"And God said, Let there be light, and there was light."--_Genesis_
i., 3.
"God is light."--1 _John_, i. 5.
[Page 16]
"Hail! holy light, offspring of Heaven first born,
Or of the eternal, co-eternal beam,
May I express thee unblamed? since God is light,
And never but in unapproached light
Dwelt from eternity, dwelt then in thee,
Bright effluence of bright essence increate."
MILTON.
"A million torches lighted by Thy hand
Wander unwearied through the blue abyss:
They own Thy power, accomplish Thy command,
All gay with life, all eloquent with bliss.
What shall we call them? Piles of crystal light--
A glorious company of golden streams--
Lamps of celestial ether burning bright--
Suns lighting systems with their joyous beams?
But 'Thou to these art as the noon to night."
DERZHAVIN, trans. by BOWRING.
[Page 17]
II.
_CREATIVE PROGRESS._
Worlds would be very imperfect and useless when simply endowed
with attraction and inertia, if no time were allowed for these
forces to work out their legitimate results. We want something
more than swirling seas of attracted gases, something more than
compacted rocks. We look for soil, verdure, a paradise of beauty,
animal life, and immortal minds. Let us go on with the process.
Light is the child of force, and the child, like its father, is full
of power. We dowered our created world with but a single quality--a
force of attraction. It not only had attraction for its own material
substance, but sent out an all-pervasive attraction into space. By
the force of condensation it flamed like a sun, and not only lighted
its own substance, but it filled all space with the luminous outgoings
of its power. A world may be limited, but its influence cannot;
its body may have bounds, but its soul is infinite. Everywhere is
its manifestation as real, power as effective, presence as actual,
as at the central point. He that studies ponderable bodies alone
is not studying the universe, only its skeleton. Skeletons are
somewhat interesting in themselves, but far more so when covered
with flesh, flushed with beauty, and inspired with soul. The
universe [Page 18] has bones, flesh, beauty, soul, and all is one.
It can be understood only by a study of all its parts, and by
tracing effect to cause.
But how can condensation cause light? Power cannot be quiet. The
mighty locomotive trembles with its own energy. A smitten piece
of iron has all its infinitesimal atoms set in vehement commotion;
they surge back and forth among themselves, like the waves of a
storm-blown lake. Heat is a mode of motion. A heated body commences
a vigorous vibration among its particles, and communicates these
vibrations to the surrounding air and ether. When these vibrations
reach 396,000,000,000,000 per second, the human eye, fitted to be
affected by that number, discerns the emitted undulations, and the
object seems to glow with a dull red light; becoming hotter, the
vibrations increase in rapidity. When they reach 765,000,000,000,000
per second the color becomes violet, and the eye can observe them no
farther. Between these numbers are those of different rapidities,
which affect the eye--as orange, yellow, green, blue, indigo, in an
almost infinite number of shades--according to the sensitiveness
of the eye.
We now see how our dark immensity of attractive atoms can become
luminous. A force of compression results in vibrations within,
communicated to the ether, discerned by the eye. Illustrations are
numerous. If we suddenly push a piston into a cylinder of brass,
the force produces heat enough to set fire to an inflammable substance
within. Strike a half-inch cube of iron a moderate blow and it becomes
warm; a sufficient blow, and its vibrations become quick enough to
be seen--it is red-hot. Attach a thermometer to an extended [Page
19] arm of a whirling wheel; drive it against the air five hundred
feet per second, the mercury rises 16°. The earth goes 98,000 feet
per second, or one thousand miles a minute. If it come to an
aerolite or mass of metallic rock, or even a cloudlet of gas,
standing still in space, its contact with our air evolves 600,000°
of heat. And when the meteor comes toward the world twenty-six miles
a second, the heat would become proportionally greater if the meteor
could abide it, and not be consumed in fervent heat. It vanishes
almost as soon as seen. If there were meteoric masses enough lying
in our path, our sky would blaze with myriads of flashes of light.
Enough have been seen to enable a person to read by them at night.
If a sufficient number were present, we should miss their individual
flashes as they blend their separate fires in one sea of
insufferable glory. The sun is 1,300,000 times as large as our
planet; its attraction proportionally greater; the aerolites more
numerous; and hence an infinite hail of stones, small masses and
little worlds, makes ceaseless trails of light, whose individuality
is lost in one dazzling sea of glory.
On the 1st day of September, 1859, two astronomers, independently
of each other, saw a sudden brightening on the surface of the sun.
