called the object-glass; the other N O, acts on the principle of the singie microscope, and is called the eye-glass. 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 shines, 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 XXIII. fig. 1.) which is a small insect, before the lens CD, 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 diminishing 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 the whole of the light admitted upon it ? 1014. What does Fig. 1, plate XXIII represent?-1015. How would you describe a solar microscope by the use of this figure? 1016. Where must an object be placed in regard to a lens, so that the object be magnified? - 1017. Where, so that the object be diminished?-1018. Where must all the light fall, used in the solar microscope, so that the effect be the most favourable? Mrs. B. Very well. We shall enlarge the opening and place the lens XY (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 ob ject; 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. PQ, (fig. 2.) is a small mirror placed on the outside of the window shutter, which receives the incident rays SS, 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 would 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 pass 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 lantern constructed on the same principles ? * * The magic lantern is an instrument used for magnifying paintings on glass, and throwing their images upon a white screen in a darkened chamber. 1019. What does Fig. 2, plate XXIII represent? 1020. What is the use of the mirror in the solar microscope? 1021. For what objects can the solar microscope be used? -1022. How çan opaque objects be exhibited? -1023. Which figure illus trates this? 1024. What is a magic lantern? 1 • Mrs. B. Yes; with this difference, that 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 approach the object to 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 ? a Mrs. B. Very well, Emily; I congratulate you on having invented telescope. In figure 4, the lens C D, forms an image E F, of the object A B; and the lens XY, 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 usinverted; 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 glasses are used to view terrestrial objects; for no inconvenience arises from seeing the celestial bodies inverted. 1025. How does a magic lantern differ from a solar microscope? 1026. What is the reason that the solar microscope may not be used with objects at a great distance with equal effect?1027. What does Fig. 4, plate xx represent?--1028. How would you explain the principle of the common refracting telescope by the use of that figure?-1029. What is necessary when the image of an object is to be exhibited erect?---1030. Why are not these additional glasses used in viewing celestial objects? 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 farthest from the lens. Mrs. B. Your observation applies only to the lens C D, or object glass, which serves to bring an image of the object nearer the eye; for the lens X Y, or eye-glass, 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. 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-glass the same as in the refracting telescope to magnify the image. The advantage of the reflecting telescope is, that mirrors whose focus is six six feet will magnify as much as lenses of a hundred feet. Caroline. But I thought it was the eye-glass only which magnified the image; and that the other lens served to bring a diminished image nearer to the eye. Mrs. B. The image is diminished in comparison to the object, it is true; but it is magnified if you compare it to the dimensions of which it would appear without the intervention of any optical instrument; and this magnifying power is greater in reflecting than in refracting telescopes. We must now bring our observations to a conclusion, for I have communicated to you the whole of my very limited stock of knowledge of Natural Philosophy. If it will enable you to make further progress in that science, my wishes will be satisfied; but remember that, in order that the study of nature may be productive of happiness, it must lead to an entire confidence in the wisdom and goodness of its bounteous Author. 1031. What part of the telescope exhibited in the figure may be considered as a simple microscope? -1032. When a very great magnifying power is required, how must telescopes be constructed? -1033. In the reflecting telescopes why are mirrors used? 1034. How great is the advantage of the reflecting telescope? ABERRATION, in Astronomy, an apparent motion of the heavenly bodies, produced by the progressive motion of light and the earth's an sual motion. ACCELERATION, in mechanics, denotes the augmentation or increase of motion in accelerated bodies. ACOUSTICS is the science which treats of the nature, phenomena, and laws of the sense of sound. It extends to the theory of musical concord and harmony, and is, therefore, a valuable and interesting science. AIR, a thin, elastic fluid, surrounding the globe of the earth. The air, together with the clouds and vapors