Pg. 172) By what experiment can you prove that a separate image of an object is formed in each eye?

18. (Pg. 172) Under what circumstances are objects seen double?

19. (Pg. 172) Why is not the image of an object inverted in the common mirror?

20. (Pg. 172) Your whole figure may be seen in a looking-glass, which is not more than half your height; how is this shown in fig. 3. plate 17?

21. (Pg. 173) Why is the image invisible to the person, when not standing directly before the glass?

22. (Pg. 173) In what situation may a second person see the image reflected?

23. (Pg. 173) In what direction will an object always appear to the eye?

24. (Pg. 173) How is this explained by fig. 4, plate 17?

25. (Pg. 173) What is it that reflects the rays in a looking-glass?

26. (Pg. 174) All opaque bodies reflect some light, why do they not all act as mirrors?

27. (Pg. 174) What substances form the most perfect mirrors, and for what reason?

28. (Pg. 174) What are the three kinds of mirrors usually employed for optical purposes?

29. (Pg. 174) How are the rays of light affected by them?

30. (Pg. 175) What is the form of a convex mirror, and how do parallel rays fall upon it, as represented in fig. 1, plate 18?

31. (Pg. 175) What is represented by the dotted line in the same figure?

32. (Pg. 175) Explain by the figure, how the parallel rays will be reflected.

33. (Pg. 175) At what distance behind such a mirror, would an image, produced by parallel rays, be formed?

34. (Pg. 175) What is that point denominated?

35. (Pg. 176) What is meant by a focus?

36. (Pg. 176) Why is the point behind the mirror, called the _imaginary focus_?

37. (Pg. 176) Why does an object appear to be lessened by a convex mirror, (fig. 2.)?

38. (Pg. 176) What is a concave mirror, and what its peculiar property?

39. (Pg. 176) How are parallel rays reflected by a concave mirror, as explained by fig. 3, plate 18?

40. (Pg. 177) Where is the focus of parallel rays, in a concave mirror?

41. (Pg. 177) If rays fall on it convergent, how are they reflected?

42. (Pg. 177) How if divergent?

43. (Pg. 177) How, and why, may concave, become burning mirrors?

44. (Pg. 178) Why may rays of light coming from the sun, be viewed as parallel to each other?

45. (Pg. 178) If a luminous body, as a burning taper, be placed in the focus of a concave mirror, how will the rays from it, be reflected? (fig. 6.)

46. (Pg. 178) What fact is explained by fig. 7, plate 18?

CONVERSATION XVI.

ON REFRACTION AND COLOURS.

TRANSMISSION OF LIGHT BY TRANSPARENT BODIES. REFRACTION. REFRACTION BY THE ATMOSPHERE. REFRACTION BY A LENS. REFRACTION BY THE PRISM. OF COLOUR FROM THE RAYS OF LIGHT. OF THE COLOURS OF BODIES.

MRS. B.

The refraction of light will furnish the subject of to-day's lesson.

_Caroline._ That is a property of which I have not the faintest idea.

_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. The ray of light is then said to be refracted._

_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._ The ray is attracted perpendicularly towards the water, in the same manner in which bodies are acted upon by gravity.

If then a ray, A B, (fig. 1, plate 19.) 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 the water?

_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 refraction takes place, when a ray passes from a dense into a rare medium; I should suppose that it would be less, attracted by the latter, than by the former.

_Mrs. B._ And it is precisely on that account that the ray is refracted. Let the upper half of fig. 2, represent glass, and the lower half water, let C B represent a ray, passing obliquely from the 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.

_Mrs. B._ The rule upon this subject is this; _when a ray of light passes from a rare into a dense medium, it is refracted towards the perpendicular; when from a dense into a rare medium, it is refracted from the perpendicular_. By the perpendicular is meant a line, at right angle with the refracting surface. This may be seen in fig. 1, and fig. 2, where the lines A E, are the perpendiculars.

_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 into the other.

Now that you understand the principle of refraction, I will show you the real refraction of a 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, and bent downwards when passing out of the water, into the air, so as again to enter my eyes.

