Part 19

_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.

Questions

1. (Pg. 157) What is optics?

2. (Pg. 157) What is meant by a luminous body?

3. (Pg. 157) What is meant by a dark body, and what by an opaque body?

4. (Pg. 157) What are transparent bodies?

5. (Pg. 157) What is a medium?

6. (Pg. 158) How is light projected from luminous bodies, and how, from every point of such bodies, (fig. 1, plate 15?)

7. (Pg. 158) Why do not the rays of light from different points, stop each other's progress?

8. (Pg. 158) What is a ray, and what a pencil of rays? fig. 2, plate 15.

9. (Pg. 158) Do we know whether light is a substance, similar to bodies in general?

10. (Pg. 158) When a ray of light falls upon an opaque body, what is the result?

11. (Pg. 159) In what does shadow consist?

12. (Pg. 159) Why are they, in general, but partially dark?

13. (Pg. 159) Upon what does the intensity of a shadow depend?

14. (Pg. 159) How are shadows affected by the size of the luminous body, as represented in plate 15, fig. 3?

15. (Pg. 159) When is the shadow larger than the intercepting body?

16. (Pg. 160) What is explained by fig. 4, plate 15?

17. (Pg. 160) What will be the effect of several lights, as in fig. 5, plate 15?

18. (Pg. 160) Why will neither of these shadows be very dark?

19. (Pg. 160) What becomes of the light which falls upon an opaque body?

20. (Pg. 160) What is meant by reflection?

21. (Pg. 161) What is meant by the incident, and reflected rays?

22. (Pg. 161) What is the result, when the incident ray falls perpendicularly, and what, when it falls obliquely?

23. (Pg. 161) What two angles are always equal in this case?

24. (Pg. 161) To what law in mechanics, is this analogous, as represented in fig. 4, plate 2?

25. (Pg. 161) What is represented by fig. 6, plate 15?

26. (Pg. 161) By what light are we enabled to see opaque, and by what, luminous bodies?

27. (Pg. 161) What enables us to see a ray of light in its passage, through a darkened room?

28. (Pg. 162) By what reasoning would you prove that an object, such, for example, as a house, is seen by reflected light?

29. (Pg. 162) Why may one side of such object appear more bright than another side?

30. (Pg. 162) How is the fact exemplified by the sun, or moon, shining upon water?

31. (Pg. 162) Why is this best evinced by moonlight?

32. (Pg. 163) By what light do we see the moon, and why is it comparatively feeble?

33. (Pg. 163) What circumstance, renders objects seen by moonlight, still less vivid?

34. (Pg. 164) What is meant by the pupil of the eye?

35. (Pg. 164) What by the retina?

36. (Pg. 164) How do the rays of light operate on the eye in producing vision?

37. (Pg. 164) How may this be exemplified, in a darkened room?

38. (Pg. 164) What is meant by a _camera obscura_?

39. (Pg. 164) How is it explained in plate 16?

40. (Pg. 165) Why are the objects inverted and reversed?

41. (Pg. 165) What analogy is there between the camera obscura, and the eye?

42. (Pg. 165) Is it the object, or its picture on the retina, which presents to the mind an idea of the object seen?

43. (Pg. 166) By what organs is sensation produced, and how must these organs be affected?

44. (Pg. 166) How will the idea of contact, apply to objects not touching the eye?

45. (Pg. 167) Why do not objects appear reversed to the eye, as in the camera obscura?

CONVERSATION XV.

OPTICS--_continued_.

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

ANGLE OF VISION. REFLECTION OF PLAIN MIRRORS. REFLECTION OF CONVEX MIRRORS. REFLECTION OF CONCAVE MIRRORS.

CAROLINE.

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

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

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

_Caroline._ I know no other reason than their distance.