that float in it, is called the atmosphere. The height to which the atmosphere extends has never been ascertained; but, at a greater height than 45 miles, it ceases to reflect the rays of light from the sun. AIR-PUMPS are machines made for exhausting the air from certain glass vessels, adapted to the purpose of experiments on air. ANGLE is the inclination of two lines meeting one another in a point, and called the legs of the angle. Angles, in Geometry, are called rigbi, acute, and obtuse. A right angle contains just 90 degrees, or the quarter part of a circle. Acute angles contain less, and obtuse angle more than 90 degrees. ANGLE OF INCIDENCE is that which is contained between the line described by the incident ray, and a line perpendicular to the surface on which the ray strikes, raised from the point of incidence. ANGLE OF REFLECTION is contained between the line described by the reflected ray, and a line perpendicular to the reflecting surface, at the point from which the ray is reflected. ANGLE OF REFRACTION is that which is contained between the line described by the refracted ray, and a line perpendicular to the refracting surface at the point in which the ray passes through that surface. ANGLE OF VISION is that which is contained between lines coming from opposite parts of an object and meeting in the eye. ANTARCTIC CIRCLE, in Astronomy, is an imaginary line extending round the south pole, 60 degrees from the equator and parallel to it. APHELION, in Astronomy, is that point in any planet's orbit in which the orbit is most distant from the sun. AQUEOUS HUMOR, or watery humor of the eye; it is the first and outermost, and that which is less dense than either the vitreous or crystalline. It is transparent and colorless like water, and fills up the space that lies between the cornea and the crystalline humor. ARCTIC CIRCLE, in Astronomy, is an imaginary line extending round the north pole, 66.1 degrees from the equator and parallel to it. AREOMETER, an instrument by which the density and gravity of fluids are measured. ARIES, in astronomy, a constellation of fixed stars, drawn on the globe in the figure of a ram. It is the first of the twelve signs of the zodiac from which a twelfth part of the ecliptic takes its name. It consists of sixty-six stars. W ASCENSION, in astronomy, the rising of the sun or star, or any part of the equinoctial with it, above the horizon. ASTERIODS, a name given by Dr. Herschell to the new planets, Ceres, Juno, Paltas, and Vesta, lately discovered. ASTRONOMY is the science which teaches the motions of the earth, the sun, moon, planets, comets, and stars, and explains the phe nomena occasioned by those motions. ATMOSPHERE, or atmospheric air, the fluid that surrounds our earth. Without this fluid no animal could exist; vegetation would cease, and there would be neither rain nor refreshing dews to moisten the face of the ground; and though the sun and stars might be seen as bright specks, yet there would be little enjoy. ment of light, could we exist without it. ATTRACTION, a general term, used to denote the power or principle by which bodies mutually tend towards each other, without regarding the cause or action that may be the means of producing the effect. ATTRACTION OF COHESION takes place between the constituent particles of the same body. By this principle bodies preserve their forms and are prevented from falling to pieces. ATTRACTION OF GRAVITATION, or gravity, is the name of that force by which distant bodies tend towards one another. AXIS of a planet is an imaginary line which passes through its centre, and on which it turns; and it is this motion which produces day and night. With that side of the planet facing the sun it is day; and with the opposite side, which remains in darkness, it is night. AXIS OF MOTION, in mechanics, is the line about which a revolving body moves. Philo. sophically speaking, the axis of motion is said to be at rest, whilst the other parts of a body move round it, and the further any part of a body is from the axis of motion, the greater is its velocity. AXIS OF THE EARTH is an imaginary line conceived to pass through the centre of it from one pole to the other, about which is performed its diurnal rotation. AXIS, în optics, is that ray, among all others that are sent to the eye, which falls perpendicu larly upon it, and which consequently passes through the centre of the eye. AXIS OF A GLASS, OR LENS, is a right line joining the middle points of the two opposite surfaces of the glass. BALANCE, or BALLANCE, in mechanics, one of the simple powers which serves to find out the equality or difference of weight in heavy bodies. BALLOON, a machine used in navigation through the air. It takes its name from the form of the machine, the word balloon signifying any spherical hollow body, of whatever matter it be composed, or for whatever purposes it be designed. BAROMETER, an instrument for measuring the weight or pressure of the atmosphere; and by that means measuring heights and depths, determining variations in the state of the air, and foretelling the changes in the weather. |