_Mrs. B._ You have explained it perfectly: fig. 3. will help to imprint it on your memory. You must observe 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 the image of the flower appears 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 at B, in the direction of the refracted ray.

_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.

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 19., the dotted line represents the extent of the atmosphere, above a portion of the earth, E B E: 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 its real situation, at 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, therefore, are always refracted, in this latitude. 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 a quarter, 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 quarter, 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._ Certainly; 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; when below our 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, previously to his rising, the rays that fall upon the atmosphere being refracted to the earth.

_Caroline._ On the other hand, we must recollect that light is eight minutes and a quarter on its journey; so that, by the time it reaches the earth, the sun may, perhaps, have risen above the horizon.

_Emily._ Pray, do not glass windows, refract the light?

_Mrs. B._ They do; but this refraction would not be perceptible, were the surfaces of the glass, perfectly flat and parallel, because, in passing through a pane of glass, the rays suffer two refractions, which, being in contrary directions, produce nearly the same effect as if no refraction had taken place.

_Emily._ I do not understand that.

_Mrs. B:_ Fig. 5, plate 19, 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 is 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: the apparent, differing so little from the true place of any object, when seen through glass of ordinary thickness.

_Emily._ So that the effect which takes place on the ray entering the glass, is undone on its quitting it. Or, to express myself 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.

_Caroline._ I think the effect is very sensible, for, in looking through the glass of the window, I see objects very much distorted; articles which I know to be straight, appear bent and broken, and sometimes the parts seem to be separated to a distance from each other.

_Mrs. B._ That is because common window glass is not flat, its whole surface being uneven. Rays from any object, falling upon it under different angles, are, consequently, refracted in various ways, and thus produce the distortion you have observed.

_Emily._ Is it not in consequence of refraction, that the glasses in common spectacles, magnify objects seen through them?

_Mrs. B._ Yes. Glasses of this description are called _lenses_; of these, there are several kinds, the names of which it will be necessary for you to learn. Every lens is formed of glass, ground so as to form a segment of a sphere, on one, or both sides. They are all represented at fig. 1, plate 20. The most common, is the _double convex_ lens, D. This is thick in the middle, and thin at the edges, like common spectacles, or reading glasses. A B, is a _plano-convex_ lens, being flat on one side, and convex on the other. E is a _double concave_, being, in all respects, the reverse of D. C is a _plano-concave_, flat on one side, and concave on the other. F is called a _meniscus_, or _concavo-convex_, being concave on one, and convex on the other side. A line passing through the centre of a lens, is called its _axis_.

_Caroline._ I should like to understand how the rays of light are refracted, by means of a lens.

_Mrs. B._ When parallel rays (fig. 6) fall on a double convex _lens_, that only, which falls in the direction of the axis of the lens, is perpendicular to the surface; the other rays, falling obliquely, are refracted towards the axis, and will meet at a point beyond the lens, called its _focus_.

Of the three rays, A B C, which fall on the lens D E, the rays A and C are refracted in their passage through it, to _a_, and _c_; and on quitting the lens, they undergo a second refraction in the same direction, which unites them with the ray B, at the focus F.

_Emily._ And what is the distance of the focus, from the surface of the lens?

_Mrs. B._ The focal distance depends both upon the form of the lens, and on the refracting power of the substance of which it is made: in a glass lens, both sides of which are equally convex, the focus is situated nearly at the centre of the sphere, of which the surface of the lens forms a portion; it is at the distance, therefore, of the radius of the sphere.

The property of those lenses which have a convex surface, is to collect the rays of light to a focus; and of those which have a concave surface, on the contrary, to disperse them. For the rays A and C, falling on the concave lens X Y, (fig. 7, plate 19.) instead of converging towards the ray B, in the axis of the lens, will each be attracted towards the thick edges of the lens, both on entering and quitting it, and will, therefore, by the first refraction, be made to diverge to _a_, _c_, and by the seconds, to _d_, _e_.

_Caroline._ And lenses which have one side flat, and the other convex, or concave, as A and B, (fig. 1, plate 20.) are, I suppose, less powerful in their refractions?

