472.What is the refraction of light?
When rays of light fallobliquelyupon the surface of anytransparent medium, they are slightly diverted from their course. This alteration of the course of the rays is calledrefraction, and the degree of refraction is influenced by the difference between thedensitiesof the mediumsthrough which light is transmitted.
"Let your light so shine before men, that they may see your good works, and glorify your father which is in heaven."—Matt. v.
473.If a ray of light falls in a straight line upon a transparent surface, is it then refracted?
In that case the ray pursues its course—there is no refraction.
474.Is the direction in which the rays are bent, or refracted, influenced by the relative densities of the media?
A ray of light falling slantingly upon awindow, in passing through it is slightly brought to theperpendicular; and if it then falls upon the surface of water, it is still further brought to the perpendicular inpassing through the water.
475.Is light refracted in passing from a dense medium to a thinner one?
It is; but thedirection of the refractionis just the opposite to the instance just given; a ray of light passing throughwaterintoair, does not take a moreperpendicular course, but becomes moreoblique.
Fig. 11.
Fig. 11.
476.Why, if a rod or a spoon be set in an empty basin, will it appear straight, or of its usual shape?
Because the rays of light that are reflected from it all pass through the same medium, theair.
477.Why if water be poured into the basin will the rod or spoon appear bent?
Because the rays of light that pass through thewaterarereflected in a different degreeto those that pass through the air.
"Evening, and morning, and at noon, will I pray, and cry aloud; and he shall hear my voice."—Psalm lv.
Place in the bottom of an empty basin (Fig. 11.) a shilling; then stand in such a position at the point B that the line of sight, over the edge of the basin, just excludes the shilling from view. Then request some one to pour water into the basin, until it is filled to C (Fig. 12.), keeping your eye fixed upon the spot. The shilling will gradually appear, and will soon come entirely in view. Not only will the shilling be brought in view, but also portions of the basin before concealed. This is owing to the rays of light passing from the bottom through the water in a directionmore perpendicularthan they would have done through the air; but on leaving the water they become moreoblique, and hence they convey the image of the shillingover the edge of the basin, which otherwise would have obstructed the view.
Fig. 12.
Fig. 12.
478.Why is it that in cloudy and showery days we see the sun's rays bursting through the clouds in different directions?
Because, in passing through clouds ofdifferent densitiesthe rays arebent out of their course.
479.Why is the apparent depth of water always deceptive?
Because the light reflected from the objects at the bottom isrefractedas it leaves the water.
480.How much deeper is water than it appears to be?
Aboutone-third. A person bathing, and being unable to swim, should calculate before jumping into the water, that if itlooks two feet deep, it is quitethree feet.
481.Why can we seldom at the first attempt touch anything lying at the bottom of the water with a stick?
Because we do not allow for thedifferent refractive powersof water and of air.
"I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth."
482.Why do we see the sun before sunrise, and after sunset?
Because of the refractive effects of the atmosphere. Rays of light, passing obliquely from the sun through the air to the earth, are refracted three or four times by the varying density of the medium. Each refraction bends the rays towards theperpendicular; and hence we see the sunbefore it risesandafter it sets.
Fig. 13.—DIAGRAM EXHIBITING THE REFRACTION OF THE SUN'S RAYS IN PASSING THROUGH THE ATMOSPHERE.
Fig. 13.—DIAGRAM EXHIBITING THE REFRACTION OF THE SUN'S RAYS IN PASSING THROUGH THE ATMOSPHERE.
483.Why do figures, viewed through the hot air proceeding from furnaces, and from lime-kilns, appear distorted and tremulous?
Because the ever varying density of the air which is flying away in hot currents, and succeeded by cold,constantly changes the refractive powerof the medium through which the figures are viewed.
484.Why do the stars twinkle?
Because their light reaches us throughvariously heated and moving currents of air. In this case the earth is thekiln, and thestarstheobjectthat isviewed through the refractive medium.
485.Why does much twinkling of the stars foretell bad weather?
Because it denotes that there arevarious ærial currentsof different temperatures and densities, producingatmospheric disturbance.
"And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud."—Genesis ix.
486.What causes the rainbow?
Therefractionof the sun's rays by thefalling rain.
487.Why does the rainbow exhibit various colours?
The colours belong to theelementary rays of light; and these rays havingdifferent degrees of refrangibility, some of them are bent more than others; they are therefore separated intodistinct rays of different colours.
488.Why are there sometimes two rainbows?