Probably two large meteoric masses were travelling side by side
at two or three hundred miles per second, and striking the sun's
atmosphere, suddenly blazed into light bright enough to be seen
on the intolerable light of the photosphere as a background. The
earth responded to this new cause of brilliance and heat in the
sun. Vivid auroras appeared, not only at the north and south poles,
but even where such spectacles are seldom seen. The electro-magnetic
[Page 20] disturbances were more distinctly marked. "In many places
the telegraphic wires struck work. In Washington and Philadelphia
the electric signalmen received severe electric shocks; at a station
in Norway the telegraphic apparatus was set fire to; and at Boston a
flame of fire followed the pen of Bain's electric telegraph." There
is the best of reason for believing that a continuous succession of
such bodies might have gone far toward rendering the earth
uncomfortable as a place of residence.
Of course, the same result of heat and light would follow from
compression, if a body had the power of contraction in itself. We
endowed every particle of our gas, myriads of miles in extent, with an
attraction for every other particle. It immediately compressed itself
into a light-giving body, which flamed out through the interstellar
spaces, flushing all the celestial regions with exuberant light.
But heat exerts a repellent force among particles, and soon an
equilibrium is reached, for there comes a time when the contracting
body can contract no farther. But heat and light radiate away into
cold space, then contraction goes on evolving more light, and so
the suns flame on through the millions of years unquenched. It is
estimated that the contraction of our sun, from filling immensity
of space to its present size, could not afford heat enough to last
more than 18,000,000 years, and that its contraction from its present
density (that of a swamp) to such rock as that of which our earth
is composed, could supply heat enough for 17,000,000 years longer.
But the far-seeing mind of man knows a time must come when the
present force of attraction [Page 21] shall have produced all the
heat it can, and a new force of attraction must be added, or the sun
itself will become cold as a cinder, dead as a burned-out char.
Since light and heat are the product of such enormous cosmic forces,
they must partake of their nature, and be force. So they are. The
sun has long arms, and they are full of unconquerable strength
ninety-two millions, or any other number of millions, of miles
away. All this light and heat comes through space that is 200°
below zero, through utter darkness, and appears only on the earth.
So the gas is darkness in the underground pipes, but light at the
burner. So the electric power is unfelt by the cable in the bosom
of the deep, but is expressive of thought and feeling at the end.
Having found the cause of light, we will commence a study of its
qualities and powers.
Light is the astronomer's necessity. When the sublime word was
uttered, "Let there be light!" the study of astronomy was made
possible. Man can gather but little of it with his eye; so he takes
a lens twenty-six inches in diameter, and bends all the light that
passes through it to a focus, then magnifies the image and takes
it into his eye. Or he takes a mirror, six feet in diameter, so
hollowed in the middle as to reflect all the rays falling upon it
to one point, and makes this larger eye fill his own with light.
By this larger light-gathering he discerns things for which the
light falling on his pupil one-fifth of an inch in diameter would
not be sufficient. We never have seen any sun or stars; we have
only seen the light that left them fifty minutes or years ago, more
or less. Light is the aërial sprite that carries our measuring-rods
across the infinite [Page 22] spaces; light spreads out the history
of that far-off beginning; brings us the measure of stars a thousand
times brighter than our sun; takes up into itself evidences of the
very constitutional elements of the far-off suns, and spreads them
at our feet. It is of such capacity that the Divine nature, looking
for an expression of its own omnipotence, omniscience, and power of
revelation, was content to say, "God is Light." We shall need all
our delicacy of analysis and measurement when we seek to determine
the activities of matter so fine and near to spirit as light.
[Illustration: Fig. 4.--Velocity of Light measured by Eclipses of
Jupiter's Moons.]
We first seek the velocity of light. In Fig. 4 the earth is 92,500,000
miles from the sun at E; Jupiter is 480,000,000 miles from the sun
at J. It has four moons: the inner one goes around the central
body in forty-two hours, and is eclipsed at every revolution. The
light that went out from the sun to M ceases to be reflected back
to the earth by the intervention of the planet Jupiter. We know
to a second when these eclipses take place, and they can be seen
with a small telescope. But when the earth is on the opposite side
of the sun [Page 23] from Jupiter, at E', these eclipses at J' take
place sixteen and a half minutes too late. What is the reason? Is
the celestial chronometry getting deranged? No, indeed; these great
worlds swing never an inch out of place, nor a second out of time.
By going to the other side of the sun the earth is 184,000,000 miles
farther from Jupiter, and the light that brings the intelligence of
that eclipse consumes the extra time in going over the extra
distance. Divide one by the other and we get the velocity, 185,000
miles per second. That is probably correct to within a thousand
miles. Methods of measurement by the toothed wheel of Fizeau confirm
this result. Suppose the wheel, Fig. 5, to have one thousand teeth,
making five revolutions to the second. Five thousand flashes of
light each second will dart out. Let each flash travel nine miles to
a mirror and return. If it goes that distance in 1/10000 of a
second, or at the rate of 180,000 miles a second, the next tooth
will have arrived before the eye, and each returning ray be cut off.