_Mrs. B._ Yes; the focus of the plano-convex, is at the distance of the diameter of a sphere, of which the convex surface of the lens, forms a portion; as represented in figure 2, plate 20. The three parallel rays, A B C, are brought to a focus by the plano-convex lens, X Y, at F.

_Emily._ You have not explained to us, Mrs. B., how the lens serves to magnify objects.

_Mrs. B._ By turning again to fig. 6, plate 19. you will readily understand this. Let A C, be an object placed before the lens, and suppose it to be seen by an eye at F; the ray from the point A, will be seen in the direction F G, that from C, in the direction F H; the visual angle, therefore, will be greatly increased, and the object must appear larger, in proportion.

I must now explain to you the refraction of a ray of light, by a triangular piece of glass, called a prism. (Fig. 3.)

_Emily._ The three sides of this glass are flat; it cannot, therefore, bring the rays to a focus; nor do I suppose that its refraction will be similar to that of a flat pane of glass, because it has not two sides parallel; I cannot, therefore, conjecture what effect the refraction by a prism, can produce.

_Mrs. B._ The refractions of the ray, both on entering and on quitting the prism, are in the same direction, (Fig. 3.) On entering the prism P, the ray A is refracted from B to C, and on quitting it from C to D. In the first instance it is refracted towards, and in the last, from the perpendicular; each causing it to deviate in the same way, from its original course, A B.

I will show you this by experiment; but for this purpose it will be advisable to close the window-shutters, and admit, through the small aperture, a ray of light, which I shall refract, by means of this prism.

_Caroline._ Oh, what beautiful colours are represented on the opposite wall! There are all the colours of the rainbow, and with a brightness, I never saw equalled. (Fig. 4, plate 20.)

_Emily._ I have seen an effect, in some respects similar to this, produced by the rays of the sun shining upon glass lustres; but how is it possible that a piece of white glass can produce such a variety of brilliant colours?

_Mrs. B._ The colours are not formed by the prism, but existed in the ray previously to its refraction.

_Caroline._ Yet, before its refraction, it appeared perfectly white.

_Mrs. B._ The white rays of the sun, are composed of rays, which, when separated, produce all these colours, although when blended together, they appear colourless or white.

Sir Isaac Newton, to whom we are indebted for the most important discoveries respecting light and colours, was the first who divided a white ray of light, and found it to consist of an assemblage of coloured rays, which formed an image upon the wall, such as you now see exhibited, (fig. 4.) in which are displayed the following series of colours: red, orange, yellow, green, blue, indigo, and violet.

_Emily._ But how does a prism separate these coloured rays?

_Mrs. B._ By refraction. It appears that the coloured rays have different degrees of refrangibility; in passing through the prism, therefore, they take different directions according to their susceptibility of refraction. The violet rays deviate most from their original course; they appear at one of the ends of the spectrum, A B: contiguous to the violet, are the blue rays, being those which have somewhat less refrangibility; then follow, in succession, the green, yellow, orange, and lastly, the red, which are the least refrangible of the coloured rays.

_Caroline._ I cannot conceive how these colours, mixed together, can become white?

_Mrs. B._ That I cannot pretend to explain: but it is a fact that the union of these colours, in the proportions in which they appear in the spectrum, produce in us the idea of whiteness. If you paint a circular piece of card, in compartments, with these seven colours, as nearly as possible in the proportion, and of the shade exhibited in the spectrum, and whirl it rapidly on a pin, it will appear white; as the velocity of the motion, will have the effect of blending the colours, in the impression which they make upon the eye.

But a more decisive proof of the composition of a white ray is afforded, by reuniting these coloured rays, and forming with them, a ray of white light.

_Caroline._ If you can take a ray of white light to pieces, and put it together again, I shall be quite satisfied.

_Mrs. B._ This can be done by letting the coloured rays, which have been separated by a prism, fall upon a lens, which will converge them to a focus; and if, when thus reunited, we find that they appear white as they did before refraction, I hope you will be convinced that the white rays, are a compound of the several coloured rays. The prism P, you see, (fig. 5.) separates a ray of white light, into seven coloured rays, and the lens L L brings them to a focus at F, where they again appear white.

_Caroline._ You succeed to perfection: this is indeed a most interesting and conclusive experiment.

