[Ill.u.s.tration: Fig. 6.--White Light resolved into Colors.]

Light was once supposed to be corpuscular, or consisting of transmitted particles. It is now known to be the result of undulations in ether.

Reference has been made to the minuteness of these undulations.

Their velocity is equally wonderful. Put a prism of gla.s.s into a ray of light coming into a dark room, and it is [Page 25]

instantly turned out of its course, some parts more and some less, according to the number of vibrations, and appears as the seven colors on different parts of the screen. Fig. 6 shows the arrangement of colors, and the number of millions of millions of vibrations per second of each. But the different divisions we call colors are not colors in themselves at all, but simply a different number of vibrations. Color is all in the eye. Violet has in different places from 716 to 765,000,000,000,000 of vibrations per second; red has, in different places, from 396 to 470,000,000,000,000 vibrations per second. None of these in any sense are color, but affect the eye differently, and we call these different effects color. They are simply various velocities of vibration. An object, like one kind of stripe in our flag, which absorbs all kinds of vibrations except those between 396 and 470,000,000,000,000, and reflects those, appears red to us. The field for the stars absorbs and destroys all but those vibrations numbering about 653,000,000,000,000 of [Page 25] vibrations per second. A color is a constant creation. Light makes momentary color in the flag. Drake might have written, in the continuous present as well as in the past,

"Freedom mingles with its gorgeous dyes The milky baldrick of the skies, And stripes its pore celestial white With streakings of the morning light."

Every little pansy, tender as fancy, pearled with evanescent dew, fresh as a new creation of sunbeams, has power to suppress in one part of its petals all vibrations we call red, in another those we call yellow, and purple, and reflect each of these in other parts of the same tender petal. "Pansies are for thoughts," even more thoughts than poor Ophelia knew. An evening cloud that is dense enough to absorb all the faster and weaker vibrations, leaving only the stronger to come through, will be said to be red; because the vibrations that produce the impression we have so named are the only ones that have vigor enough to get through. It is like an army charging upon a fortress. Under the deadly fire and fearful obstructions six-sevenths go down, but one-seventh comes through with the glory of victory upon its face.

Light comes in undulations to the eye, as tones of sound to the ear. Must not light also sing? The lowest tone we can hear is made by 16.5 vibrations of air per second; the highest, so shrill and "fine that nothing lives 'twixt it and silence," is made by 38,000 vibrations per second. Between these extremes lie eleven octaves; C of the G clef having 258-7/8 vibrations to the second, and its octave above 517-1/2. Not that sound vibrations cease [Page 27] at 38,000, but our organs are not fitted to hear beyond those limitations. If our ears were delicate enough, we could hear even up to the almost infinite vibrations of light. In one of those semi-inspirations we find in Shakspeare's works, he says--

"There's not the smallest orb which thou beholdest, But in his motion like an angel sings, Still quiring to the young-eyed cherubim.

Such harmony is in immortal souls; But, whilst this muddy vesture of decay Doth grossly close it in, we cannot hear it."

And that older poetry which is always highest truth says, "The morning stars sing together." We misconstrued another pa.s.sage which we could not understand, and did not dare translate as it was written, till science crept up to a perception of the truth that had been standing there for ages, waiting a mind that could take it in.

Now we read as it is written--"Thou makest the out-goings of the morning and evening to sing." Were our senses fine enough, we could hear the separate keynote of every individual star. Stars differ in glory and in power, and so in the volume and pitch of their song. Were our hearing sensitive enough, we could hear not only the separate key-notes but the infinite swelling harmony of these myriad stars of the sky, as they pour their mighty tide of united anthems in the ear of G.o.d:

"In reason's ear they all rejoice, And utter forth a glorious voice.

Forever singing, as they shine, The hand that made us is divine."

This music is not monotonous. Stars draw near each other, and make a light that is unapproachable by mortals; [Page 28] then the music swells beyond our ability to endure. They recede far away, making a light so dim that the music dies away, so near to silence that only spirits can perceive it. No wonder G.o.d rejoices in his works. They pour into his ear one ceaseless tide of rapturous song.

Our senses are limited--we have only five, but there is room for many more. Some time we shall be taken out of "this muddy vesture of decay," no longer see the universe through crevices of our prison-house, but shall range through wider fields, explore deeper mysteries, and discover new worlds, hints of which have never yet been blown across the wide Atlantic that rolls between them and men abiding in the flesh.

