Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 50

_Mrs. B._ Certainly. But short-sighted persons have another resource, for objects which they can not bring near to their eyes; this is, to place a concave lens, C D, (fig. 1, plate 22.) before the eye, in order to increase the divergence of the rays. The effect of a concave lens, is, you know, exactly the reverse of a convex one: it renders parallel rays divergent, and those which are already divergent, still more so. By the a.s.sistance of such gla.s.ses, therefore, the rays from a distant object, fall on the pupil, as divergent as those from a less distant object; and, with short-sighted people, they throw the image of a distant object, back, as far as the retina.

_Caroline._ This is an excellent contrivance, indeed.

_Mrs. B._ And tell me, what remedy would you devise for such persons as have a contrary defect in their sight; that is to say, who are long-sighted, in whom the crystalline humour, being too flat, does not refract the rays sufficiently, so that they reach the retina before they are converged to a point?

_Caroline._ I suppose that a contrary remedy must be applied to this defect; that is to say, a convex lens, L M, fig. 2, to make up for the deficiency of convexity of the crystalline humour, O P. For the convex lens would bring the rays nearer together, so that they would fall, either less divergent, or parallel, on the crystalline humour; and, by being sooner converged to a focus, would fall on the retina.

_Mrs. B._ Very well, Caroline. This is the reason why elderly people, the humours of whose eyes are decayed by age, are under the necessity of using convex spectacles. And when deprived of that resource, they hold the object at a distance from their eyes, as in fig. 3, in order to bring the focus more forward.

_Caroline._ I have often been surprised, when my grandfather reads without his spectacles, to see him hold the book at a considerable distance from his eyes. But I now understand the cause; the more distant the object is from the crystalline lens, the nearer to it, will the image be formed.

_Emily._ I comprehend the nature of these two opposite defects very well; but I cannot now conceive, how any sight can be perfect: for, if the crystalline humour is of a proper degree of convexity, to bring the image of distant objects to a focus on the retina, it will not represent near objects distinctly; and if, on the contrary, it is adapted to give a clear image of near objects, it will produce a very imperfect one, of distant objects.

_Mrs. B._ Your observation is very good, Emily; and it is true, that every person would be subject to one of these two defects, if we had it not in our power to adapt the eye, to the distance of the object; it is believed that this is accomplished, by our having a command over the crystalline lens, so as to project it towards, or draw it back from the object, as circ.u.mstances require, by means of the two muscles, to which the crystalline humour is attached; so that the focus of the rays, constantly falls on the retina, and an image is formed equally distinct, either of distant objects, or of those which are near.

_Caroline._ In the eyes of fishes, which are the only eyes I have ever seen separate from the head, the cornea does not protrude, in that part of the eye which is exposed to view.

_Mrs. B._ The cornea of the eye of a fish is not more convex than the rest of the ball of the eye; but to supply this deficiency, their crystalline humour is spherical, and refracts the rays so much, that it does not require the a.s.sistance of the cornea to bring them to a focus on the retina.

_Emily._ Pray, what is the reason that we cannot see an object distinctly, if we place it very near to the eye?

_Mrs. B._ Because the rays fall on the crystalline humour, too divergent to be refracted to a focus on the retina; the confusion, therefore, arising from viewing an object too near the eye, is similar to that which proceeds from a flattened crystalline humour; the rays reach the retina before they are collected to a focus, (fig. 4.) If it were not for this imperfection, we should be able to see and distinguish the parts of objects, which, from their minuteness, are now invisible to us; for, could we place them very near the eye, the image on the retina would be so much magnified, as to render them visible.

_Emily._ And could there be no contrivance, to convey the rays of objects viewed, close to the eye, so that they should be refracted to a focus on the retina?

_Mrs. B._ The microscope is constructed for this purpose. The single microscope (fig. 5.) consists simply of a convex lens, commonly called a magnifying gla.s.s; in the focus of which the object is placed, and through which it is viewed: by this means, you are enabled to place your eye very near to the object, for the lens A B, by diminis.h.i.+ng the divergence of the rays, before they enter the pupil C, makes them fall parallel on the crystalline humour D, by which they are refracted to a focus on the retina, at R R.

_Emily._ This is a most admirable invention, and nothing can be more simple; for the lens magnifies the object, merely by allowing us to bring it nearer to the eye.

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

_Mrs. B._ Those lenses, therefore, which have the shortest focus will magnify the object most, because they enable us to place it nearest to the eye.

_Emily._ But a lens, that has the shortest focus, is most bulging or convex; and the protuberance of the lens will prevent the eye from approaching very near to the object.

_Mrs. B._ This is remedied by making the lens extremely small: it may then be spherical without occupying much s.p.a.ce, and thus unite the advantages of a short focus, and of allowing the eye to approach the object.

There is a mode of magnifying objects, without the use of a lens: if you look through a hole, not larger than a small pin, you may place a minute object near to the eye, and it will be distinct, and greatly enlarged.

This piece of tin has been perforated for the purpose; place it close to your eye, and this small print before it.

_Caroline._ Astonis.h.i.+ng! the letters appear ten times as large as they do without it: I cannot conceive how this effect is produced.

_Mrs. B._ The smallness of the hole, prevents the entrance into the eye, of those parts of every pencil of rays which diverge much; so that, notwithstanding the nearness of the object, those rays from it, which enter the eye, are nearly parallel, and are, therefore, brought to a focus by the humours of the eye.

_Caroline._ We have a microscope at home, which is a much more complicated instrument than that you have described.

_Mrs. B._ It is a double microscope, (fig. 6.) in which you see, not the object A B, but a magnified image of it, _a b_. In this microscope, two lenses are employed; the one, L M, for the purpose of magnifying the object, is called the object-gla.s.s, the other, N O, acts on the principle of the single microscope, and is called the eye-gla.s.s.