_Emily._ Yet, Mrs. B., I cannot help thinking, that there may, perhaps, be but three distinct colours in the spectrum, red, yellow, and blue; and that the four others may consist of two of these colours blended together; for, in painting, we find, that by mixing red and yellow, we produce orange; with different proportions of red and blue, we make violet or any shade of purple; and yellow, and blue, form green. Now, it is very natural to suppose, that the refraction of a prism, may not be so perfect as to separate the coloured rays of light completely, and that those which are contiguous, in order of refrangibility, may encroach on each other, and by mixing, produce the intermediate colours, orange, green, violet, and indigo.

_Mrs. B._ Your observation is, I believe, neither quite wrong, nor quite right. Dr. Wollaston, who has performed many experiments on the refraction of light, in a more accurate manner than had been previously done, by receiving a very narrow line of light on a prism, found that it formed a spectrum, consisting of rays of four colours only; but they were not exactly those you have named as primitive colours, for they consisted of red, green, blue, and violet. A very narrow line of yellow was visible, at the limit of the red and green, which Dr. Wollaston attributed to the overlapping of the edges of the red and green light.

_Caroline._ But red and green mixed together, do not produce yellow?

_Mrs. B._ Not in painting; but it may be so in the primitive rays of the spectrum. Dr. Wollaston observed, that, by increasing the breadth of the aperture, by which the line of light was admitted, the space occupied by each coloured ray in the spectrum, was augmented, in proportion as each portion encroached on the neighbouring colour, and mixed with it; so that the intervention of orange and yellow, between the red and green, is owing, he supposes, to the mixture of these two colours; and the blue is blended on the one side with the green, and on the other with the violet, forming the spectrum, as it was originally observed by Sir Isaac Newton, and which I have just shown you.

The rainbow, which exhibits a series of colours, so analogous to those of the spectrum, is formed by the refraction of the sun's rays, in their passage through a shower of rain; every drop of which acts as a prism, in separating the coloured rays as they pass through it; the combined effect of innumerable drops, produces the bow, which you know can be seen, only when there are both rain, and sunshine.

_Emily._ Pray, Mrs. B., cannot the sun's rays be collected to a focus by a lens, in the same manner as they are by a concave mirror?

_Mrs. B._ The same effect in concentrating the rays, is produced by the refraction with a lens, as by the reflection from a concave mirror: in the first, the rays pass through the glass and converge to a focus, behind it, in the latter, they are reflected from the mirror, and brought to a focus, before it. A lens, when used for the purpose of collecting the sun's rays, is called a burning glass. I have before explained to you, the manner in which a convex lens, refracts the rays, and brings them to a focus; (fig. 6, plate 19.) as these rays contain both light and heat, the latter, as well as the former, is refracted; and intense heat, as well as light, will be found in the focal point. The sun now shines very bright; if we let the rays fall on this lens, you will perceive the focus.

_Emily._ Oh yes: the point of union of the rays, is very luminous. I will hold a piece of paper in the focus, and see if it will take fire. The spot of light is extremely brilliant, but the paper does not burn?

_Mrs. B._ Try a piece of brown paper;--that, you see, takes fire almost immediately.

_Caroline._ This is surprising; for the light appeared to shine more intensely, on the white, than on the brown paper.

_Mrs. B._ The lens collects an equal number of rays to a focus, whether you hold the white or the brown paper, there; but the white paper appears more luminous in the focus, because most of the rays, instead of entering into the paper, are reflected by it; and this is the reason that the paper does not readily take fire: whilst, on the contrary, the brown paper, which absorbs more light and heat than it reflects, soon becomes heated and takes fire.

_Caroline._ This is extremely curious; but why should brown paper, absorb more rays, than white paper?

_Mrs. B._ I am far from being able to give a satisfactory answer to that question. We can form but mere conjecture on this point; it is supposed that the tendency to absorb, or reflect rays, depends on the arrangement of the minute particles of the body, and that this diversity of arrangement renders some bodies susceptible of reflecting one coloured ray, and absorbing the others; whilst other bodies, have a tendency to reflect all the colours, and others again, to absorb them all.