Because the rays ofrefractedlight, reflected upon other drops of rain, areagainrefracted, and thenreflected again, forming a secondary bow.
489.Why are the colours of the secondary bow arrayed in the reverse order of the primary bow?
Because the secondary bow isa reflectionof the primary bow, and, like all reflections, is reversed.
490.Why are reflections reversed?
Because those rays whichfirst reachthe reflecting surface are thefirst returned. If you hold your open hand towards the looking-glass, the light passing from the point of your finger will reach the reflector and be returned before the rays that pass from the back parts of the hand. Hence the image of the hand will present the reflection of the finger point towards the point of the finger.
491.Why are the colours of the secondary rainbow fainter than those of the primary?
Because they are derived from therefraction and reflectionof rays which havealreadybeen refracted and reflected, and therebytheir intensity has been diminished.
492.What is a lunar rainbow?
Alunar rainbowis caused by the light of themoon, in the same manner as thesolar rainbowis caused by the light of thesun.
"I am come a light into the world, that whosoever believeth in me should not abide in darkness."—John xiii.
493.Why is the lunar rainbow fainter than a solar rainbow?
Because thelight of the moonis thereflected light of the sun, and is thereforeless intense.
494.What is a halo?
A halo is aluminous ring, which forms between the eye of the observer and a luminous body.
Haloes may appear around the disc of the sun, moon, or stars. But in this country thelunarhaloes are the most remarkable and frequent.
495.What is the cause of the luminous ring?
Therefraction of lightas it passes through an interveningcloud, or a stratum ofmoistandcold air.
496.Why are haloes sometimes large and at other times small?
Because they are sometimes formedvery highin the atmosphere, at other timesvery low. Being high, and farther removed from the spectator, and nearer the source of light, they appearsmaller; while the nearer they are, thelarger they appear.
497.Why do haloes foretell wet weather?
Because they show that there is a great amount of atmospheric moisture, which will probably formrain.
498.Why do glass lustres and chandeliers exhibit "rainbow colours"?
Because theyrefract the rays of lightin the same manner as the rain drops.
499.Why does a soap bubble show the prismatic colours?
Because, like a large rain drop, itrefracts the rays of light, and shows the elementary rays.
500.What causes the rich tints displayed by "mother-of-pearl?"
Therefraction of the lightthat falls upon the surface of the pearl.
"Light is sown for the righteous, and gladness for the upright in heart."—Psalm xcvii.
501.What causes the brilliant colours of the diamond?
Therefractionof the rays of light by the variousfacetsof the diamond.
The refraction of light, and the production of prismatic colours, surrounds us with most interesting phenomena. The laundress, whose active labours raise over the wash-tub a soapy froth, performs inadvertently one of the most delicate operations of chemistry—the chemistry of the imponderable agents—and the result of her manipulations manifests itself in the delicate colours that dance like a fairy light over the glassy films that follow the motion of her arms. The laughing child, throwing a bubble from the bowl of a tobacco pipe into the air, performs the same experiment, and produces a result such as that which filled the philosophic Newton with unbounded joy. The foam of the seashore, the plumage of birds, the various films that float upon the surface of waters, the delicate tints of flowers, and the rich hues of luscious fruits, all combine to remind us, that every ray of light comes like an angelic artist sent from heaven, bearing upon his palette the most celestial tints, with which to beautify the earth, and show the illimitable glory of God.
502.What is the difference between the refraction and the reflection of light?
Refractionis the deviation of rays of light from their course through the interference of adifferentmedium;reflectionis the return of rays of light which, having fallen upon a surface, are repelled by it.
503.What is the radiation of light?
Theradiationof light is itsemission in raysfrom the surface of aluminous body.
504.Do all bodies radiate light?
All bodies radiate light; but those that are not in themselves primary sources of light, are said toreflect it.
505.Do black bodies reflect any light?
Black bodiesabsorbthe light that falls upon them. But they reflect avery smalldegree of light.
506.Why is glass transparent?
Because its atoms are so arranged that they allow the vibrations of light to continue through their substance.
"As in water face answereth to face, so the heart of man to man."—Proverbs xxvii.
507.Does glass obstruct the passage of any portion of light?
Glassreflects(sends back) a very small portion of light. This may be observed by holding a piece of paper, or a hand, a few inches from a window, when a faint reflection of it will be visible. Probably the small amount of lightreflected by transparent glass, which gives a passage to the greater part of the rays, may serve to illustrate the small amount of light reflected fromblack surfaces, whichabsorbs the greater portion of light.