_Mrs. B._ It is a fact, that distance causes objects to appear smaller, but to state the fact, is not to give the reason. We must refer again to the camera obscura, to account for this circumstance; and you will find, that the different apparent dimensions of objects at different distances, proceed from our seeing, not the objects themselves, but merely their image on the retina. Fig. 1, plate 17, represents a row of trees, as viewed in the camera obscura. I have expressed the direction of the rays, from the objects to the image, by lines. Now, observe, the ray which comes from the top of the nearest tree, and that which comes from the foot of the same tree, meet at the aperture, forming an angle of about twenty-five degrees; the angle under which we see any object, is called, the visual angle, or, angle of vision. These rays cross each other at the aperture, forming equal angles on each side of it, and represent the tree inverted in the camera obscura. The degrees of the image, are considerably smaller than those of the object, but the proportions are perfectly preserved.

Now, let us notice the upper and lower ray, from the most distant tree; they form an angle of not more than twelve or fifteen degrees, and an image of proportional dimensions. Thus, two objects of the same size, as the two trees of the avenue, form figures of different sizes in the camera obscura, according to their distance; or, in other words, according to the angle of vision under which they are seen. Do you understand this?

_Caroline._ Perfectly.

_Mrs. B._ Then you have only to suppose, that the representation in the camera obscura, is similar to that on the retina.

Now, since objects of the same magnitudes, appear to be of different dimensions, when at different distances from us, let me ask you which it is, that you see; the real objects, which, we know, do not vary in size, or the images, which, we know, do vary, according to the angle of vision under which we see them?

_Caroline._ I must confess, that reason is in favour of the latter. But does that chair, at the further end of the room, form an image on my retina, much smaller than this which is close to me? they appear exactly of the same size.

_Mrs. B._ Our senses are imperfect, but the experience we acquire by the sense of touch, corrects the illusions of our sight, with regard to objects within our reach. You are so perfectly convinced, of the real size of objects, which you can handle, that you do not attend to the apparent difference.

Does that house appear to you much smaller, than when you are close to it?

_Caroline._ No, because it is very near us.

_Mrs. B._ And yet you can see the whole of it, through one of the windows of this room. The image of the house on your retina must, therefore, be smaller than that of the window through which you see it. It is your knowledge of the real magnitude of the house which prevents your attending to its apparent size. If you were accustomed to draw from nature, you would be fully aware of this difference.

_Emily._ And pray, what is the reason that, when we look up an avenue, the trees not only appear smaller as they are more distant, but seem gradually to approach each other, till they meet in a point?

_Mrs. B._ Not only the trees, but the road which separates the two rows, forms a smaller visual angle, in proportion as it is more distant from us; therefore, the width of the road gradually diminishes, as well as the size of the trees, till at length the road apparently terminates in a point, at which the trees seem to meet.

_Emily._ I am very glad to understand this, for I have lately begun to learn perspective, which appeared to me a very dry study; but now that I am acquainted with some of the principles on which it is founded, I shall find it much more interesting.

_Caroline._ In drawing a view from nature, it seems that we do not copy the real objects, but the image they form on the retina of our eyes?

_Mrs. B._ Certainly. In sculpture, we copy nature as she really exists; in painting, we represent her, as she appears to us.

We must now conclude the observations that remain to be made, on the angle of vision.

If the rays, proceeding from the extremities of an object, with an ordinary degree of illumination, do not enter the eye under an angle of more than two seconds, which is the 1-1800th part of a degree, it is invisible. There are, consequently, two cases in which objects may be invisible; if they are either so small, or so distant, as to form an angle of less than two seconds of a degree.

In like manner, if the velocity of a body does not exceed 20 degrees in an hour, its motion is imperceptible.

_Caroline._ A very rapid motion may then be imperceptible, provided the distance of the moving body, is sufficiently great.

_Mrs. B._ Undoubtedly; for the greater its distance, the smaller will be the angle, under which its motion will appear to the eye. It is for this reason, that the motion of the celestial bodies is invisible, although inconceivably rapid.

_Emily._ I am surprised, that so great a velocity as 20 degrees an hour, should be invisible.

