called the object-glass; the other N O, acts on the principle of the single 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, 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 further 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 purposé 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 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 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 magick lantern constructed on the same principles ?*

The magick 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?what objects can the solar microscope be used?

-1021. For -1022. How

can opaque objects be exhibited?1023. Which figure illus trates this ?- -1024. What is a magick lantern?

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?

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 glasses are used to view terrestrial objects; for no inconvenience arises from seeing the celestial bodies inverted.

1025. How does a magick 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 XXIII. 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 objects 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 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 appa- pole, 66 1-2 degrees from the equator and rent motion of the heavenly bodies, pro- parallel to it.

duced by the progressive motion of light AREOMETER, an instrument by which and the earth's annual motion. the density and gravity of fluids are mea

ACCELERATION, in mechanicks, de-sured. notes the augmentation or increase of mo- ARIES, in astronomy, a constellation of tion in accelerated bodies. fixed stars, drawn on the globe in the figure ACOUSTICKS is the science which treats of a ram. It is the first of the twelve signs of the nature, phenomena, and laws of the of the zodiac from which a twelfth part of sense of sound. It extends to the theory of the ecliptick takes its name. It consists of musical concord and harmony, and is, there- sixty-six stars. fore, a valuable and interesting science.

ASCENSION, in astronomy, the rising AIR, a thin, elastick fluid, surrounding the of the sun or star, or any part of the equiglobe of the earth. The air, together with noctial with it, above the horizon. the clouds and vapours that float in it, is ASTERIODS, a name given by Dr. Hercalled the atmosphere. The height to which schel to the new planets, Ceres, Juno, Palthe atmosphere extends has never been as-las, and Vesta, lately discovered. certained; but, at a greater height than 45 ASTRONOMY is the science which miles, it ceases to reflect the rays of light teaches the motions of the earth, the sun, from the sun. moon, planets, comets, and stars, and exAIR-PUMPS are machines made for ex-plains the phenomena occasioned by those hausting the air from certain glass vessels, motions. adapted to the purpose of experiments on ATMOSPHERE, or atmospherick air, the air. fluid that surrounds our earth. Without ANGLE is the inclination of two lines this fluid no animal could exist; vegetation meeting one another in a point, and called would cease, and there would be neither the legs of the angle. Angles, in geometry, rain nor refreshing dews to moisten the are called right, acute, and obtuse. A right face of the ground; and though the sun and angle contains just 90 degrees, or the quar-stars might be seen as bright specks, yet ter part of a circle. Acute angles contain there would be little enjoyment of light, less, and obtuse angles more than 90 degrees. could we exist without it.

ANGLE OF INCIDENCE is that which ATTRACTION, a general term, used to is contained between the line described by denote the power or principle by which bothe incident ray, and a line perpendicular dies mutually tend towards each other, to the surface on which the ray strikes, without regarding the cause or action that raised from the point of incidence. may be the means of producing the effect. ANGLE OF REFLECTION is contain- ATTRACTION OF COHESION takes ed between the line described by the re-place between the constituent particles of flected ray, and a line perpendicular to the the same body. By this principle bodies reflecting surface, at the point from which preserve their forms and are prevented from the ray is reflected. falling to pieces.

ANGLE OF REFRACTION is that ATTRACTION OF GRAVITATION, which is contained between the line describ- or gravity, is the name of that force by ed by the refracted ray, and a line perpendi- which distant bodies tend towards one cular to the refracting surface at the point another. in which the ray passes through that sur- AXIS of a planet is an imaginary line face. which passes through its centre, and on ANGLE OF VISION is that which is which it turns; and it is this motion which contained between lines coming from oppo-produces day and night. With that side of site parts of an object and meeting in the the planet facing the sun it is day; and with the opposite side, which remains in ANTARCTICK CIRCLE, in astronomy, darkness, it is night.

eye.

is an imaginary line extending round the AXIS OF MOTION, in mechanicks, is the south pole, 66 1-2 degrees from the equator line about which a revolving body moves. and parallel to it. Philosophically speaking, the axis of moAPHELION, in astronomy, is that point tion is said to be at rest, whilst the other in any planet's orbit in which the orbit is parts of a body move round it; and the most distant from the sun. further any part of a body is from the axis AQUEOUS HUMOUR, or watery hu- of motion, the greater is its velocity. mour of the eye; it is the first and outermost, AXIS OF THE EARTH is an imaginaand that which is less dense than either ry line conceived to pass through the centre the vitreous or crystalline. It is transpa-of it from one pole to the other, about which rent and colourless like water, and fills up is performed its diurnal rotation. the space that lies between the cornea and AXIS, in opticks, is that ray, among all the crystalline humour. others that are sent to the eye, which falls ARCTICK CIRCLE, in astronomy, is an perpendicularly upon it, and which conseimaginary line extending round the north quently passes through the centre of the eye.