  Q. What is light? A. Rapid undulations of a fluid called ether, striking on the optic nerve of the eye. (See p.46.) The heat of fire or of the sun sets the atoms of matter in motion; and these atoms, striking against the fluid ether, cause it to undulate. Q. How fast does light travel? A. Light travels so fast, that it would go eight times round the earth, while a person counts “one.” Q. Does all light travel equally fast? A. Yes; the light of the sun, or the light of a candle, or the light from houses, trees, and fields. Q. Where does the light of houses, trees, and fields come from? A. The light of the sun (or of some lamp or candle) is reflected from their surfaces. Q. Why are some surfaces brilliant like glass and steel, and others dull like lead? A. Those surfaces which reflect the most light, are the most brilliant; and those which absorb light are dull. Q. What is meant by reflecting light? A. Throwing the rays of light back again, from the surface on which they light. Q. What is meant by absorbing light? A. Letting the rays of light sink below the surface which they touch, so as not to be seen. Q. Why can a thousand persons see the same object at the same time? A. Because it throws off from its surface an infinite number of rays in all directions; and one person sees one portion of these rays, and another person another. Q. Why is the eye pained by a sudden light? A. Because the pupil of the eye is burdened with rays, before it has had time to contract. Q. Why does it give us pain, if a candle be brought suddenly towards our bed at night time? A. In the dark the pupils of the eyes dilate very much, in order to admit more rays. When a candle is brought before them, the enlarged pupil is overladen with rays, and feels pained. Q. Why can we bear the candle-light after a few moments? A, Because the pupil contracts again almost instantly, and adjusts itself to the quantity of light which falls upon it. Q. Why can we see nothing, when we leave a well-lighted room, and go into the dark road or street? A. Because the pupil (which contracted in the bright room) does not dilate instantaneously; and the contracted pupil is not able to collect rays enough (from the dark road or street) to enable us to see before us. Q. Why do we see better, when we get used to the dark? A. Because the pupil dilates again, and is able to gather together more rays; in consequence of which, we see more distinctly. Q. If we look at the sun for a few moments, why do all other things appear dark? A. Because the pupil of the eye (which was very much contracted by looking at the sun) is too small to collect sufficient rays from other objects, to enable us to distinguish their colours. (See “accidental colours.”) Q. If we watch a bright fire for a few moments, why does the room seem dark? A. Because the pupil of the eye (which was very much contracted by looking at the fire) is too small to collect sufficient rays from the objects around, to enable us to distinguish their colours. Q. Why can we see the proper colour of every object again, after a few minutes? A. Because the pupil dilates again, and accommodates itself to the light around. Q. Why can tigers, cats, and owls see in the dark? A. Because they have the power of enlarging the pupil of their eyes, so as to collect several scattered rays of light; in consequence of which, they can see distinctly when it is not light enough for us to see any thing at all. Q. Why do cats and owls sleep almost all day? A. As the pupil of their eyes is very broad, daylight fatigues them; so they close their eyes for relief. Q. Why do cats keep winking, when they sit before a fire? A. As the pupil of their eyes is very broad, the light of the fire pains them; and they keep shutting their eyes to relieve the sensation of too much light. Q. Why do tigers, cats, owls, &c. prowl by night for prey? A. As these animals cannot see distinctly in strong daylight, they sleep during the day: and as they can see clearly in the dark, they prowl then for prey. Q. Why do glow-worms glisten by night only? A. Because the light of day is so much stronger, that it eclipses the feeble light of a glow-worm; in consequence of which, glow-worms are invisible by day. Q. Why can we not see the stars in the day-time? A. Because the light of day is so powerful, that it eclipses the feeble light of the stars: in consequence of which, they are invisible by day. Q. Why can we see the stars even at mid-day, from the bottom of a deep well? A. As the rays of the sun never come directly over a well, but the rays of the stars do; therefore the light from those stars (in such a situation) is more clear than the light of the sun. Q. What is the use of two eyes, since they present only one image of any object? A. The use of two eyes is to increase the