_Mrs. B._ The real velocity depends upon the space comprehended in each degree, and upon the time, in which the moving body, passes over that space. But we can only know the extent of this space, by knowing the distance of the moving body, from its centre of motion; for supposing two men to set off at the same moment from A and B, (fig. 2.) to walk each to the end of their respective lines, C and D; if they perform their walk in the same space of time, they must have proceeded at a very different rate; and yet to an eye situated at E, they will appear to have moved with equal velocity, because they will both have gone through an equal number of degrees, though over a very unequal length of ground. The number of degrees over which a body moves in a given time, is called its angular velocity; two bodies, you see, may have the same angular, or apparent velocity, whilst their real velocities may differ almost infinitely. Sight is an extremely useful sense, no doubt, but it cannot always be relied on, it deceives us both in regard to the size and the distance of objects; indeed, our senses would be very liable to lead us into error, if experience did not set us right.

_Emily._ Between the two, I think that we contrive to acquire a tolerably accurate idea of objects.

_Mrs. B._ At least sufficiently so, for the general purposes of life. To convince you how requisite experience is, to correct the errors of sight, I shall relate to you, the case of a young man, who was blind from his infancy, and who recovered his sight at the age of fourteen, by the operation of couching. At first, he had no idea, either of the size, or distance of objects, but imagined that every thing he saw touched his eyes; and it was not, till after having repeatedly felt them, and walked from one object to another, that he acquired an idea of their respective dimensions, their relative situations, and their distances.

_Caroline._ The idea that objects touched his eyes, is, however, not so absurd, as it at first appears; for if we consider that we see only the image of objects, this image actually touches our eyes.

_Mrs. B._ That is, doubtless, the reason of the opinion he formed, before the sense of touch had corrected his judgment.

_Caroline._ But since an image must be formed on the retina of each of our eyes, why do we not see objects double?

_Mrs. B._ The action of the rays, on the optic nerve of each eye, is so perfectly similar, that they produce but a single sensation; the mind, therefore, receives the same idea, from the retina of both eyes, and conceives the object to be single.

_Caroline._ This is difficult to comprehend, and I should think, can be but conjectural.

_Mrs. B._ I can easily convince you, that you have a distinct image of an object formed on the retina of each eye. Look through the window, with both eyes open, at some object exactly opposite to one of the upright bars of the sash.

_Caroline._ I now see a tree, the body of which, appears to be in a line exactly opposite to one of the bars.

_Mrs. B._ If you now shut your right eye, and look with the left, it will appear to the left of the bar; then by closing the left eye, and looking with the other, it will appear to the right of the bar.

_Caroline._ That is true, indeed!

_Mrs. B._ There are, evidently, two representations of the tree in different situations, which must be owing to an image of it being formed on each eye; if the action of the rays, therefore, on each retina, were not so perfectly similar as to produce but one sensation, we should see double; and we find that to be the case with some persons, who are afflicted with a disease in one eye, which prevents the rays of light from affecting it in the same manner as the other.

_Emily._ Pray, Mrs. B., when we see the image of an object in a looking-glass, why is it not inverted, as in the camera obscura, and on the retina of the eye?

_Mrs. B._ Because the rays do not enter the mirror by a small aperture, and cross each other, as they do at the orifice of a camera obscura, or the pupil of the eye.

When you view yourself in a mirror, the rays from your eyes fall perpendicularly upon it, and are reflected in the same line; the image is, therefore, described behind the glass, and is situated in the same manner as the object before it.

_Emily._ Yes, I see that it is; but the looking-glass is not nearly so tall as I am, how is it, therefore, that I can see the whole of my figure in it?

_Mrs. B._ It is not necessary that the mirror should be more than half your height, in order that you may see the whole of your person in it, (fig. 3.) The ray of light A B, from your eye, which falls perpendicularly on the mirror B D, will be reflected back, in the same line; but the ray from your feet, will fall obliquely on the mirror, for it must ascend in order to reach it; it will, therefore, be reflected in the line A D: and since we view objects in the direction of the reflected rays, which reach the eye, and since the image appears at the same distance, behind the mirror, that the object is before it, we must continue the line A D to E, and the line C D to F, at the termination of which, the image will be represented.

_Emily._ Then I do not understand why I should not see the whole of my person in a much smaller mirror, for a ray of light from my feet would always reach it, though more obliquely.

