A Guide to the Scientific Knowledge of Things Familiar
CHAPTER XXVII.
LIGHT.
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_.
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_.
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 equals the _angle of reflection_, therefore the rays of the noon-day sun enter the glass _too perpendicularly_ for their reflection to be seen.
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.
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 so _oblique_ as those of the sun, they will often reach the water.
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_.
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.
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 collect from the window sufficient rays to enable them to _see into the room_.
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.
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.
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.
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_.
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.
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 _edge_, and those which issue from the _centre_.
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_.
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.
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.
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 eye: and (as a bent stick is not so far from end to end as a straight one) so the fishes appear nearer our eye 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_.
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 glasses_, to enable them to see objects near at hand.
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_.
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.
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.
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_.
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_.
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 petals) produces an _acid_ which turns them _red_.
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_ (or blue colour); hence the _yellow sap_ of the potato is not converted to green by 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 sand _reflects the rays of the sun_ like a mirror.
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 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. " " " of orange " blue. " " " of violet " yellow. " " " of black " white.
And the converse of this is true:--
The accidental colour of bluish green is red. " " " of blue " orange. " " " of yellow " violet. " " " of white " 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.