Instead of a piece of white paper, hold a piece ofblack clothtwo or three inches from the window-pane, and you will have two reflections so weak that the image of the cloth will be almost lost. The first reflection is that of the very small amount of light from the black surface on to the glass, and the second reflection is that of the inconceivably small amount returned by the glass, and by which the faint image of the black cloth is produced. But put the black cloth outside of the window-pane, and then hold an object before them, and you will find that thetwo weak reflectors, acting together, produce an improved image, or reflection.
508.Why, if a book is held between a candle-light and the wall, does a shadow fall upon the wall?
Because the rays of light areinterceptedby the book.
509.Why do the rays pass over the edges of the book in a direct line with the flame of the candle?
Because light always travels instraight lines.
510.Why is there some amount of light even where shadows fall?
Because,as all objects reflect light, some of them throw their light into the field of the shadow.
511.Why are some substances opaque to light?
Because the arrangement of their particles will not admit of thevibrations of the luminous etherpassing through them.
Opaque—impervious to rays of light.
512.Why do we see our faces reflected in mirrors?
Because the rays of light from our faces arereflectedby the surface of thequicksilverat the back of the glass.
"The day is thine, the night also is thine: thou hast prepared the light and the sun."—Psalm lxxiv.
513.Why does the quicksilver reflect the rays of light?
Because, beingdensely opaque to light, and presenting also a bright surface, it is a good reflector, and itthrows back the whole of the rays.
514.What has the glass to do with the reflection?
The glass hasnothing to do with the reflection, except that it affords a field upon which the reflecting surface of the quicksilver is spread; and it keeps the air and dirt fromdulling the quicksilver.
The parts of a mirror from which the quicksilver is rubbed away give no reflection that could assist the reflecting power of the quicksilver. That the surface of the glass does not reflect the image, is shown by the fact, that if you put the point of any object against the glass, the thickness between the point and the place where the reflection of it begins, willshow the exact thickness of the glass.
515.Why does a compound mirror (a multiplying mirror) exhibit a large number of images of one object.
Because all objects reflect rays of light inevery direction, and therefore the different mirrors, being atvarious angles, receiveeach a reflectionof the same object.
516.Why does a window-pane appear to be a better reflector by candle-light than by day-light?
The reflecting power of glass is precisely the same by night as by day, and is always very feeble. But it appears to be greater by night,because the surrounding darkness increases the apparent strength of the reflection.
517.How do we know that objects reflect light in every direction?
Because if weprick a hole in a card with a pin, and then look through that small hole upon alandscape, we can see some miles of country, and some thousands of objects; every part of every object throughout the whole scene, must have sent rays of light the small hole pricked in the card.
"Such knowledge is too wonderful for me; it is high, I cannot attain unto it."—Psalm cxxxix.
At one extremity of the landscape, viewed through the hole in the card, there may be a forest of trees; in the distance there may be hills bathed in golden light, and overhung with glittering clouds; in the mid-distance there may be a river winding its course along, as though it loved the earth through which it ran, and wished, by wandering to and fro, to refresh the thirsty soil; in the foreground may be a church, covered by a million ivy leaves; and grouping towards the sacred edifice may be hundreds of intending worshippers, old and young, rich and poor; flowers may adorn the path-ways, and butterflies spangle the air with their beauties; yet every one of those objects—the forest, the hills, the clouds, the river, the church, the ivy, the people, the flowers, the butterflies—must have sent rays of light, which found their way through the little hole in the card, and entered to paint the picture upon the curtain of the eye.
This is one of the most striking instances that can be afforded of the wonderful properties of light, and of the infinitude of those luminous rays that attend the majestic rising of the sun. Not only does light fly from the grand "ruler of the day" with a velocity which is a million and a half times greater than the speed of a cannon-ball, but it darts from every reflecting surface with a like velocity, and reaches the tender structure of the eye so gently that, as it falls upon the little curtain of nerves which is there spread to receive it, it imparts the most pleasing sensations, and tells its story of the outer world with a minuteness of detail, and a holiness of truth. Philosophers once sought toweighthesunbeam; they constructed a most delicate balance, and suddenly let in upon it a beam of light; the lever of the balance was so delicately hung that the fluttering of a fly would have disturbed it. Everything prepared, the grave men took their places, and with keen eyes watched the result. The sunbeam that was to decide the experiment had left the sun eight minutes prior to pass the ordeal. It had flown through ninety-five millions of miles of space in that short measure of time, and it shot upon the balance with unabated velocity: but the lever moved not, and the philosophers were mute.