_Chemistry of Suns revealed by Light._

When we examine the a.s.semblage of colors spread from the white ray of sunlight, we do not find red simple red, yellow yellow, etc., but there is a vast number of fine microscopic lines of various lengths, parallel--here near together, there far apart, always the same number and the same relative distance, when the same light and prism are used. What new alphabets to new realms of knowledge are these! Remember, that what we call colors are only various numbers of vibrations of ether. Remember, that every little group in the infinite variety of these vibrations may be affected differently from every other group. One number of these is bent by the prism to where we see what we call the violet, another number to the place we call red. All of the vibrations are destroyed when they strike a surface we call black. A part of them are destroyed when [Page 29] they strike a substance we call colored. The rest are reflected, and give the impression of color. In one place on the flag of our nation all vibrations are destroyed except the red; in another, all but the blue. Perhaps on that other gorgeous flag, not of our country but of our sun, the flag we call the solar spectrum, all vibrations are destroyed where these dark lines appear. Perhaps this effect is not produced by the surface upon which the rays fall, but by some specific substance in the sun. This is just the truth.

Light pa.s.sing through vapor of sodium has the vibrations that would fall on two narrow lines in the yellow utterly destroyed, leaving two black s.p.a.ces. Light pa.s.sing through vapor of burning iron has some four hundred numbers or kinds of vibrations destroyed, leaving that number of black lines; but if the salt or iron be glowing gas, in the source of the light itself the same lines are bright instead of dark.

Thus we have brought to our doors a readable record of the very substances composing every world hot enough to shine by its own light. Thus, while our flag means all we have of liberty, free as the winds that kiss it, and bright as the stars that shine in it, the flag of the sun means all that it is in const.i.tuent elements, all that it is in condition.

We find in our sun many substances known to exist in the earth, and some that we had not discovered when the sun wrote their names, or rather made their mark, in the spectrum. Thus, also, we find that Betelguese and Algol are without any perceivable indications of hydrogen, and Sirius has it in abundance. What a sense of acquaintanceship it gives us to look up and recognize [Page 30] the stars whose very substance we know! If we were transported thither, or beyond, we should not be altogether strangers in an unknown realm.

But the stars differ in their const.i.tuent elements; every ray that flashes from them bears in its very being proofs of what they are.

Hence the eye of Omniscience, seeing a ray of light anywhere in the universe, though gone from its source a thousand years, would be able to tell from what orb it originally came.

_Creative Force of Light._

Just above the color vibrations of the unbraided sunbeam, above the violet, which is the highest number our eyes can detect, is a chemical force; it works the changes on the gla.s.s plate in photography; it transfigures the dark, cold soil into woody fibre, green leaf, downy rose petals, luscious fruit, and far pervasive odor; it flushes the wide acres of the prairie with gra.s.s and flowers, fills the valleys with trees, and covers the hills with corn, a single blade of which all the power of man could not make.

This power is also fit and able to survive. The engineer Stephenson once asked Dr. Buckland, "What is the power that drives that train?"

pointing to one thundering by. "Well, I suppose it is one of your big engines." "But what drives the engine?" "Oh, very likely a canny Newcastle driver." "No, sir," said the engineer, "it is sunshine."

The doctor was too dull to take it in. Let us see if we can trace such an evident effect to that distant cause. Ages ago the warm sunshine, falling on the scarcely lifted hills of Pennsylvania, caused the reedy vegetation to grow along the banks of [Page 31]

shallow seas, acc.u.mulated vast amounts of this vegetation, sunk it beneath the sea, roofed it over with sand, compacted the sand into rock, and changed this vegetable matter--the products of the sunshine--into coal; and when it was ready, lifted it once more, all garnered for the use of men, roofed over with mighty mountains. We mine the coal, bring out the heat, raise the steam, drive the train, so that in the ultimate a.n.a.lyses it is sunshine that drives the train. These great beds of coal are nothing but condensed sunshine--the sun's great force, through ages gone, preserved for our use to-day. And it is so full of force that a piece of coal that will weigh three pounds (as big as a large pair of fists) has as much power in it as the average man puts into a day's work. Three tons of coal will pump as much water or shovel as much sand as the average man will pump or shovel in a lifetime; so that if a man proposes to do nothing but work with his muscles, he had better dig three tons of coal and set that to do his work and then die, because his work will be better done, and without any cost for the maintenance of the doer.

Come down below the color vibrations, and we shall find that those which are too infrequent to be visible, manifest as heat. Naturally there will be as many different kinds of heat as tints of color, because there is as great a range of numbers of vibration. It is our privilege to sift them apart and sort them over, and find what kinds are best adapted to our various uses.