light, or take in more rays of light from the object looked at, in order that it may appear more distinct. Q. Why do we not see things double, with two eyes? A. 1st—Because the axis of both eyes is turned to one object; and, therefore, the same impression is made on the ret´ina of each eye. 2ndly—The nerves (which receive the impression) have one point of union, before they reach the brain. Q. Why do we see ourselves in a glass? A. The rays of light from our face strike against the surface of the glass, and (instead of being absorbed) are reflected, or sent back again to our eye. Q. Why are the rays of light reflected by a mirror? A. Because they cannot pass through the impenetrable metal with which the back of the glass is covered; so they rebound back, just as a marble would do if it struck against a wall. Q. When a marble is rolled towards a wall, what is that path through which it runs called? A. The line of the angle of incidence. Q. When a marble rebounds back again, what is the path it then describes called? A. The line of the angle of reflection. Q. When the light of our face goes to the glass, what is the path through which it goes called? A. The line of the angle of incidence. Q. When the light of our face is reflected back again from the mirror, what is this returning path called? A. The line of the angle of reflection. Q. Why does our reflection in a mirror seem to approach us as we walk towards it, and to retire from us as we retire? A. Because the line of the angle of incidence is always equal to the line and angle of reflection. Here CA, EA and DB, FB are the lines of the angle of incidence; and GA, KA and HB, LB are the lines of the angle of reflection. When the arrow is at CD, its shadow will appear at GH, because the line CA=GA and the angle CAB=angle GAB, &c.; and the same may be said about the point D. Q. Why can a man see his whole person reflected in a little mirror not 6 inches in length? A. Because the line of the angle of incidence is always equal to the line and angle of reflection. Take the last figure—CD is much larger than the mirror AB; but the head of the arrow C is reflected obliquely behind the mirror to G; and the barb D appears at H.—Why? Because the line CA=AG and the angle CAB=angle GAB, &c. The same may be said of the point D. Q. Why does a shadow in water always appear topsy-turvy? A. Because the line of the angle of incidence is always equal to the line and angle of reflection. Here the arrow-head A strikes the water at F, and is reflected to D; and the barb B strikes the water at E, and is reflected to C. Q. When we see our shadow in water, why do we seem to stand on our head? A. Because the line of the angle of incidence is always equal to the line and angle of reflection. Suppose our head to be at A, and our feet at B; then the shadow of our head will be seen at D, and the shadow of our feet at C. (See last figure.) Q. Why do windows seem to blaze at sun-rise and sun-set? A. Because glass is a good reflector of light; and the rays of the sun (striking against the window glass) are reflected, or thrown back. Q. Why do not windows reflect the noon-day rays also? A. They do, but the reflection is not seen. Q. Why is the reflection of the rising and setting sun seen in the window, and not that of the noon-day sun? A. As the angle of incidence always Here AB represents a ray of the noon-day sun striking the window at B; its reflection will be at C: Q. Why can we not see the reflection of the sun in a well, during the day-time? A. Because the rays of the sun fall so obliquely, that they never reach the surface of the water at all, but strike against the brick sides. Let BDEC be the well, and DE the water. Q. Why do we see the moon reflected in a well very often? A. As the rays of the moon are not so oblique as those of the sun, they will often reach the water. (See next figure.) Q. Why are the stars reflected in a well, although the sun is not? A. As the rays of the stars are not Here the moon's rays AB, AC, both strike the water DE, and are reflected by it. Q. In a sheet of water at noon, the sun appears to shine upon only one spot, and all the rest of the water seems dark,—Why is this? A. Because the rays (which fall at various degrees of obliquity on the water) are reflected at similar angles; but as only those which meet the eye of the spectator are visible, all the sea will appear dark but that one spot. Here of the rays SA, SB, and SC, only the ray SC meets the eye of the spectator D. Q. At night the moon seems to be reflected from only one spot of a lake of water, while all the rest seems dark,—Why is this? A. Because the rays (which fall at various degrees of obliquity on the lake) are reflected at similar angles; but as only those which enter the eye of the spectator will be