518.Why, when we move before a mirror, does the image draw near to the reflecting surface as we draw near to it, and retire when we retire?
Because the lines and angles ofreflectionare always equal to the lines and angles ofincidence.
519.What is the line of incidence?
If a person stands in a direct line before a mirror, the line through which the light travels from him to the mirror isthe line of incidence.
Incidence—falling on.
"Blessed be the Lord, who daily loadeth us with benefits, even the God of our salvation."—Psalm lxviii.
520.What is the line of reflection?
Theline of reflectionis the line in which the rays of light are returned from the image formed in the glass to the eye of the observer.
Reflection—a turning back.
521.What is the angle of incidence?
Theangleof incidence is the angle which rays of light, falling on a reflecting surface, make with a line perpendicular to that surface.
Fig. 14.—EXPLAINING THE LINES AND ANGLES OF INCIDENCE AND OF REFLECTION.
Fig. 14.—EXPLAINING THE LINES AND ANGLES OF INCIDENCE AND OF REFLECTION.
522.What is the angle of reflection?
Theangleof reflection is the angle which is formed by the returning rays of light, and a line perpendicular to the reflecting surface. It is alwaysequivalentto the angle of incidence.
Take a marble and roll it across the floor, so that it shall strike the wainscot obliquely. Let A in the diagram represent the point from which the marble is sent. The marble will not return to the hand, nor will it travel to the line B, but will bound off, or bereflected, to C. Now B is an imaginary line,perpendicular to the reflecting surface; and it will be found that the path described by the marble inrolling to the surface and rebounding from it, form, with the line B, two angles that areequal. These represent the angles ofincidenceand ofreflection, and explain why the reflection of a person standing at A before a mirror, would be seen by another person standing at C. This simple law in optics explains a great many interesting phenomena, and therefore it should be clearly impressed upon the memory.
"And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also."—Gen. i.
523.Why do windows reflect the sun in the evening?
Because the eye of the observer is in theline of the reflection.
524.Why do windows not reflect the sun at noon?
They do, but our eyes are not then in theline of the reflection.
Fig. 15.—SHOWING THE LINES OF INCIDENCE AND REFLECTION OF THE SUN'S RAYS AT NOON AND AT EVENING.
Fig. 15.—SHOWING THE LINES OF INCIDENCE AND REFLECTION OF THE SUN'S RAYS AT NOON AND AT EVENING.
It is obvious from the foregoing diagram that the evening rays of reflection fall upon the eyes of spectators, while the reflections at noon are so perpendicular that they are lost.
525.Why do the sun and moon appear smaller when near the meridian, than when near the horizon?
Because, when near the horizon, they are brought intocomparison with the sizes of terrestrial objects; but when near the meridian they occupy the centre of a vast field of sky, and as there are no objects of comparison surrounding them, theyappear smaller.
This is one "Reason Why," assigned by some observers. But there is also another reason to be found in the fact that, when the sun or moon is near the horizon, we view it through agreater depth of atmospherethan we do when atthe meridian. (SeeFig. 13.) A straight line passed upward through the air, would not be so long as that which passes to S. Consequently, as the air is generally impregnated with moisture, at the time when these effects are observed, the rays of light are caused to diverge more, and the disc of the sun or moonappears magnified. Probably both of these reasons contribute to the effect. This latter reason also explains why the disc of the sun or moon may sometimes appearovalin shape, the lower stratum of air being more loaded with moisture than that through which we view the upper part of the disc.
"There is no darkness nor shadow of death, where the workers of iniquity may hide themselves."—Job xxxiv.
526.Why do our shadows lengthen as the sun goes down?
Because light travels only instraight lines, and as the sun descends, the direction of his rays becomes moreoblique, thereby causing longer shadows.
527.What is the cause of the optical illusions frequently observed in nature?
There are various kinds of natural optical illusions:—
Themirage, in which landscapes are seen reflected in burning sands.
Thefata morgana, in which two or three reflections of objects occur at the same time.
Theærial spectra, or ærial reflections, &c.
Fig. 16.—ILLUSTRATING THE APPEARANCE OF PHANTOM SHIPS.
Fig. 16.—ILLUSTRATING THE APPEARANCE OF PHANTOM SHIPS.