Take an electric lamp, giving a strong beam of light and heat, and with a plano-convex lens gather it into a single beam and direct it upon a thermometer, twenty feet away, that is made of gla.s.s and filled with air. The [Page 32] expansion or contraction of this air will indicate the varying amounts of heat. Watch your air-thermometer, on which the beam of heat is pouring, for the result. There is none. And yet there is a strong current of heat there. Put another kind of test of heat beyond it and it appears; coat the air-thermometer with a bit of black cloth, and that will absorb heat and reveal it. But why not at first? Because the gla.s.s lens stops all the heat that can affect gla.s.s. The twenty feet of air absorbs all the heat that affects air, and no kind of heat is left to affect an instrument made of gla.s.s and air; but there are kinds of heat enough to affect instruments made of other things.

A very strong current of heat may be sent right through the heart of a block of ice without melting the ice at all or cooling off the heat in the least. It is done in this way: Send the beam of heat through water in a gla.s.s trough, and this absorbs all the heat that can affect water or ice, getting itself hot, and leaving all other kinds of heat to go through the ice beyond; and appropriate tests show that as much heat comes out on the other side as goes in on this side, and it does not melt the ice at all. Gunpowder may be exploded by heat sent through ice. Dr. Kane, years ago, made this experiment. He was coming down from the north, and fell in with some Esquimaux, whom he was anxious to conciliate. He said to the old wizard of the tribe, "I am a wizard; I can bring the sun down out of the heavens with a piece of ice." That was a good, deal to say in a country where there was so little sun. "So," he writes, "I took my hatchet, chipped a small piece of ice into the form of a double-convex lens, [Page 33] smoothed it with my warm hands, held it up to the sun, and, as the old man was blind, I kindly burned a blister on the back of his hand to show him I could do it."

These are simple ill.u.s.trations of the various kinds of heat. The best furnace or stove ever invented consumes fifteen times as much fuel to produce a given amount of heat as the furnace in our bodies consumes to produce a similar amount. We lay in our supplies of carbon at the breakfast, dinner, and supper table, and keep ourselves warm by economically burning it with the oxygen we breathe.

Heat a.s.sociated with light has very different qualities from that which is not. Sunlight melts ice in the middle, bottom, and top at once. Ice in the spring-time is honey-combed throughout. A piece of ice set in the summer sunshine crumbles into separate crystals.

Dark heat only melts the surface.

Nearly all the heat of the sun pa.s.ses through gla.s.s without hinderance; but take heat from white-hot platinum and only seventy-six per cent.

of it goes through gla.s.s, twenty-four per cent. being so const.i.tuted that it cannot pa.s.s with facility. Of heat from copper at 752 only six per cent. can go through gla.s.s, the other ninety-four per cent. being absorbed by it.

The heat of the sun beam goes through gla.s.s without [Page 34] any hinderance whatever. It streams into the room as freely as if there were no gla.s.s there. But what if the furnace or stove heat went through gla.s.s with equal facility? We might as well try to heat our rooms with the window-panes all out, and the blast of winter sweeping through them.

The heat of the sun, by its intense vibrations, comes to the earth dowered with a power which pierces the miles of our atmosphere, but if our air were as pervious to the heat of the earth, this heat would flyaway every night, and our temperature would go down to 200 below zero. This heat comes with the light, and then, dissociated from it, the number of its vibrations lessened, it is robbed of its power to get away, and remains to work its beneficent ends for our good.

Worlds that are so distant as to receive only 1/1000 of the heat we enjoy, may have atmospheres that retain it all. Indeed it is probable that Mars, that receives but one-quarter as much heat as the earth, has a temperature as high as ours. The poet drew on his imagination when he wrote:

"Who there inhabit must have other powers, Juices, and veins, and sense and life than ours; One moment's cold like theirs would pierce the bone, Freeze the heart's-blood, and turn us all to stone."

The power that journeys along the celestial s.p.a.ces in the flashing sunshine is beyond our comprehension. It accomplishes with ease what man strives in vain to do with all his strength. At West Point there are some links of a chain that was stretched across the river to prevent British ships from ascending; these links were made of two-and-a-quarter-inch iron. A powerful locomotive might tug in vain at one of them and not stretch it the thousandth part of an inch. But the heat of a single gas-burner, that glows with the preserved sunlight of other ages, when suitably applied to the link, stretches it with ease; such enormous power has a little heat. There is a certain iron bridge across the Thames at London, resting on arches. The warm sunshine, acting [Page 35] upon the iron, stations its particles farther and farther apart. Since the bottom cannot give way the arches must rise in the middle. As they become longer they lift the whole bridge, and all the thundering locomotives and miles of goods-trains cannot bring that bridge down again until the power of the sunshine has been withdrawn. There is Bunker Hill Monument, thirty-two feet square at the base, with an elevation of two hundred and twenty feet. The sunshine of every summer's day takes hold of that mighty pile of granite with its aerial fingers, lengthens the side affected, and bends the whole great ma.s.s as easily as one would bend a whipstock. A few years ago we hung a plummet from the top of this monument to the bottom. At 9 A.M. it began to move toward the west; at noon it swung round toward the north; in the afternoon it went east of where it first was, and in the night it settled back to its original place.

