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at the bottom of the eye, we can never reflect, without wonder, upon the smallness, yet correctness of the picture, the subtilty of the touch, the fineness of the lines. A landscape of five or six square leagues is brought into a space of half an inch diameter; yet the multitude of objects which it contains are all preserved; are all discriminated in their magnitudes, positions, figures, colours.

Microscopes are instruments for viewing small objects; and they apparently magnify them, because they enable us to see them nearer than with the naked eye, without affecting the distinctness of vision. The distance from the naked eye, at which most persons are supposed to see small objects best, is about seven inches; but by the help of convex glasses, we are enabled to view things clearly at a much shorter distance than this; for the nature of a convex lens is, to render an object distinctly visible to the eye at the distance of its focus. With a knowledge of this fact, we may easily determine the magnifying powers of glasses employed in Single Microscopes, which are small double convex lenses, having the object placed in the focus, and the eye at the same distance on the other side. If rays of light from an object are converged to a point at the distance of one inch from the centre of the glass, or, in other words, if the focal distance of the lens is one inch, an object may be seen through that lens at one inch distance from the eye, and it will appear, in its diameter,-since the natural sight is seven inches,-seven times larger than to the naked eye. But as the object is magnified every way equally, in length as well as breadth, we must square this diameter, to know really how much the object appears enlarged; and we shall thus find that its surface is magnified forty-nine times. If we suppose the focus of a convex lens to be at one-tenth of an inch distant from its centre, in seven inches there are seventy such tenths of an inch; and an object therefore may be seen through this lens seventy times nearer than it can, distinctly, by the naked eye. It will consequently appear seventy times longer and seventy times broader than it does to common sight; and as seventy multiplied by seventy makes four thousand nine hundred; so many times it really appears magnified. Those lenses, therefore, which have the shortest focus, will magnify the object most. Single microscopes of the greatest power may be made with a very small

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globule of glass, fixed in a thin plate of metal, so that the middle of it may be directly over the centre of an extremely small hole made in the plate.

The compound microscope consists of at least two lenses, by one of which an image is formed within the tube of the microscope; and this image is viewed through the eye-glass, instead of the object itself. The solar microscope is a kind of camera obscura, which, in a darkened chamber, throws the image on a wall or skreen. It consists of two lenses fixed opposite to a hole in a board or window-shutter. There is also a plane reflector or mirror placed without, which may be so regulated as to throw the sun's rays upon the outer lens. A magic lantern is constructed on the same principles. The light is supplied by a lamp instead of the sun, and it is used for magnifying paintings on glass, and throwing their images upon a white skreen in a darkened chamber.

QUESTIONS.-1. In what ways may sight be defective? 2. For what are spectacles intended? 3. How do they assist eyes that are too flat? 4. Too convex or round? 5. Why do some persons bring objects close to their eyes, and others hold them at a distance? 6. What are microscopes? 7. Single microscopes? 8. How is their magnifying power calculated? 9. Describe the compound microscope. 10. Solar microscope. 11. Magic lantern. 12. Look on fig. 35. and describe the single microscope. 13. On fig. 34. and describe the compound microscope.

LESSON 38.

Microscopic Discoveries.

Miniature, (pronounced min'e-türe,) representation in a small compass. Filament, a slender thread.

Ped'icle, a footstalk. Animal'cule, a small animal.

Con'ical, consisting of a circular base or bottom and ending in a point.

Tissue, (pron. tish'ū,) a substance interwoven with threads, or variegated.

THE microscope has opened to us a new world of insects and vegetables; it has taught us that objects, invisible to the naked eye, exist, having figure, extension, and different parts; some examples of which we shall produce, that we may have more reasons for admiring and praising the wis dom and power of God. A grain of sand when examined by the eye appears round, but with the help of a glass we

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observe that each grain differs from the other, both in size and figure; some of them are perfectly round, others square, some conical, and the greater part of an irregular form. By microscopes which magnify objects millions of times, we can discover in the grains of sand a new animal world; for within their cavity dwell various insects. In the vegetable kingdom we are presented with a thick forest of trees and plants, bearing leaves, branches, flowers, and fruits. Mouldi ness, when looked at by the naked eye, seems nothing but an irregular tissue of filaments; but the magnifying glass shows it to be a forest of small plants, which derive their nourishment from the moist substance which serves them as a base. The stems of these plants may be plainly distin guished, and sometimes their buds, some shut and some open. They have much similarity to mushrooms, which, it is well known, are the growth of a single night; but those in miniature, of which we are speaking, seem to come to perfection in a much less space of time; hence we account for the extraordinary progress which mouldiness makes in a few hours. A sort of dust, which covers some stones, has been found to consist of small mushrooms, raised on pedicles, the heads of which, round the middle, were turned up at the edges. Above their covering a multitude of small grains appear, shaped like cherries somewhat flattened; and among them several small red insects, which probably feed upon them. A small drop of the green surface of water, that has stood for some time, has been found to be altogether composed of animalcules of several shapes and magnitudes. The most remarkable were those that gave the water the green colour; they were oval creatures; they could contract and dilate themselves, tumble over many times together, and then shoot away like fishes.