There is another kind of microscope, called the solar microscope, which is the most wonderful from its great magnifying power: in this we also view an image formed by a lens, not the object itself. As the sun s.h.i.+nes, I can show you the effect of this microscope; but for this purpose, we must close the shutters, and admit only a small portion of light, through the hole in the window-shutter, which we used for the camera obscura. We shall now place the object A B, (plate 23, fig. 1.) which is a small insect, before the lens C D, and nearly at its focus: the image E F, will then be represented on the opposite wall, in the same manner, as the landscape was in the camera obscura; with this difference, that it will be magnified, instead of being diminished. I shall leave you to account for this, by examining the figure.

_Emily._ I see it at once. The image E F is magnified, because it is farther from the lens, than the object A B; while the representation of the landscape was diminished, because it was nearer the lens, than the landscape was. A lens, then, answers the purpose equally well, either for magnifying or diminis.h.i.+ng objects?

_Mrs. B._ Yes: if you wish to magnify the image, you place the object near the focus of the lens; if you wish to produce a diminished image, you place the object at a distance from the lens, in order that the image may be formed in, or near the focus.

_Caroline._ The magnifying power of this microscope is prodigious: but the indistinctness of the image, for want of light, is a great imperfection. Would it not be clearer, if the opening in the shutter were enlarged, so as to admit more light?

_Mrs. B._ If the whole of the light admitted, does not fall upon the object, the effect will only be to make the room lighter, and the image consequently less distinct.

_Emily._ But could you not by means of another lens, bring a large pencil of rays to a focus on the object, and thus concentrate upon it the whole of the light admitted?

_Mrs. B._ Very well. We shall enlarge the opening, and place the lens X Y (fig. 2.) in it, to converge the rays to a focus on the object A B.

There is but one thing more wanting to complete the solar microscope, which I shall leave to Caroline's sagacity to discover.

_Caroline._ Our microscope has a small mirror attached to it, upon a moveable joint, which can be so adjusted as to receive the sun's rays, and reflect them upon the object: if a similar mirror were placed to reflect light upon the lens, would it not be a means of illuminating the object more perfectly?

_Mrs. B._ You are quite right. P Q (fig. 2.) is a small mirror, placed on the outside of the window-shutter, which receives the incident rays S S, and reflects them on the lens X Y. Now that we have completed the apparatus, let us examine the mites on this piece of cheese, which I place near the focus of the lens.

_Caroline._ Oh, how much more distinct the image now is, and how wonderfully magnified! The mites on the cheese look like a drove of pigs scrambling over rocks.

_Emily._ I never saw any thing so curious. Now, an immense piece of cheese has fallen: one might imagine it an earthquake: some of the poor mites must have been crushed; how fast they run--they absolutely seem to gallop.

But this microscope can be used only for transparent objects; as the light must pa.s.s through them, to form the image on the wall?

_Mrs. B._ Very minute objects, such as are viewed in a microscope, are generally transparent, but when opaque objects are to be exhibited, a mirror M N (fig. 3.) is used to reflect the light on the side of the object next the wall: the image is then formed by light reflected from the object, instead of being transmitted through it.

_Emily._ Pray, is not a magic lanthorn constructed on the same principles?

_Mrs. B._ Yes, with this difference; the objects to be magnified, are painted upon pieces of gla.s.s, and the light is supplied by a lamp, instead of the sun.

The microscope is an excellent invention to enable us to see and distinguish objects, which are too small to be visible to the naked eye.

But there are objects, which, though not really small, appear so to us, from their distance; to these, we cannot apply the same remedy; for when a house is so far distant, as to be seen under the same angle as a mite which is close to us, the effect produced on the retina is the same: the angle it subtends is not large enough for it to form a distinct image on the retina.

_Emily._ Since it is impossible, in this case, to make the object approach the eye, cannot we by means of a lens bring an image of it, nearer to us?

_Mrs. B._ Yes; but then the object being very distant from the focus of the lens, the image would be too small to be visible to the naked eye.

_Emily._ Then, why not look at the image through another lens, which will act as a microscope, enable us to bring the image close to the eye, and thus render it visible?

_Mrs. B._ Very well, Emily; I congratulate you on having invented a telescope. In figure 4, the lens C D, forms an image E F, of the object A B; and the lens X Y, serves the purpose of magnifying that image; and this is all that is required in a common refracting telescope.

_Emily._ But in fig. 4, the image is not inverted on the retina, as objects usually are: it should therefore appear to us inverted; and that is not the case in the telescopes I have looked through.

_Mrs. B._ When it is necessary to represent the image erect, two other lenses are required; by which means a second image is formed, the reverse of the first, and consequently upright. These additional gla.s.ses are used to view terrestrial objects; for no inconvenience arises from seeing the celestial bodies inverted.

_Emily._ The difference between a microscope and a telescope, seems to be this:--a microscope produces a magnified image, because the object is nearest the lens; and a telescope produces a diminished image, because the object is furthest from the lens.

_Mrs. B._ Your observation applies only to the lens C D, or object-gla.s.s, which serves to bring an image of the object nearer the eye; for the lens X Y, or eye-gla.s.s, is, in fact, a microscope, as its purpose is to magnify the image.

When a very great magnifying power is required, telescopes are constructed with concave mirrors, instead of lenses. These are called reflecting telescopes, because the image is reflected by metallic mirrors. Concave mirrors, you know, produce by reflection, an effect similar to that of convex lenses, by refraction. In reflecting telescopes, therefore, mirrors are used in order to bring the image nearer the eye; and a lens, or eye-gla.s.s, the same as in the refracting telescope, to magnify the image.