visible, all the water will appear dark but that one spot. (See last figure.) Q. Why are more stars visible from a mountain, than from a plain? A. As the air absorbs and diminishes light, the higher we ascend, the less light will be absorbed. Q. Why does the sun seem larger at his rise and set, than it does at noon? A. Because the earth is surrounded by air, which acts like a magnifying glass; and when the sun is near the horizon (as its rays pass through more of this air), it is more magnified. Here SC represents a ray of the sun at noon, and MC a ray of the sun near the horizon. DEG represents the air or atmosphere around the earth. Q. Why does the rising and setting moon appear so much larger, than after it is risen higher above our heads? A. Because the earth is surrounded by air, which acts like a magnifying glass; and when the moon is near the horizon (as its rays pass through more of this air) it is more magnified. (See last figure.) Q. When candles are lighted, we cannot see into the street or road,—Why is this? A. 1st—Because glass is a reflector, and throws the candle-light back into the room again; and 2ndly—The pupil of the eye (which has become contracted by the light of the room) is too small to collect rays enough from the dark street, to enable us to see into it. Q. Why can’t persons in the street see into a well-lighted room? A. Because the pupil of their eyes is much dilated by the dark, and cannot Q. Why do we often see the fire reflected in our parlour window in winter time? A. Because glass is a good reflector; and the rays of the fire (striking against the window-glass) are reflected back into the room again. Q. Why do we often see the shadow of our candles in the window, while we are sitting in our parlour? A. Because the rays of the candle (striking against the glass) are reflected back into the room: and the darker the night, the clearer the reflection. Q. Why is this reflection more clear, if the external air be dark? A. Because the reflection is not then eclipsed by the brighter rays of the sun striking on the other side of the window. Q. Why is the shadow of an object (thrown on the wall) larger and larger, the closer any object be held to the candle? A. Because the rays of light diverge (from the flame of a candle) in straight lines, like lines drawn from the centre of a circle. Here the arrow A held close to the candle, will cast the shadow BF on the wall: while the same arrow held at C, would cast only the little shadow D E. Q. When we enter a long avenue of trees, why does the avenue seem to get narrower and narrower till it appears to meet? A. Because the further the trees are off, the more acute will be the angle that any two will make with our eye. Here the width between the trees A and B will seem to be as great as the line AB: But the width between the trees C and D will seem to be no more than EF. Q. In a long straight street, why do the houses seem to approach nearer and nearer as they are more distant? A. Because the more distant the houses are, the more acute will be the angle which any two make with our eye. Thus in the last figure— If A and B were two houses at the top of the street, the street would seem to be as wide as the line A B: And if C and D were two houses at the bottom of the street, the street at the bottom would seem to be no wider than E F. Q. In an avenue of trees, why do they seem to be smaller as their distance increases? A. Because the further the trees are off, the more acute will be the angle made by their perpendicular height with our eye. Here the first tree A B will appear the height of the line A B; but the last tree C D will appear only as high as the line E F. Q. In a long straight street, why do the houses seem to be smaller and smaller the further they are off? A. Because the further any house is off, the more acute will be the angle made by its perpendicular height with our eye. Thus in the last figure— If A B be a house at the top of the street, its perpendicular height will be that of the line A B. If C D be a house at the bottom of the street, its perpendicular height will appear to be that of E F. Q. Why does a man on the top of a mountain or church spire seem to be no bigger than a crow? A. Because the angle made by the perpendicular height of the man (at that distance) with our eye, is no bigger than the perpendicular height of a crow close by. Let AB be a man on a distant mountain or spire, and CD a crow close by: Q. Why does the moon appear to us so much bigger than the stars, though in fact it is a great deal smaller? A. Because the moon is very much nearer to us than any of the stars. Let AB represent a fixed star, and CD the moon. Q. Why does the moon (which is a sphere) appear to be a flat surface? A. It is so far off, that we cannot distinguish