If you slightly bruise some corns of pepper, and infuse them in water for a few days, and then expose a drop of it to the microscope, a number of animalcules will be visible, in continual motion, going backwards and forwards in all directions, turning aside when they meet each other, or when their passage is stopped by some obstacle. In other infusions, as in that of new hay, differently shaped animalcules will be found. When the drop in which they swim, and which to them is like a pond, becomes diminished by evaporation, they gradually retire towards the middle, where they

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accumulate, and at length, when entirely deprived of moisture, perish. Previously to this they appear in great distress, writhe their bodies, and endeavour to escape from that state of uneasiness which they evidently feel. If the smallest quantity of sulphuric acid be put into a drop of the infusion which swarms with these insects, they immediately throw themselves on their backs and expire.

Upon examining the edge of a very sharp lancet with a microscope, it will appear as broad as the back of a knife; rough, uneven, full of notches and furrows. An exceedingly small needle resembles a rough iron bar. But the sting of a bee, seen though the same instrument, exhibits every where a most beautiful polish, without the least flaw, blemish, or inequality, and it ends in a point too fine to be discerned. The threads of fine lawn seem coarser than the yarn with which ropes are made for anchors. But a silkworm's web appears perfectly smooth and shining, and every where equal. The smallest dot, that can be made with a pen, appears irregular and uneven. But the little specks on the wings or bodies of insects are found to be most accurately circular. The finest miniature paintings appear before the microscope rugged and uneven, entirely void of beauty, either in the drawing or colouring. The most even and beautiful varnishes will be found to be mere roughness. But the nearer we examine the works of God, even in the least of his productions, the more sensible shall we be of his wisdom and power. In the numberless species of insects, what proportion, exactness, uniformity, and symmetry do we perceive in all their organs! what a profusion of colouring! azure, green, and vermilion, gold, silver, pearls, rubies, and diamonds; fringe and embroidery on their bodies, wings, heads, and every other part! how high the finishing, how inimitable the polish we every where behold!

On the gay bosom of some fragrant flower
They, idly fluttering, live their little hour;
Their life all pleasure, and their task all play,
All spring their age, and sunshine all their day.
Not so the child of sorrow, wretched man,
His course with toil concludes, with pain began;
That his high destiny he might discern,
And in misfortune's school this lesson learn;

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Pleasure's the portion of th' inferior kind,
But glory, virtue, Heaven for MAN designed.

BARBAULD.

QUESTIONS.-1. What has the microscope done for us? 2. What is the appearance of grains of sand when examined by the eye, and by the microscope? 3. Mouldiness? 4. What is said of the green surface of standing water? 5. What is the appearance of animalcules in the infusions of pepper?-new hay? 6. What appearance has the edge of a lancet? 7. Sting of a bee? 8. Fine lawn? 9. Silk worm's web?

LESSON 39.

The Telescope and Telegraph.

Satellite, a small planet revolving round a larger, a moon.
Octag'onal, having eight angles and sides.

O'ral, delivered verbally, not written.

No invention in the mechanic arts has ever proved more useful and entertaining than the production of the telescope; its utility both by sea and land is too well known to need observation; and without such assistance the science of astronomy must have been far short of its present state. A telescope is useful, not only for discovering those distant objects that are invisible to the naked eye, but for rendering more clear and distinct those that are discernible; it is constructed to act either by refraction or reflection. It is the sole business of all telescopes to enable the eye to see the object under a larger angle. For this purpose a new image of an object is produced by the object-glass of the telescope, and then this image is viewed by means of the eye-glasses. The first impression, conveyed to the mind by a telescope, is that of bringing the object nearer, which is only another mode of declaring that it is enlarged, or seen under a larger angle. To show objects in their natural posture, a telescope must have three eye-glasses. The two additional lenses simply give an erect position to objects. If you remove one of the eye-glasses from a common telescope, every thing will appear in an inverted position. The three eye-glasses have all their focal distances equal, and the magnifying power is found by dividing the focal distance of the object-glass by the focal distance of one of the eye-glasses. The two additional lenses

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