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 exceed ingly 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 68 REFRACTION OF LIGHT. perpendicular. If you put a piece of money into an empty basin, and stand at such a distance that it may not be visible; then let another person pour water into the basin, and the money will be seen; for the rays of light, in passing from a denser into a rarer medium, are bent from the perpendicular, and thus are directed to your eye. The following, therefore, may be established as a sort of axiom in optics we see every thing in the direction of that line in which the rays approach us last. If you place a candle before a looking-glass, and stand before it, the image of the candle appears behind it; but if another looking-glass be so placed as to receive the reflected rays of the candle, and you stand before this second glass, the candle will appear behind that; because the mind imagines every object to be in the direction from which the rays come to the eye last. Hence, when the rays of light coming from the celestial bodies, arrive at our atmosphere, they are bent downward; and those bodies appear, when in the horizon, higher than they are. The effect of this refraction is about six minutes of time, but the higher they rise, the less are the rays refracted; and when they are in the zenith, they suffer no refraction. The sun is visible about three minutes before he rises, and about the same time after he sets; making in the course of a year about a day and a half. Twilight is occasioned partly by refraction, but chiefly by reflection of the sun's rays by the atmosphere, and it lasts till the sun is eighteen degrees below the horizon. Were there no atmosphere to reflect and refract the sun's rays, only that part of the heavens would be luminous in which the sun is placed; and if we could live without air, and should turn our backs to the sun, the whole heavens would appear as dark as in the night. In this case also, a sudden transition from the brightest sunshine to dark night would immediately take place upon the setting of the sun. QUESTIONS.-1. What is said of optics? 2. In what manner is light projected from luminous bodies? 3. What is still a disputed point, and what is said of it? 4. How are rays of light reflected? 5. How is it shown that the angle of reflection is equal to the angle of incidence? 6. What is meant by the refraction of rays of light? 7. How are they refracted in passing from a rarer into a denser medium? 3. From a denser into a rarer? 9. What is the example for illustration? 10. What may be established as a sort of axiom in optics? 11. Give the illustration. 12. What is the effect of rays of light, coming from celestial bodies, being refracted by the atmosphere? 13. What occasions twilight? 14. How would the heavens appear if there were no atmosphere? 15. Illustrate the reflection of light by fig. 29. Engr. III. 16. Refraction of light by fig. 29. [NOTE. Fig. 31. is a vessel with a flower in water at the bottom, seen by the eye in the direction of the rays which enter it. This experiment, and many others, may be easily performed.] LESSON 32. Different kinds of Lenses. Diverge', rays of light coming from a point, and continually separating as they proceed, are said to diverge; the point is called the radiant point. Converge', rays which tend to a common point are said to converge. A Beam of light is a body of parallel rays; a Pencil of rays is a body of diverging or converging rays. Cam'era obscu'ra, a chamber darkened; an optical machine used in a darkened chamber. If A LENS is a glass ground into such a form as to collect or disperse the rays of light which pass through it. They are of different shapes, from which they take their names. rays proceed from a radiant point distant as far as the sun, their divergency is so trifling that they may be considered as parallel. When parallel rays fall on a piece of glass having a double convex surface, that ray only, which falls in the direction of the axis of the lens, is perpendicular to the surface; the other rays falling obliquely, are refracted towards the axis, and they will meet beyond the lens at a point called its focus. The distance of the focus from the centre of the lens depends both upon the form of the lens, and upon the refractive power of the substance of which it is made; in a glass lens, both sides of which are equally convex, the focus is situated nearly at the centre of the sphere of which the surface of the lens forms a portion; it is at the distance, therefore, of half the diameter of the sphere. The property of a lens which has a double concave surface is to disperse the rays of light. Instead of converging towards the ray, which falls on the axis of the lens, they will be attracted towards its thick edges, both on entering and quitting it, and will, therefore, be made to diverge. Lenses which have one side flat and the other convex or concave are less powerful in their refractions, than those which have been described. They are called plano-convex and plano-concave. The focus of the former is at the distance of the diameter of a sphere, of which the convex surface of the lens forms a portion. The last kind of lens is called a menis'cus, being convex on one side and concave on the other, like the glass or crystal of a watch. All the parallel rays of the sun which pass through a convex glass are collected in its focus, and the force of the heat there is to the common heat of the sun, as the surface of the glass is to the surface of the focus. If a lens four inches in diameter collect the sun's rays into a focus at the distance of twelve inches, the image will not be more than one tenth of an inch in diameter: the surface of this little circle is one thousand six hundred times less than the surface of the lens, and consequently the heat will be one thousand six hundred times greater at the focus than at the lens. A globular decanter of water acts as a double convex lens, and furniture has been set on fire by leaving one incautiously exposed to the rays of the sun. A gentleman of London formed a burning-glass three feet in diameter, and when fixed in its frame, it exposed clear surface of more than two feet eight inches in diameter, and its focus, by means of another lens, was reduced to a diameter of half an inch. The heat produced by this was so great that iron plates were melted in a few seconds; tiles and slates became red-hot in a moment, and were vitrified, or changed into glass; sulphur, pitch, and other resinous bodies, were melted under water; gold was rendered fluid in a few seconds. But notwithstanding this intense heat at the focus, the finger might, without the smallest injury, be placed in the cone of rays within an inch of the focus. On bringing the finger nearer, a sensation was felt like that produced by a sharp lancet, and not at all similar to the pain occasioned by the heat of fire or a candle. Substances of a white colour were difficult to be acted upon. Pure water in a clear glass decanter will not be warmed by the most powerful lens, but a piece of wood placed in the water may be burned to a coal. If a cavity be made in a piece of charcoal, and the substance to be acted on be put in it, the effect produced by the lens will be much increased. Any metal thus enclosed melts in a |