any difference between the length of the rays which issue from the The rays AD and CD appear to be no longer than the ray BD; but if all the rays seem of the same length, the part B will not seem to be nearer to us than A and C, and therefore ABC will look like a flat or straight line. Q. Why do the sun and stars (which are spheres) appear to be flat surfaces? A. Because they are such an immense way off, that we can discern no difference of length between the rays which issue from the edge, and those which issue from the centre of these bodies. The rays AD and CD appear no longer than BD; and as B appears to be no nearer than A or C, therefore ABC must all seem equally distant; and ABC will seem a flat or straight line. (See last figure.) Q. Why does distance make an object invisible? A. Because the angle (made by the perpendicular height of the distant object with our eye) is so very acute, that one line of the angle merges in the other. Here the tree AD would not be visible to the spectator C, even if he were to approach as far as B; because no visible perpendicular can be inserted between the two lines AC, DC, till after the point B is past; when the tree will appear like a very little speck. Q. Why do telescopes enable us to see objects invisible to the naked eye? A. Because they concentrate several rays within the tube of the telescope, and bend them upon the mirror or lens, which acts as a magnifying glass. Q. When a ship (out at sea) is approaching the shore, why do we see the small masts before we see the bulky hull? A. Because the earth is round, and the curve of the sea hides the hull from our eyes, after the tall masts have become visible. Here only that part of the ship above the line AC can be seen by the spectator A; the rest of the ship is hidden by the swell of the curve DE. Q. What is meant by refraction? A. The bending of a ray of light, as it passes from one medium to another. Q. How is a ray of light bent, as it passes from one medium to another? A. When a ray of light passes into a denser medium, it is bent towards the perpendicular. When it passes into a rarer medium, it is bent from the perpendicular. Suppose DE to be a perpendicular line. Q. Why does a spoon (in a glass of water) always appear bent? A. Because as the light of the spoon emerges from the water, it is refracted. And the spoon looks like ABC. (See the last figure.) Q. Why does a river always appear more shallow than it really is? A. Because the light of the bottom of the river is refracted as it emerges out of the water: and (as a stick is not so long when it is bent, as it is when it is straight) so the river seems less deep than it really is. Q. How much deeper is a river than it seems to be? A. One-third. If, therefore, a river seems only 4 feet deep, it is really 6 feet deep. N. B. Many boys get out of their depth in bathing, in consequence of this deception. Remember, a river is always one-third deeper than it appears to be:—thus, if a river seems to be 4 feet deep, it is in reality 6 feet deep, and so on. Q. Why do fishes always seem to be nearer the surface of a river than they really are? A. Because the rays of light from the fish are refracted as they emerge from the Q. Why are some persons near-sighted? A. Because the COR´NEA of their eye is so prominent, that the image of distant objects is reflected before it reaches the ret’ina; and, therefore, is not distinctly seen. N.B. The cor´nea shields the crystalline lens, and is more or less convex according to the lens which it covers. Q. What is meant by the “cor’nea of the eye?” A. All the outside of the visible part of the eye-ball. The curve A B C is called the cor'nea. Q. What is meant by the “ret’ina of the eye?” A. The net-work which lines the back of the eye, is so called.
Q. What sort of glasses do near-sighted persons wear? A. If the cor’nea be too convex (or projecting), the person must wear double concave glasses, to counteract it. Q. What is meant by “double concave glasses?” A. Glasses hollowed in on both sides.
Q. What is meant by the “image of objects being reflected before it reaches the ret’ina?” A. If the cor’nea be too convex, the image of a distant object is reflected (on the vitreous humours of the eye) before it reaches the ret’ina.
Q. What is the use of double concave spectacle glasses? A. Near-sighted spectacles cast the reflection further back; and the image (being thrown upon the ret’ina) becomes visible. Q. Why are old people far-sighted? A. Because the humours of their eyes are dried up by age, and the cor’nea sinks in, or becomes flattened. Q. Why does the flattening of the cor’nea prevent persons seeing objects which are near? A. As the cor’nea is too flat, the image of any near object is formed behind the ret’ina of the eye, and is not seen at all.