_Emily._ And how do you know which colours bodies have a tendency to reflect, or which to absorb?

_Mrs. B._ Because a body always appears to be of the colour which it reflects; for, as we see only by reflected rays, it can appear of the colour of those rays, only.

_Caroline._ But we see all bodies of their own natural colour, Mrs. B.; the grass and trees, green; the sky, blue; the flowers of various hues.

_Mrs. B._ True; but why is the grass green?--because it absorbs all, except the green rays; it is, therefore, these only which the grass and trees reflect to our eyes, and this makes them appear green. The flowers, in the same manner, reflect the various colours of which they appear to us; the rose, the red rays; the violet, the blue; the jonquil, the yellow, &c.

_Caroline._ But these are the permanent colours of the grass and flowers, whether the sun's rays shine on them or not.

_Mrs. B._ Whenever you see those colours, the flowers must be illumined by some light; and light, from whatever source it proceeds, is of the same nature; composed of the various coloured rays which paint the grass, the flowers, and every coloured object in nature.

_Caroline._ But, Mrs. B., the grass is green, and the flowers are coloured, whether in the dark, or exposed to the light?

_Mrs. B._ Why should you think so?

_Caroline._ It cannot be otherwise.

_Mrs. B._ A most philosophical reason indeed! But, as I never saw them in the dark, you will allow me to dissent from your opinion.

_Caroline._ What colour do you suppose them to be, then, in the dark?

_Mrs. B._ None at all; or black, which is the same thing. You can never see objects, without light. White light is compounded of rays, from which all the colours in nature are produced; there, therefore, can be no colour without light; and though a substance is black, or without colour, in the dark, it may become coloured, as soon as it becomes visible. It is visible, indeed, only by the coloured rays which it reflects; therefore, we can see it only when coloured.

_Caroline._ All you say seems very true, and I know not what to object to it; yet it appears at the same time incredible! What, Mrs. B., are we all as black as negroes in the dark? you make me shudder at the thought.

_Mrs. B._ Your vanity need not be alarmed at the idea, as you are certain of never being seen, in that state.

_Caroline._ That is some consolation, undoubtedly; but what a melancholy reflection it is, that all nature which appears so beautifully diversified with colours, is really one uniform mass of blackness!

_Mrs. B._ Is nature less pleasing for being coloured, as well as illumined, by the rays of light? and are colours less beautiful, for being accidental, rather than essential properties of bodies?

Providence seems to have decorated nature with the enchanting diversity of colours, which we so much admire, for the sole purpose of beautifying the scene, and rendering it a source of sensible gratification: it is an ornament which embellishes nature, whenever we behold her. What reason is there to regret, that she does not wear it when she is invisible?

_Emily._ I confess, Mrs. B., that I have had my doubts, as well as Caroline, though she has spared me the pains of expressing them: but I have just thought of an experiment, which, if it succeed, will, I am sure, satisfy us both. It is certain, that we cannot see bodies in the dark, to know whether they have then any colour. But we may place a coloured body in a ray of light, which has been refracted by a prism; and if your theory is true, the body, of whatever colour it naturally is, must appear of the colour of the ray in which it is placed; for since it receives no other coloured rays, it can reflect no others.

_Caroline._ Oh! that is an excellent thought, Emily; will you stand the test, Mrs. B.?

_Mrs. B._ I consent: but we must darken the room, and admit only the ray which is to be refracted; otherwise, the white rays will be reflected on the body under trial, from various parts of the room. With what do you choose to make the experiment?

_Caroline._ This rose: look at it, Mrs. B., and tell me whether it is possible to deprive it of its beautiful colour?

_Mrs. B._ We shall see.--I expose it first to the red rays, and the flower appears of a more brilliant hue; but observe the green leaves----

_Caroline._ They appear neither red nor green; but of a dingy brown with a reddish glow?