The optical illusions above enumerated owe their origin to variousatmospheric conditions, in whichrefractionsandreflectionsaremultipliedby the different densities of atmospheric layers. They chiefly occur in hot countries, where, from the varying effects of heat, the conditions of atmospheric refraction and reflection frequently prevail in their highest degree.
"In the morning ye say, it will be foul weather to-day, for the sky is red and lowering."—Matt. xvi.
528.Why do we have twilight mornings and evenings?
Because the coming and the departing rays of the sun arerefractedandreflectedby the upper portions of the atmosphere. (SeeFig. 13.)
529.How long before the sun appears above the horizon does the reflection of his light reach us?
The timevarieswith the refracting and reflecting power of the atmosphere, fromtwenty minutestosixty minutes. But the sun's position is usuallyeighteen degreesbelow the horizon when twilight begins or ends.
530.Why is the sky blue?
The white light of the sun falls upon the earth without change; it is then reflected back by the earth, and as it passes through the atmosphere portions of it are again returned to us, and this double reflection produces apolarisedcondition of light which imparts to vision the sensation of adelicate blue. (See549.)
531.Why do the clouds appear white?
Because they reflect back to us the solar beamunchanged.
532.Why does the sky appear red at sunset?
Because the light vapours of the air, which are condensed as the sun sets, refract the rays of light, and produce red rays. The refraction which producesredrequires only amoderate degree of density.
533.Why do the clouds sometimes appear yellow?
Because there is a larger amount of vapour in the air, which produces a different degree of refraction,resulting in yellow.
534.Why does a yellow sunset foretell wet weather?
Because it shows that the air is heavy with vapours. The refraction that producesyellowrequires a greater degree of density.
"When it is evening ye say it will be fair weather, for the sky is red."—Matt. xvi.
535.Why does a red sunset foretell fine weather?
Because the redness shows that the vapours in the airtowards the West, or wet quarter, arelight, as is evidenced by the degree of refraction of the sun's rays.
536.Why does a red sunrise foretell wet?
Because it shows thattowards the East, or dry quarter, the air is charged with vapour, and therefore probably at other points the air has reachedsaturation.
537.Why does a grey sunrise foretell a dry day?
Because it shows that the vapours in the air arenotvery dense.
538.Why is "a rainbow in the morning the shepherd's warning?"
Because it shows thatin the West, or wet quarter, the air issaturatedto the rain point.
539.Why is "a rainbow at night the shepherd's delight?"
Because it shows that therain is falling in the East, and as that is a dry quarter, it willsoon be over. Rainbows are always seen in opposition to the sun.
540.What is the difference between light and heat?
The most obvious distinction is, that light acts uponvision, and heat uponsensation, or feeling.
Another distinction is, thatheat expands all bodies, and alters their atomic condition; whilelight, though usually attended by heat, does not display the same expansive force, but produces various effects which arepeculiar to itself.
"Ye are the light of the world. A city that is set on a hill cannot be hid."—Matthew v.
541.Are light and heat combined in the solar ray?
Yes. A ray of light, as well as containing elementary rays that produce colours under refraction, contains alsochemical rays, andheat rays.
542.How do we know that light and heat are separate elements?
Because we haveheat rays, as from dark hot iron, from various chemical actions, and from friction, which areunattended by the development of light. And we have light, or luminosity, such as that ofphosophoresence, which is unaccompanied by any appreciable degree of heat.
But, besides this confirmation, further proof is afforded by the fact, that in passing rays of solar light through media that aretransparent to heat, but not tolight, the heat rays may beseparatedfrom the luminous rays, andvice versa.
Black glass, and black mica, which are nearlyopaque to light, aretransparent to heatto the extent of ninety degrees out of a hundred. While pale green glass, coloured by oxide of copper, and covered with a coating of water, or a thin coating of alum, will be perfectlytransparent to light, but will be almost quiteopaque to heat. These remarks apply, in a greater or less degree, to various other substances.
543.In what respects are light and heat similar?
Both heat and light have been referred to minute vibratory motions which occur, under exciting causes, in a very subtile elastic medium.
They are both united in the sun's rays.
They are both subject to laws of absorption, radiation, reflection, and refraction.
They are both essential to life, whether animal or vegetable.
Both may be developed in their greatest intensity by electricity.
They are both imponderable.
"When I consider thy heavens, the work of thy fingers, the moon and the stars which thou hast ordained:"
544.In what respects are light and heat dissimilar?