Q. What sort of glasses do old people wear? A. As their cor’nea is not sufficiently convex, they must use double convex Q. What sort of glasses are double convex spectacle-glasses? A. Glasses which curve outwards on both sides.
Q. What is the use of double convex spectacle-glasses? A. As the image of near objects is reflected behind the ret’ina, these double convex glasses shorten the focus of the eye, and bring the image into the eye (upon the ret’ina). Q. Why do near-sighted persons bring objects close to the eye, in order to see them? A. As the distance between the front and back of their eye is too great, distant objects are reflected before they reach the ret’ina; therefore, near-sighted persons bring the objects closer, in order that the reflection may be cast further back, (to reach the ret’ina). Q. Why do old people hold objects further off, in order to see them better? A. As the distance between the front and back of their eye is not great enough, the reflection of near objects is thrown beyond the ret’ina; therefore, they hold objects a long way off, in order to bring their images forward (so as to cast it on the ret’ina). Q. Why are hawks able to see such an immense way off? A. Because they have a muscle in the eye which enables them to flatten their cor’nea, by drawing back the crystalline lens. This muscle is called the “marsupium.” Q. Why can hawks not only see such a long way off, but also objects within half-an-inch of their eye? A. Because their eyes are furnished with a broad circular rim which confines the action of this muscle, and throws the cor’nea forward. Q. Into how many parts may a ray of light be divided? A. Into three parts: Blue, Yellow, and Red. N.B. These 3 colours, by combination, make seven. 1.—Red. 2.—Red and yellow form orange. 3.—Yellow. 4.—Yellow and blue make green. 5.—Blue. 6 and 7.—Shades of blue called indigo and violet. Q. How is it known, that a ray of light consists of several different colours? A. Because, if a ray of light be cast upon a triangular piece of glass (called a prism), it will be distinctly divided into seven colours: 1.—Red; 2.—Orange; 3.—Yellow; 4.—Green; 5.—Blue; 6.—Indigo; and 7.—Violet. Q. Why does a prism divide a ray of light into various colours? A. Because all these colours have different refractive powers. Red is refracted least, and blue the most; therefore, the blue colour of the ray will be bent to the top of the prism, and the red will remain at the bottom. Here the ray AB received on a prism, would have the blue part bent up to C; the yellow part to D; and the red part no further than E. Q. What is meant by the refraction of a ray? A. Bending it from its straight line. Thus the ray AB of the last figure is refracted at B into three courses, C, D, and E. Q. What is the cause of a rainbow? A. When the clouds opposite the sun are very dark, and rain is still falling from them, the rays of the bright sun are divided by the rain-drops, as they would be by a prism. Let A, B, and C be three drops of rain; SA, SB, and SC three rays of the sun. SA is divided into the 3 colours; the blue and yellow are bent above the eye D, and the red enters it. Q. Does every person see the same colours from the same drops? A. No; no two persons see the same rainbow. To another spectator the rays from SB might be red instead of yellow; the ray from SC, yellow; and the blue might be reflected from some drop below C. To a third person the red may issue from a drop above A, and then A would reflect the yellow, and B the blue, and so on. Q. Why are there often two rainbows at one and the same time? A. In one rainbow we see the rays of the sun entering the rain-drops at the top, and reflected to the eye from the bottom. In the other rainbow, we see the rays of the sun entering the rain-drops at the bottom, and reflected to the top, whence they reach the eye. Here the ray SA strikes the drop at A,—is refracted or bent to B,—is then reflected to C, where it is refracted again, and reaches the eye of the spectator. Here the ray SB strikes the drop at B,—is refracted to A,—is then reflected to C,—is again reflected to D, when it is again refracted or bent till it reaches the eye of the spectator. Q. Why are the colours of the second bow all reversed? A. Because in one bow we see the But in the other bow we see the rays which enter at the bottom of the raindrops (after two reflections), refracted from the top. Here A, B, C, represent three drops of rain in the primary (or inner) rainbow. Here also the least refracted ray is red, and the most refracted blue (as in the former case); but the position of each is reversed. Q. Why does a soap bubble exhibit such variety of