_Mrs. B._ They cannot appear green, because they have no green rays to reflect; neither are they red, because green bodies absorb most of the red rays. But though bodies, from the arrangement of their particles, have a tendency to absorb some rays, and reflect others, yet it is not natural to suppose, that bodies are so perfectly uniform in their arrangement, as to reflect only pure rays of one colour, and perfectly to absorb the others; it is found, on the contrary, that a body reflects, in great abundance, the rays which determine its colour, and the others in a greater or less degree, in proportion as they are nearer to or further from its own colour, in the order of refrangibility. The green leaves of the rose, therefore, will reflect a few of the red rays, which, blended with their natural blackness, give them that brown tinge: if they reflected none of the red rays, they would appear perfectly black. Now I shall hold the rose in the blue rays----

_Caroline._ Oh, Emily, Mrs. B. is right! look at the rose: it is no longer red, but of a dingy blue colour.

_Emily._ This is the most wonderful, of any thing we have yet learnt. But, Mrs. B., what is the reason that the green leaves, are of a brighter blue than the rose?

_Mrs. B._ The green leaves reflect both blue and yellow rays, which produce a green colour. They are now in a coloured ray, which they have a tendency to reflect; they, therefore, reflect more of the blue rays than the rose, (which naturally absorbs that colour,) and will, of course, appear of a brighter blue.

_Emily._ Yet, in passing the rose through the different colours of the spectrum, the flower takes them more readily than the leaves.

_Mrs. B._ Because the flower is of a paler hue. Bodies which reflect all the rays, are white; those which absorb them all, are black: between these extremes, bodies appear lighter or darker, in proportion to the quantity of rays they reflect or absorb. This rose is of a pale red; it approaches nearer to white than to black, and therefore, reflects rays, more abundantly than it absorbs them.

_Emily._ But if a rose has so strong a tendency to reflect rays, I should imagine that it would be of a deep red colour.

_Mrs. B._ I mean to say, that it has a general tendency to reflect rays. Pale coloured bodies, reflect all the coloured rays to a certain degree, their paleness, being an approach towards whiteness: but they reflect one colour more than the rest: this predominates over the white, and determines the colour of the body. Since, then, bodies of a pale colour, in some degree reflect all the rays of light, in passing through the various colours of the spectrum, they will reflect them all, with tolerable brilliancy; but will appear most vivid, in the ray of their natural colour. The green leaves, on the contrary, are of a dark colour, bearing a stronger resemblance to black, than to white; they have, therefore, a greater tendency to absorb, than to reflect rays; and reflecting very few of any, but the blue, and yellow rays, they will appear dingy, in passing through the other colours of the spectrum.

_Caroline._ They must, however, reflect great quantities of the green rays, to produce so deep a colour.

_Mrs. B._ Deepness or darkness of colour, proceeds rather from a deficiency, than an abundance of reflected rays. Remember, that if bodies reflected none of the rays, they would be black; and if a body reflects only a few green rays, it will appear of a dark green; it is the brightness, and intensity of the colour, which show that a great quantity of rays are reflected.

_Emily._ A white body, then, which reflects all the rays, will appear equally bright in all the colours of the spectrum.

_Mrs. B._ Certainly. And this is easily proved by passing a sheet of white paper, through the rays of the spectrum.

White, you perceive, results from a body reflecting all the rays which fall upon it; black, is produced, when they are all absorbed; and colour, arises from a body possessing the power to decompose the solar ray, by absorbing some parts, and reflecting others.

_Caroline._ What is the reason that articles which are blue, often appear green, by candle-light?

_Mrs. B._ The light of a candle, is not of so pure a white as that of the sun: it has a yellowish tinge, and when refracted by the prism, the yellow rays predominate; and blue bodies reflect some of the yellow rays, from their being next to the blue, in the order of refrangibility; the superabundance of yellow rays, which is supplied by the candle, gives to blue bodies, a greenish hue.

_Caroline._ Candle-light must then give to all bodies, a yellowish tinge, from the excess of yellow rays; and yet it is a common remark, that people of a sallow complexion, appear fairer, or whiter, by candle-light.

_Mrs. B._ The yellow cast of their complexion is not so striking, when every surrounding object has a yellow tinge.

_Emily._ Pray, why does the sun appear red, through a fog?

_Mrs. B._ It is supposed to be owing to the rays, which are most refrangible, being also the most easily reflected: in passing through an atmosphere, loaded with moisture, as in foggy weather, and also in the morning and evening, when mists prevail, the _violet_, _indigo_, _blue_, and _green_ rays, are reflected back by the particles which load the air; whilst the _yellow_, _orange_, and _red_ rays, being less susceptible of reflection, pass on, and reach the eye.

