Q. Why do fishes always seem to be nearer the surface of a river than they really are?
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.
If this curve be too prominent (or convex), the eye is near-sighted.
If too flat (or concave), the eye is far-sighted.
Q. What is meant by the “ret’ina of the eye?”
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.
The figure A is double concave, or concave on both
sides.
Q. What is meant by the “image of objects being reflected 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.
The reflection is made at DE, instead of at ABC (the
retina).
Q. What sort of glasses do old people wear?
Q. What sort of glasses are double convex spectacle-glasses?
A. Glasses which curve outwards on both sides.
The figure A is double convex, or convex 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?
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?
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.
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The ray SB is divided into the three colours; the
blue is bent above the eye, and the red falls below the
eye D; but the yellow enters it.
The ray SC is also divided into the three colours.
The blue (which is bent most) enters the eye;
and the other two fall below it. Thus the eye sees
the blue of C, and all drops in the position of C;
the yellow of B, and of all drops in the position of
B; and the red of A, &c.; and thus it sees a
rainbow.
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 rays which enter at the top of the raindrops, refracted from the bottom:
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.
The least refracted line is red, and blue the most.
So the red (or least refracted rays) of all the drops in the position
of A,—the yellow of those in the position of B,—and the blue (or the
most refracted rays) of the lowest drops, all meet the eye D, and form
a rainbow to the spectator.
The reason why the primary bow exhibits the stronger colours is
this—because the colours are seen after one reflection and two
refractions; but the colours of the secondary (or upper) rainbow undergo
two reflections and three refractions.
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.
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.
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?
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?
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?
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?
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?
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?
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?
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 of the sun, and does not adjust itself immediately to the feebler light of terrestrial objects; but, independent of this, the “accidental colour” of the sun being dark violet, would tend to throw a shadow upon all things. (See p. 366.)
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.
The accidental colour of orange is blue.
The accidental colour of violet is yellow.
The accidental colour of of black is white.
And the converse of this is true:—
The accidental colour of bluish green is red.
The accidental colour of of blue is orange.
The accidental colour of of yellow is violet.
The accidental colour of of white is black.
(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.
Q. What is sound?
A. The vibration of some sonorous substance produces motion in the air called sound waves, which strike upon the drum of the ear, and give the sensation of sound.
Q. What are musical sounds?
Q. How fast does sound travel?
A. About 13 miles in a minute, or 1142 feet in a second of time.
Q. How fast does light travel?
A. Light would go 8 times round the whole earth, while sound is going its 13 miles.
Q. Why are some things sonorous, and others not?
A. The sonorous quality of any substance depends upon its hardness and elasticity.
Q. Why are copper and iron sonorous, and not lead?
A. Copper and iron are hard and elastic; but as lead is neither hard nor yet elastic, it is not sonorous.
Q. Of what is bell-metal made?
A. Of copper and tin in the following proportions:—In every 5 pounds of bell-metal, there should be 1 lb. of tin, and 4 lbs. of copper.
Q. Why is this mixture of tin and copper used for bell-metal?
Q. Why is the sound of a bell stopped by touching the bell with our finger?
A. The weight of the finger stops the vibrations of the bell; and as soon as the bell ceases to vibrate, it ceases to make sound-waves in the air.
Q. Why does a split bell make a hoarse disagreeable sound?
A. The split of the bell causes a double vibration; and as the sound-waves clash and jar, they impede each other’s motion, and produce discordant sounds.
Q. Why does a fiddle-string give a musical sound?
A. The bow drawn across the string causes it to vibrate, and this vibration of the string sets in motion the sound-waves of the air, and produces musical notes.
Q. Why does a drum sound?
A. The parchment head of the drum vibrates from the blow of the drum-stick, and sets in motion the sound-waves of the air.
Q. Why do musical glasses give sounds?
Q. Why do flutes, &c. produce musical sounds?
A. The breath of the performer causes the air in the flute to vibrate, and sets in motion the sound-waves of the air.
Q. Why do piano-fortes produce musical sounds?
A. The keys of the piano (being struck with the finger) lift up a little hammer which knocks against a string; and the vibration thus produced, sets in motion the sound-waves of the air.
Q. Why are some notes bass and some treble?
A. Slow vibrations produce bass or deep sounds; whereas, quick vibrations produce shrill or treble sounds.
Q. Why is an instrument flat when the strings are unstrung?
A. Because the vibrations are too slow; in consequence of which, the sounds produced are not shrill or sharp enough.
Q. Why can persons living a mile or two from
a town hear the bells of the town-church some times, and not at others?
A. Fogs, rain, and snow, obstruct the passage of sound; but when the air is cold and clear, sound is propagated more easily.
Q. Why can we not hear sounds (as distant church bells) in rainy weather, so well as in fine weather?
A. Because the falling rain interferes with the undulations of the sound-waves, and breaks them up.
Q. Why can we not hear sounds (as distant church bells) in snowy weather, so well as in fine weather?
A. Because the falling snow interferes with the undulations of the sound-waves, and stops their progress.
Q. Why can we hear distant clocks most distinctly in clear cold weather?
A. Because the air is most uniform then: there are not two currents of air (one up and one down) to interrupt the sound-waves.
Q. Why can persons hear the voices of men in conversation for a mile distant, near the poles, in winter time?
A. Because the air is very cold and very clear; in consequence of which, there are not two currents of air (one up and one down) to interrupt the sound-waves.
Captain Ross heard the voices of his men in conversation, a mile and a half from the spot where they stood.
Q. Why are not sounds (such as distant church bells) heard so distinctly on a hot day as in frosty weather?
A. Because there are two currents of air; the current of hot air ascending from the earth, and the current of colder air falling towards the earth; and these two currents break up the sound-waves.
Q. Why can we not hear sounds (such as distant clocks) so distinctly in a thick mist or haze, as in a clear night?
A. Because the mist diminishes the velocity of the sound-waves, and (by overburdening them with vapour) limits their length.
Q. Why do we hear sounds better by night than by day?
Q. How should partition walls be made to prevent the voices in adjoining rooms from being heard?
A. The space between the laths (or canvass) should be filled with shavings or saw-dust; and then no sound would ever pass from one room to another.
Q. Why would shavings or saw-dust prevent the transmission of sound from room to room?
A. Because there would be several different media for the sound to pass through: 1st—the air;
2ndly—the laths and paper;
3rdly—the saw-dust or shavings;
4thly—the air again: and every variety diminishes the strength of the sound-waves.