Heat frequently exists without light.
Light is usually attended with heat.
Light may be instantly extinguished, butHeat can only be more gradually reduced, by diffusion.
The solar rays deliver heat to the earth by day, and the heat remains with the earth when the light has departed.
Heat diffuses itself in all directions.
Light travels only in straight lines.
The colours that absorb and radiate both light and heat do not act in the same degree upon them both. Black, which does not radiate light, is a goodradiator of heat, &c., &c.
The oxy-hydrogenlightemits a most intense heat, but glass which will transmit the rays of light, will afford no passage to the rays of theheat.
Heat is latent in all bodies, but no satisfactory proof has been found that light is latent in substances.
These are only a few of the analogies and distinctions that exist between the two mysterious agents, light and heat. But they are sufficient to supply the starting points of investigation.
The importance of the heat that attends the solar rays may be illustrated by the experiments performed a few years ago, by Mr. Baker, of Fleet-street, London, who made a large burning lens, three feet and a half in diameter, and employed another lens to reduce the rays of the first to a focus of half an inch in diameter. The heat produced was so great that iron plates, gold, and stones wereinstantly melted; and sulphur, pitch, and resinous bodies,were melted under water.
545.What is the point of heat at which bodies become luminous?
The point of heat at which the eye begins to discover luminosity has been estimated at 1,000 deg.
546.What is the velocity of artificial light?
The light of a fire, or of a candle, or gas, travels with the same velocity as the light of the sun,—a velocity which would convey light eight times round the world while a person could count "one."
547.At what rate of velocity does the light of the stars travel?
At the same velocity as all other light. And yet there are stars so distant that, although the light of the sun reaches the earth in eight minutes and a half, it requireshundreds of yearsto bring their light to us.
"What is man, that thou art mindful of him? and the son of man that thou visitest him?"—Psalm viii.
548.What is the relative intensity of primary and reflected light?
The intensity of a reflection depends upon the power of the reflecting surface. But, taking the sun and moon as the great examples of primary and reflected light, the intensity of thesun's lightis 801,072 timesgreater than that of the moon.
549.What is polarized light?
Polarized lightis light which has been subjected tocompound refraction, and which, after polarization, exhibits a new series of phenomena, differing materially from those that pertain to the primary conditions of light.
550.What are the chief deductions from the phenomena observed under the polarization of light?
The polarization of light appears to confirm in a high degree the vibratorytheory of light; and to show that the vibrations of light have two planes or directions of motion. The mast of a ship, for instance, has two motions: it progressesverticallyas the ship is impelled forward, and it rollslaterallythrough the motion of the billows.
Something like this occurs in the vibrations of light, only thevertical vibrationis the condition ofone ray, and thelateral vibrationis the condition of another ray, and the vibrations of these two rays intersect each other in the solar ray. When these vibrations occur together, the ray has certain properties and powers. But by polarization the rays may beseparated, and the result is two distinct rays, havingdifferent vibrations.
It then appears that various bodies are transparent to these polarized raysonly in certain directions. And this fact is supposed to show that bodies are made up of their atoms arranged in certain planes, through or between which thelateralor theverticalwaves of light, together or singly, can or cannot pass; and that the transparency or the opacity of a body is determined by therelation of its atomic planestothe planes of the vibrations of light.
Ordinary light, passing through transparent media, produces no very remarkable effect in its course; butpolarized lightappears toilluminate every atom of the permeated substance, and by surrounding it with a prismatic clothing, to afford an illustration of itsmolecular arrangement.
"A man that is called Jesus made clay, and anointed mine eyes, and said unto me, Go to the pool of Siloam, and wash: and I went and washed, and I received sight."—John ix.
551.Why are two persons able to see each other?
Because rays of lightflow from their bodies to each other's eyes, and convey an impression of their respective conditions.
In some popular works that have come under our notice, we find that the student is told that "we cannot absolutely see each other—we onlysee the rays of light reflected from each other." The statement is erroneous as expressed. We do not see theraysof light, for if we did so, the effect of vision would be destroyed, and all bodies wouldappearto be in a state ofincandesence, or ofphosphoresence. Rays of light, which are in themselvesinvisible, radiate from the objects we look upon, enter the pupil of the eye, and impress the seat of vision in a manner which conveys to the mind a knowledge of the form, colour, and relative size and position of the figure we look upon. If this is not seeing the object—what is? It would be just as reasonable to say, that we cannotheara person speak—that we only hear thevibrations of the air. But as the vibrations are imparted to the air by the organs of voice of the speaker, as he sets the air in motion, and makes the air his messenger to us, we certainly hearhim, and can dispense with any logical myths that confound the understanding, and contribute to no good result.