_Caroline._ And, pray, why is the sky of a blue colour?

_Mrs. B._ You should rather say, the atmosphere; for the sky is a very vague term, the meaning of which, it would be difficult to define, philosophically.

_Caroline._ But the colour of the atmosphere should be white, since all the rays traverse it, in their passage to the earth.

_Mrs. B._ Do not forget that the direct rays of light which pass from the sun to the earth, do not meet our eyes, excepting when we are looking at that luminary, and thus intercept them; in which case, you know, that the sun appears white. The atmosphere is a transparent medium, through which the sun's rays pass freely to the earth; but the particles of which it is composed, also reflect the rays of light, and it appears that they possess the property of reflecting the blue rays, the most copiously: the light, therefore, which is reflected back into the atmosphere, from the surface of the earth, falls upon these particles of air, and the blue rays are returned by reflection: this reflection is performed in every possible direction; so that whenever we look at the atmosphere, some of these rays fall upon our eyes; hence we see the air of a blue colour. If the atmosphere did not reflect any rays, though the objects, on the surface of the earth, would be illuminated, the sky would appear perfectly black.

_Caroline._ Oh, how melancholy would that be; and how pernicious to the sight, to be constantly viewing bright objects against a black sky. But what is the reason that bodies often change their colour; as leaves, which wither in autumn, or a spot of ink, which produces an iron-mould on linen?

_Mrs. B._ It arises from some chemical change, which takes place in the arrangement of the component parts; by which they lose their tendency to reflect certain colours, and acquire the power of reflecting others. A withered leaf thus no linger reflects the blue rays; it appears, therefore, yellow, or has a slight tendency to reflect several rays, which produce a dingy brown colour.

An ink spot on linen, at first absorbs all the rays; but, from the action of soap, or of some other agent, it undergoes a chemical change, and the spot partially regains its tendency to reflect colours, but with a preference to reflect the yellow rays, and such is the colour of the iron-mould.

_Emily._ Bodies, then, far from being of the colour which they appear to possess, are of that colour to which they have the greatest aversion, with which they will not incorporate, but reject, and drive from them.

_Mrs. B._ It certainly is so; though I scarcely dare venture to advance such an opinion, whilst Caroline is contemplating her beautiful rose.

_Caroline._ My poor rose! you are not satisfied with depriving it of colour, but even make it have an aversion to it; and I am unable to contradict you.

_Emily._ Since dark bodies, absorb more solar rays than light ones, the former should sooner be heated if exposed to the sun?

_Mrs. B._ And they are found, by experience, to be so. Have you never observed a black dress, to be warmer than a white one?

_Emily._ Yes, and a white one more dazzling: the black is heated by absorbing the rays, the white is dazzling, by reflecting them.

_Caroline._ And this was the reason that the brown paper was burnt in the focus of the lens, whilst the white paper exhibited the most luminous spot, but did not take fire.

_Mrs. B._ It was so. It is now full time to conclude our lesson. At our next meeting, I shall give you a description of the eye.

Questions

1. (Pg. 179) What is meant by the refraction of light?

2. (Pg. 179) What is believed to be the cause of refraction?

3. (Pg. 180) How is a ray refracted in passing obliquely from air into water?

4. (Pg. 180) How is this refraction explained in fig. 1, plate 19?

5. (Pg. 180) What is fig. 2 intended to explain?

6. (Pg. 180) What is the rule respecting refraction, by different mediums?

7. (Pg. 181) What is meant by the perpendicular?

8. (Pg. 181) How does fig. 3, plate 19, elucidate the law of refraction?

9. (Pg. 181) What will be the effect on the apparent situation of the flower?

10. (Pg. 181) What effect has refraction upon the apparent depth of a stream of water?

11. (Pg. 182) How does the atmosphere refract the rays of the sun, as represented, fig. 4?

12. (Pg. 182) Why have we the rays of the sun always refracted?

13. (Pg. 182) What length of time is required for light to travel from the sun, to the earth?