The sunshine says to the sea, held in the grasp of gravitation, "Rise from your bed! Let millions of tons of water fly on the wings of the viewless air, hundreds of miles to the distant mountains, and pour there those millions of tons that shall refresh a whole continent, and shall gather in rivers fitted to bear the commerce and the navies of nations." Gravitation says, "I will hold every particle of this ocean as near the centre of the earth as I can."

Sunshine speaks with its word of power, and says, "Up and away!"

And in the wreathing mists of morning these myriads of tons rise in the air, flyaway hundreds of miles, and supply all the Niagaras, Mississippis and Amazons of earth. The sun says to the earth, wrapped in the mantle of winter, [Page 36] "Bloom again;" and the snows melt, the ice retires, and vegetation breaks forth, birds sing, and spring is about us.

Thus it is evident that every force is const.i.tutionally arranged to be overcome by a higher, and all by the highest. Gravitation of earth naturally and legitimately yields to the power of the sun's heat, and then the waters fly into the clouds. It as naturally and legitimately yields to the power of mind, and the waters of the Red Sea are divided and stand "upright as an heap." Water naturally bursts into flame when a bit of pota.s.sium is thrown into it, and as naturally when Elijah calls the right kind of fire from above.

What seems a miracle, and in contravention of law, is only the const.i.tutional exercise of higher force over forces organized to be swayed. If law were perfectly rigid, there could be but one force; but many grades exist from cohesion to mind and spirit.

The highest forces are meant to have victory, and thus give the highest order and perfectness.

Across the astronomic s.p.a.ces reach all these powers, making creation a perpetual process rather than a single act. It almost seems as if light, in its varied capacities, were the embodiment of G.o.d's creative power; as if, having said, "Let there be light," he need do nothing else, but allow it to carry forward the creative processes to the end of time. It was Newton, one of the earliest and most acute investigators in this study of light, who said, "I seem to have wandered on the sh.o.r.e of Truth's great ocean, and to have gathered a few pebbles more beautiful than common; but the vast ocean itself rolls before me undiscovered and unexplored."

[Page 37]

EXPERIMENTS WITH LIGHT.

A light set in a room is seen from every place; hence light streams in every possible direction. If put in the centre of a hollow sphere, every point of the surface will be equally illumined. If put in a sphere of twice the diameter, the same light will fall on all the larger surface. The surfaces of spheres are as the squares of their diameters; hence, in the larger sphere the surface is illumined only one-quarter as much as the smaller. The same is true of large and small rooms. In Fig. 7 it is apparent that the light that falls on the first square is spread, at twice the distance, over the second square, which is four times as large, and at three times the distance over nine times the surface. The varying amount of light received by each planet is also shown in fractions above each world, the amount received by the earth being 1.

[Ill.u.s.tration: Fig. 7.]

[Ill.u.s.tration: Fig. 8.--Measuring Intensities of Light.]

The intensity of light is easily measured. Let two lights of different brightness, as in Fig. 8, cast shadows on the same screen. Arrange them as to distance so that both shadows shall be equally dark.

Let them fall side by side, and study them carefully. Measure the respective distances. Suppose one is twenty inches, the other forty.

Light varies as the square [Page 38] of the distance: the square of 20 is 400, of 40 is 1600. Divide 1600 by 400, and the result is that one light is four times as bright as the other.

[Ill.u.s.tration: Fig. 9.--Reflection and Diffusion of Light.]

Light can be handled, directed, and bent, as well as iron bars.

Darken a room and admit a beam of sunlight through a shutter, or a ray of lamp-light through the key-hole. If there is dust in the room it will be observed that light goes in straight lines. Because of this men are able to arrange houses and trees in rows, the hunter aims his rifle correctly, and the astronomer projects straight lines to infinity. Take a hand-mirror, or better, a piece of gla.s.s coated on one side with black varnish, and you can send your ray anywhere. By using two mirrors, or having an a.s.sistant and using several, you can cause a ray of light to turn as many corners as you please. I once saw Mr. Tyndall send a ray into a gla.s.s jar filled with smoke (Fig. 9). Admitting a slender ray through a small hole in a card over the mouth, one ray appeared; removing the cover, the whole jar was luminous; as the smoke disappeared in spots cavities of darkness appeared. Turn the same ray into a tumbler of water, [Page 39]

it becomes faintly visible; stir into it a teaspoonful of milk, then turn in the ray of sunlight, and it glows like a lamp, illuminating the whole room. These experiments show how the straight rays of the sun are diffused in every direction over the earth.