552.What is actinism?
Actinismis the chemical property of light.
Actinism—ray power.
553.Why does silver tarnish when exposed to light?
Because of theactinic, or chemical power of the rays of the sun.
554.Why do some colours fade, and others darken, when exposed to the sun?
Because of thechemicalpower of the sun's rays.
555.Why can pictures be taken by the sun's rays?
Because of the actinic powers that accompany the solar light.
556.What is the particular chemical effect of light exhibited in the production of photographic pictures?
Simply thedarkening of preparations of silver, by the actinic rays.
557.Why are photographic studios usually glazed with blue glass?
Because blue glass obstructs many of the luminous rays, but it is perfectly transparent toactinism.
"The hay appeareth, and the tender grass showeth itself, and herbs of the mountain are gathered."—Prov. xxvii.
558.Why do plants become scorched under the unclouded sun?
Because the heat rays are in excess. The clouds shut off the scorching light; but, like the blue glass of the photographer's studio, they transmitactinism.
559.What effect has actinism upon vegetation?
It quickens the germination of seeds; and assists in the formation of the colouring matter of leaves. Seeds and cuttings, which are required to germinate quickly, will do so under the effect of blue glass (which is equivalent to saying, the effect of an increased proportion ofactinism), in half the time they would otherwise require.
560.In what season of the year is the actinic power of light the greatest?
In thespring, when the germination of plants demands its vitalising aid. Insummer, when the maturing process advances,lightandheatincrease, andactinismrelatively declines. In theautumn, when the ripening periodarrives,lightandactinismgive way to a greater ratio ofheat.
"But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him."—Corinth. Book i., ii.
We shall have frequently, in the progress of our lessons, to refer tolightin its connection with the chemistry of nature, and with organic life. But let us now invite the student to pause, and for a moment contemplate the wonders of a sunbeam. How great is its velocity—how vast its power—how varied its parts—yet how ethereal! First, let us contemplate it as a simple beam in whichlightandheatare associated. How deep the darkness of the night, and how that darkness clings to the recesses of the earth. But the day beams, and darkness flies before it, until every atom that meets the face of day is lit up with radiance. That which before lay buried in the shade of night is itself now a radiator of the luminous fluid. Mark the genial warmth that comes as the sister of light; then stand by the side of the experimentalist and watch the point on which he directs the shining focus, and in an instant see iron melt and stones run like water, under the fervent heat! Now look upward to the heavens, where the falling drops of rain have formed a natural prism in the rainbow, and shown that the beam of pure whiteness, refracted into various rays, glows with all the tints that adorn the garden of nature. These are the visible effects of light. But follow it into the crust of the earth, where it is, by another power, which is neither light nor heat, quickening the seed into life; watch it as the germ springs up, and the plant puts forth its tenderparts, touching them from day to day with deeper dyes, until the floral picture is complete. Follow it unto the sea, where it gives prismatic tints to theanemone, and imparts the richest colours to the variousalgae. Think of the millions of pictures that it paints daily upon the eyes of living things. Contemplate the people of a vast city when, attracted by some floating toy in the air, a million eyes look up to watch its progress. The sun paints a million images of the same object, and each observer has a perfect picture. It makes common to all mankind the beauties of nature, and paints as richly for the peasant as for the king. The Siamese twins were united by a living cord which joined their systems, and gave unity and sympathy to their sensations. In the great flood of light that daily bathes the world, we have a bond of union, giving the like pleasures and inspirations to millions of people at the same instant. And that which floods the world with beauty, should no less be a bond of unity and love.
561.What is electricity?
Electricity is a property offorcewhich resides in all matter, and which constantly seeks to establish anequilibrium.
562.Why is it called electricity?
Because it first revealed itself to human observation through a substance called, in the Greek language,electrum. This substance is known to us asamber.
563.In what way did electrum induce attention to this property of force in matter?
Thales, a Greek philosopher, observed that, by briskly rubbingelectrum, it acquired the property ofattractinglight particles of matter, which moved towards the amber, and attached themselves to its surface, evidently under the influence of aforceexcited in the amber.
564.What is amber?
