MICROSCOPIC DISCOVERIES. 83 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'u,) 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 84 MICROSCOPIC DISCOVERIES. 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. Mouldiness, 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 distinguished, 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 MICROSCOPIC DISCOVERIES. 85 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 silk worm'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 Pleasure 's the portion of th' inferior kind, 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. O'ral, delivered verbally, not written. A 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. 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 are not necessary for astronomical telescopes; for no inconvenience arises from seeing the celestial bodies inverted. When very great magnifying power is required, telescopes are constructed with concave mirrors, and called reflecting telescopes. Mirrors are used in order to bring the image nearer the eye; and a lens or eye-glass is for the same purpose as in the refracting telescope, that is, to magnify the image. The Newtonian reflecting telescope consists of a tube, towards the end of which a concave mirror is placed. The reflected converging rays, before they reach the focus, are made to fall upon a plane mirror placed at an angle of forty-five degrees, and thus are thrown upwards to the focus of a convex lens fixed in the upper side of the telescope, through which the eye looks down on the image. In the telescopes made by Dr. Herschel there is but one mirror, which is placed at the lower end of the tube, with such an inclination, that the rays are brought to a focus and the image formed near the edge of the upper end of the tube. The image, therefore, is formed by only one reflection, and its brightness, when viewed through the lens is, on this account, greater than that in the Newtonian telescope. The head of the observer, when a large aperture is wanted, may be placed entirely at one edge of the tube, so as not to intercept any of the rays at the time of making an observation; but as the eye looks down the tube, the back must be turned to the object. Dr. Herschel's grand telescope is nearly forty feet long, and four feet ten inches in diameter. The concave polished surface of the great mirror is forty-eight inches in diameter, and it magnifies six thousand times. This noble instrument was, in all its parts, constructed under the sole direction of Dr. Herschel it was begun in the year 1785, and completed August 28th, 1789, on which day was discovered the sixth satellite of Saturn. The telegraph is a machine for communicating intelligence at a considerable distance, by making various signals, -which have been previously agreed upon between two parties, to represent letters, words, or ideas. No machine for making signals can with propriety be called a telegraph, unless it is adapted to express a sufficient number of letters or words to form a complete language, and which can be made, therefore, to communicate any information which can be expressed by oral or written language. Less perfect sys |