14. (Pg. 182) What effect has this upon his apparent place?

15. (Pg. 182) How is the length of the day affected by refraction?

16. (Pg. 183) How are rays refracted, which fall obliquely upon a flat pane of glass, (fig. 5, plate 19?)

17. (Pg. 183) What is the reason that objects are distorted, when seen through common window glass?

18. (Pg. 184) What is meant by a lens?

19. (Pg. 184) What are the five kinds called, represented at fig. 1, plate 20?

20. (Pg. 184) What is meant by the axis of a lens?

21. (Pg. 184) How are parallel rays, refracted by the double convex lens, fig. 6, plate 19?

22. (Pg. 184) What is meant by the focus of a lens?

23. (Pg. 184) What is the focal distance of parallel rays, from a double convex lens?

24. (Pg. 184) How are the rays refracted by a concave lens, fig. 7, plate 19?

25. (Pg. 185) What is the effect of one plane side in a lens?

26. (Pg. 185) How is the focus of the plano-convex lens situated, fig. 2, plate 20?

27. (Pg. 185) How does a convex lens magnify objects, fig. 6, plate 19?

28. (Pg. 185) What is the article denominated which is represented at fig. 3, plate 20?

29. (Pg. 185) How will a ray be refracted, which enters on one side of the prism, in the direction A B?

30. (Pg. 185) What effect is produced by this refraction, as represented in fig. 4, plate 20?

31. (Pg. 186) Of what are the rays of white light said to be composed?

32. (Pg. 186) What colours are produced?

33. (Pg. 186) By what property, in light, does refraction enable us to separate these different rays?

34. (Pg. 187) What experiment may be performed with a piece of card, so as to exemplify the compound nature of light?

35. (Pg. 187) How can the same be shown by a lens, fig. 5. plate 20?

36. (Pg. 187) Is it certain that there are seven primitive colours in the spectrum?

37. (Pg. 188) How is the rainbow produced, and what is necessary to its production?

38. (Pg. 188) How are the solar rays affected by a convex lens?

39. (Pg. 188) Why is such a lens, called a burning glass?

40. (Pg. 188) Why are bodies of a dark colour, more readily inflamed, than those which are white?

41. (Pg. 189) What is believed to be the reason, why some bodies absorb more rays than others?

42. (Pg. 189) What determines the colour of any particular body?

43. (Pg. 189) What exemplifications are given?

44. (Pg. 189) By what reasoning is it proved, that bodies do not retain their colours in the dark?

45. (Pg. 190) What proof of the truth of this theory of colours, may be afforded by the prism?

46. (Pg. 191) Why will green leaves, when exposed to the red ray, appear of a dingy brown?

47. (Pg. 191) Bodies, in general, when placed in a ray differing in colour from their own, appear of a mixed hue, what causes this?

48. (Pg. 191) Why will bodies of a pale, or light hue, most perfectly, assume the different colours of the spectrum?

49. (Pg. 192) Upon what property in a body, does the darkness of its colour depend?

50. (Pg. 192) Why do some bodies appear white, others black, and others of different colours?

51. (Pg. 192) From what cause do blue articles appear green, by candle-light?

52. (Pg. 193) What is believed to be the cause, of the red appearance of the sun, through a fog, or misty atmosphere?

53. (Pg. 193) From what is the blue colour of the sky, thought to arise?

54. (Pg. 193) What would be the colour of the sky, did not the atmosphere reflect light?

55. (Pg. 193) From what cause do some bodies change their colour, as leaves formerly green, become brown, and ink, yellow?

56. (Pg. 194) Why is a black dress, warmer in the sunshine, than a white one of the same texture?

CONVERSATION XVII.

ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS.

DESCRIPTION OF THE EYE. OF THE IMAGE ON THE RETINA. REFRACTION BY THE HUMOURS OF THE EYE. OF THE USE OF SPECTACLES. OF THE SINGLE MICROSCOPE. OF THE DOUBLE MICROSCOPE. OF THE SOLAR MICROSCOPE. MAGIC LANTHORN. REFRACTING TELESCOPE. REFLECTING TELESCOPE.

MRS. B.