It is aresinoussubstance, hard, bitter, tasteless, and glossy. It has been variously supposed to be a vegetable gum, a fossil, and an animal product. It is probably formed by aspecies of antthat inhabit pine forests. The bodies of ants are frequently found in its substance.
"He made darkness his secret place: his pavilion round about him were dark waters and thick clouds of the skies."
565.Why does the rubbing of a stick of sealing-wax cause it to attract small particles of matter?
Because it excites in the sealing wax thatforcewhich was first observed in theamber. Sealing-wax, therefore, is called anelectric(amber-like) body.
566.Why do we hear of the electric fluid?
Simply because the termfluidis the most convenient that can be found to express our ideas when speaking of thephenomena of electric force. But of the nature of electricity, except through its observedeffects, nothing is known.
567.What substances are electric?
All substances in nature, from themetalsto thegases. But they differ very widely in their electrical qualities.
568.What is positive electricity?
Electricity, when it exists, or is excited, in any body, to an amount which isin excessof the amount natural to that body, is calledpositive(called alsovitreous).
569.What is negative electricity?
Electricity, when it exists, or is excited, in any body, in an amount whichis lessthan is the amount natural to that body, is callednegative(called alsoresinous).
570.Why is "positive" electricity called also "vitreous," and "negative" electricity called also "resinous"?
Because some philosophers believe that there is butone electricity, but that it is liable to variations ofquantityorstate, which they distinguish bypositiveandnegative; while other philosophers believe that there aretwo electricities, which they namevitreousandresinous, because they may be induced respectively fromvitreousandresinoussubstances, and they display forces of attraction and repulsion.
571.Upon what do the electrical phenomena of nature depend?
Upon the tendency ofelectricityto find anequilibriumbetween itspositiveandnegativestates (assuming there to be butonefluid);or upon the tendency ofvitreous electricityto seek out and combine withresinous electricity(assuming that there aretwofluids).
"The Lord also thundered in the heavens, and the Highest gave his voice; hailstones and coals of fire."
572.How does the equilibrium of electricity become disturbed?
By changes in the condition of matter. As electricity resides in all substances, and is, perhaps, an essential ingredient in their condition, so every change in the state of matter—whether from heat to cold, or from cold to heat; from a state of rest to that of motion; from the solid to the liquid, or the æriform condition, orvice versa; or whether substances combine chemically and produce new compounds—in every changethe electrical equilibrium is disturbed; and, in proportion to the degree of disturbance, is the force exerted by electricity to resume its balance in the scale of nature.
573.How does electricity seek to regain equilibrium?
By passing through substances that are favourable to its diffusion; therefore they are calledconductingornon-conductingbodies, according as they favour or oppose the transmission of the electrical current.
574.What substances are conductors of electricity?
Metals, charcoal, animal fluids, water, vegetable bodies, animal bodies, flame, smoke, vapour, &c.
575.What substances are non-conductors?
Rust, oils, phosphorous, lime, chalk, caoutchouc, gutta percha, camphor, marble, porcelain, dry gases and air, feathers, hair, wool, silk, glass, transparent stones, vitrefactions, wax, amber, &c. These bodies are also calledinsulators. Some of these substances, as chalk, feathers, hair, wool, silk, &c., though non-conductors whendry, become conductors whenwetted.
Insulating—preventing from escaping.
576.Why are amber and wax classed among the non-conductors, when they have been pointed out as electrics, and used to illustrate electrical force?
It isbecausethey arenon-conductorsthat they have displayed, under excitement, the attractive force shown in respect to theparticles of matter which were drawn towards their substances. If a bar ofironwere excited, instead of a stick of wax, electricity would be equally developed; but the iron,being a good conductor, would pass the electricity to the hand of the operator as fast as it accumulated, and the equilibrium would be undisturbed.
"Yea, he sent out his arrows, and scattered them; and he shot out lightnings and discomfited them."—Psalm xviii.
577.What is the effect when electricity, in considerable force, seeks its equilibrium, but meets with insulating bodies?
The result is a violent action in which,intense heat and lightare developed, and in the evolution of whichthe electric force becomes expended.
578.What is the cause of electric sparks?
The electric force, passing through a conducting body to find itsequilibrium, is checked in its course by an insulator, and emits a spark.
579.What produces the electric light?
Currents of electricity pass towards each other along wires at the ends of which two charcoal points are placed. As long as the charcoal points remain in contact, the electric communication is complete, and no light is emitted, but, when they are drawn apart, intense heat and light are evolved.