colours? A. The changing colour of the bubble depends upon the changing thickness of the film through which the ray passes. Q. How does the thickness of the film affect the colour of the soap bubble? A. Because different degrees of thickness produce different angles of refraction, and, therefore, different colours reach the eye. Q. Why is the soap bubble so constantly changing its thickness? A. As the bubble is suspended, the water keeps running down from the top to the bottom of the bubble, till the crown becomes so thin as to burst. Q. Why are the late evening clouds red? A. Because red rays (being the least refrangible) are the last to disappear. Here it will be seen that the red ray PA, being reflected on the horizon at A, will be visible to us; but the yellow and blue rays will be hidden by the curve of the earth. Q. Why are the early morning clouds red? A. Because red rays (being the least refrangible) are the first to appear. See last figure.—It is evident that PA (the red rays) will be reflected on the horizon before either the yellow or blue ones. Q. What becomes of the blue and yellow rays? A. They are refracted below the horizon, and are soon made invisible by the curve of the earth. (See last figure.) Q. Why are the edges of clouds more luminous than their centres? A. Because the body of vapour is thinnest at the edges of the clouds. Q. What is the cause of morning and evening twilight? A. When the sun is below the horizon, the rays (which strike upon the atmosphere or clouds) are bent down towards the earth, and produce a little light called twilight. See figure on p. 399.—Here the rays of PA will give some light. Q. Why is a ray of light composed of various colours? A. If solar light were of one colour only, all objects would appear of that one colour (or else black.) Q. Why are some things of one colour, and some of another? A. As every ray of light is composed of all the colours of the rainbow, some things reflect one of these colours, and some another. Q. Why do some things reflect one colour, and some another? A. Because the surface of things is so differently constructed, both physically and chemically; and, therefore, some things reflect one ray; some two rays; some all the rays; and some none. Q. What mainly determines the colour of any object? A. The fluid or gas either in the body, or on its surface. N. B. Nitrogen gives green,—Oxygen gives red,—Hydrogen gives blue colours. Q. Why does dying a silk, &c. change its colour? A. Because the materials used in dyeing alter the chemical construction of the substance dyed. Q. Why is a rose red? A. Because the surface of a rose absorbs the blue and yellow rays of light, and reflects only the red ones. Q. Why does a rose absorb the yellow and blue rays, and reflect the red? A. Because the action of the sun’s rays on the oxygen (accumulated in the The leaves which compose a flower, are called petals. Q. Why is a violet blue? A. Because the surface of the violet absorbs the red and yellow rays of the sun, and reflects the blue only. Q. Why do violets absorb the red and yellow rays, and reflect the blue? A. Because the petals of the violet contain an alkali, which gives them a purple tinge. Q. Why is a primrose yellow? A. Because the surface of the primrose absorbs the blue and red rays of solar light, and reflects the yellow ones. All plants which have much alkali in their ash, have blue or yellow flowers. Those which have acid in their ash, have orange, pink, or red flowers. N. B. Anti-acids (like soda) are called alkalis. Q. Why are some things black? A. Because they absorb all the rays of light, and reflect none. Q. Why are some things white? A. Because they absorb none of the rays of light, but reflect them all. Q. Why are coals black? A. Because they absorb all the rays of the sun which impinge upon them, and stifle their reflection. Q. Why is snow white? A. Snow consists of a vast number of crystals (or small prisms), which separate the rays into their elemental colours; but as these crystals are very numerous, the colours unite again before they meet the eye, and appear white. N. B. The combination of all colours makes white. Q. Why is sugar white? A. Sugar consists of a vast number of small crystals, which separate the rays into their elemental colours; but as these crystals are very numerous, the colours unite again before they meet the eye, and appear white. Q. Why is salt white? A. Salt consists of a vast number of small crystals, which reflect the various rays of light from different points of the salt; and as these colours unite before they meet the eye, the salt appears to be white. N. B. The combination of all colours makes white. Q. Why