_Mrs. B._ True; but the more obliquely the ray falls on the mirror, the more obliquely it will be reflected; the ray would, therefore, be reflected above your head, and you could not see it. This is shown by the dotted line (fig. 3.)

Now stand a little to the right of the mirror, so that the rays of light from your figure may fall obliquely on it----

_Emily._ There is no image formed of me in the glass now.

_Mrs. B._ I beg your pardon, there is; but you cannot see it, because the incident rays, falling obliquely on the mirror, will be reflected obliquely, in the opposite direction; the angles of incidence, and reflection, being equal. Caroline, place yourself in the direction of the reflected rays, and tell me whether you do not see Emily's image in the glass?

_Caroline._ Let me consider.--In order to look in the direction of the reflected rays, I must place myself as much to the left of the glass, as Emily stands to the right of it.--Now I see her image, not straight before me, however, but before her; and it appears at the same distance behind the glass, that she is in front of it.

_Mrs. B._ You must recollect, that we always see objects in the direction of the last rays, which reach our eyes. Figure 4 represents an eye, looking at the image of a vase, reflected by a mirror; it must see it in the direction of the ray A B, as that is the ray which brings the image to the eye; prolong the ray to C, and in that spot will the image appear.

_Caroline._ I do not understand why a looking-glass reflects the rays of light; for glass is a transparent body, which should transmit them!

_Mrs. B._ It is not the glass that reflects the rays which form the image you behold, but the silvering behind it; this silvering is a compound of mercury and tin, which forms a brilliant metallic coating. The glass acts chiefly as a transparent case, through which the rays find an easy passage, to, and from, the quicksilver.

_Caroline._ Why then should not mirrors be made simply of mercury?

_Mrs. B._ Because mercury is a fluid. By amalgamating it with tinfoil, it becomes of the consistence of paste, attaches itself to the glass, and forms, in fact, a metallic mirror, which would be much more perfect without its glass cover, for the purest glass is never perfectly transparent; some of the rays, therefore, are lost during their passage through it, by being either absorbed, or irregularly reflected.

This imperfection of glass mirrors, has introduced the use of metallic mirrors, for optical purposes.

_Emily._ But since all opaque bodies reflect the rays of light, I do not understand why they are not all mirrors.

_Caroline._ A curious idea indeed, sister; it would be very gratifying to see oneself in every object at which one looked.

_Mrs. B._ It is very true that all opaque objects reflect light; but the surface of bodies, in general, is so rough and uneven, that the reflection from them is extremely irregular, and prevents the rays from forming an image on the retina. This, you will be able to understand better, when I shall explain to you the nature of vision, and the structure of the eye.

You may easily conceive the variety of directions in which rays would be reflected by a nutmeg-grater, on account of the inequality of its surface, and the number of holes with which it is pierced. All solid bodies more or less resemble the nutmeg-grater, in these respects; and it is only those which are susceptible of receiving a polish, that can be made to reflect the rays with regularity. As hard bodies are of the closest texture, the least porous, and capable of taking the highest polish, they make the best mirrors; none, therefore, are so well calculated for this purpose, as metals.

_Caroline._ But the property of regular reflection, is not confined to this class of bodies; for I have often seen myself, in a highly polished mahogany table.

_Mrs. B._ Certainly; but as that substance is less durable, and its reflection less perfect, than that of metals, I believe it would seldom be chosen, for the purpose of a mirror.

There are three kinds of mirrors used in optics; the _plain_, or _flat_, which are the common mirrors we have just mentioned; _convex_ mirrors, and _concave_ mirrors. The reflection of the two latter, is very different from that of the former. The plain mirror, we have seen, does not alter the direction of the reflected rays, and forms an image behind the glass, exactly similar to the object before it. A convex mirror has the peculiar property of making the reflected rays diverge, by which means it diminishes the image; and a concave mirror makes the rays converge, and under certain circumstances, magnifies the image.

_Emily._ We have a convex mirror in the drawing-room, which forms a beautiful miniature picture of the objects in the room; and I have often amused myself with looking at my magnified face in a concave mirror. But I hope you will explain to us, why the one enlarges, while the other diminishes the objects it reflects.