are the leaves of plants green? A. Because the carbon of the leaves is a bluish olive, and the sap and tissue of the cells, yellow; when, therefore, the yellow sap flows into the blue carbon, it produces a green leaf. Q. Why are leaves a light green in spring? A. Because the young leaves of spring have more sap than carbon; and, therefore, the yellow of the green prevails. Q. Why are leaves a yellowish brown in autumn? A. Because the carbon of the leaves is dying away, and the yellow tinge of the tissue and falling sap prevails over the blue. Q. Why are plants a pale yellow when kept in the dark? A. Solar light is essential for the production of carbon; and as plants kept in the dark lose their carbon, they lose the blue colour which should convert their yellow sap to green. Q. Why are potatoes yellow? A. Potatoes are grown underground, and, therefore, contain very little carbon Q. Why are potatoes (which grow exposed to the air and light) green? A. Because the sun-light increases their carbon; which (mingling with the yellow sap) turns the potato green. Q. Why is it dangerous to sleep in a room which contains living plants? A. Because they exhale carbon in the dark in the form of carbonic acid gas, which is destructive to animal life. Q. Why are some things (like glass) transparent? A. In transparent bodies (like glass) all the rays of light emerge on the opposite side. Q. Why are some things shining and splendid? A. Those objects which reflect the most rays are the most splendid; and those which absorb them most, are dull. Q. Why are deserts so dazzling in summer time? A. Because each separate grain of Q. If you move a stick (burnt at one end) round pretty briskly, it seems to make a circle of fire,—Why is this? A. Because the eye retains the image of any bright object, after the object itself is withdrawn; and as the spark of the stick returns before the image has faded from the eye, therefore, it seems to form a complete circle. Q. If separate figures (as a man and a horse) be drawn on separate sides of a card, and the card twisted quickly, the man seems to be seated on the horse,—Why is this? A. Because the image of the horse remains upon the eye till the man appears. The Thaumatrope is constructed on this principle. Q. Why do the stars twinkle? A. Fixed stars are so far off, that their rays of light do not strike upon the eye in a continuous flow, but at intervals: when their rays reach the eye, the star becomes visible, and then is obscured till the next batch of rays arrive; and this perpetually occurring, makes a kind of twinkling. Q. If we look at a red-hot fire for a few minutes, why does every thing seem tinged with a bluish green colour? A. Because bluish green is the “accidental colour” of red: and if we fix our eye upon any colour whatsoever, when we turn aside, we see every object tinged with its accidental colour. Q. If we wear blue glasses, (when we take them off,) every thing appears tinged with orange,—Why is this? A. Because orange is the “accidental colour” of blue: and if we look through blue glasses, we shall see its “accidental colour,” when we lay our glasses aside. Q. If we look at the sun for a few moments, every thing seems tinged with a violet colour,—Why is this? A. Because violet is the “accidental colour” of yellow light; and as the sun is yellow, we shall see its “accidental colour” blue, when we turn from gazing at it. Q. Does not the dark shadow (which seems to hang over every thing after we turn from looking at the sun) arise from our eyes being dazzled? A. Partly so: the pupil of the eye is very much contracted by the brilliant light Q. Why is black glass for spectacles the best for wear? A. Because white is the accidental colour of black; and if we wear black glasses, every thing will appear in white light, when we take them off. Q. Why does every thing seem shadowed with a black mist, when we take off our common spectacles? A. Because the glasses are white, and black being its “accidental colour,” every thing appears in a black shade, when we lay our glasses down. The accidental colour of red is bluish green. And the converse of this is true:— The accidental colour of bluish green is red. (The law of an accidental colour is this—The accidental colour is always half the spectrum. Thus, if we take half the length of the spectrum by a pair of compasses, and fix one leg in any colour, the other leg will hit upon its accidental colour.) N. B. The spectrum means the seven colours—Red, orange, yellow, green, blue, indigo, and violet, divided into seven equal bands, and placed side by side in the order just mentioned. |
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