Hasten the revolutions a little, and the next notch will then admit
the ray, on its return, that went out of each previous notch: the
eighteen miles having been traversed meanwhile. The method of
measuring by means of a revolving mirror, used by Faucault, is held
to be even more accurate.
[Illustration: Fig. 5.--Measuring the Velocity of Light.]
When we take instantaneous photographs by the exposure [Page 24] of
the sensitive plate 1/20000 part of a second, a stream of light nine
miles long dashes in upon the plate in that very brief period of
time.
The highest velocity we can give a rifle-ball is 2000 feet a second,
the next second it is only 1500 feet, and soon it comes to rest.
We cannot compact force enough behind a bit of lead to keep it
flying. But light flies unweariedly and without diminution of speed.
When it has come from the sun in eight minutes, Alpha Centauri
in three years, Polaris in forty-five years, other stars in one
thousand, its wings are in nowise fatigued, nor is the rapidity
of its flight slackened in the least.
It is not the transactions of to-day that we read in the heavens,
but it is history, some of it older than the time of Adam. Those
stars may have been smitten out of existence decades of centuries
ago, but their poured-out light is yet flooding the heavens.
It goes both ways at once in the same place, without interference.
We see the light reflected from the new moon to the earth; reflected
back from the house-tops, fields, and waters of earth, to the moon
again, and from the moon to us once more--three times in opposite
directions, in the same place, without interference, and thus we
see "the old moon in the arms of the new."
_Constitution of Light._
[Illustration: 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 glass 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 passage 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 God:
"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 God 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 assemblage 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 passing through vapor of sodium has the vibrations that would
fall on two narrow lines in the yellow utterly destroyed, leaving
two black spaces. Light passing 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 constituent 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 constituent 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 glass 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 grass 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, accumulated 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 analyses 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 glass
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 glass
lens stops all the heat that can affect glass. 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 glass 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 glass 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 illustrations 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 associated 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 passes through glass without hinderance;
but take heat from white-hot platinum and only seventy-six per cent.
of it goes through glass, twenty-four per cent. being so constituted
that it cannot pass with facility. Of heat from copper at 752°
only six per cent. can go through glass, the other ninety-four per
cent. being absorbed by it.
The heat of the sun beam goes through glass without [Page 34] any
hinderance whatever. It streams into the room as freely as if there
were no glass there. But what if the furnace or stove heat went
through glass 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 spaces 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
aërial fingers, lengthens the side affected, and bends the whole
great mass 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 constitutionally 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 potassium 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
constitutional 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 spaces 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 God'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 shore 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.
[Illustration: Fig. 7.]
[Illustration: 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.
[Illustration: 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 glass
coated on one side with black varnish, and you can send your ray
anywhere. By using two mirrors, or having an assistant 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 glass 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.
Set a small light near one edge of a mirror; then, by putting the
eye near the opposite edge, you see almost as many flames as you
please from the multiplied reflections. How can this be accounted
for?
Into your beam of sunlight, admitted through a half-inch hole,
put the mirror at an oblique angle; you can arrange it so as to
throw half a dozen bright spots on the opposite wall.
[Illustration: Fig. 10.--Manifold Reflections.]
In Fig. 10 the sunbeam enters at A, and, striking the mirror _m_
at _a_, is partly reflected to 1 on the wall, and partly enters
the glass, passes through to the silvered back at B, and is totally
reflected to _b_, where it again divides, some of it going to the
wall at 2, and the rest, continuing to make the same reflections
and divisions, causes spots 3, 4, 5, etc. The brightest spot is
at No.2, because the silvered glass at B is the best reflector
and has the most light.
When the discovery of the moons of Mars was announced in 1877,
it was also widely published that they could be seen by a mirror.
Of course this is impossible. The point of light mistaken for the
moon in this secondary reflection was caused by holding the mirror
in an oblique position.
Take a small piece of mirror, say an inch in surface, and putting
under it three little pellets of wax, putty, or clay, set it on
the wrist, with one of the pellets on the pulse. Hold the mirror
steadily in the beam of light, and the frequency and prominence of
each pulse-beat will be indicated by the tossing spot of light on
the wall. If the operator becomes excited the fact will be evident
to all observers.
[Illustration: Fig. 11.]
Place a coin in a basin (Fig. 11), and set it so that the rim will
conceal the coin from the eye. Pour in water, and the coin will
[Page 40] appear to rise into sight. When light passes from a medium
of one density to a medium of another, its direction is changed.
Thus a stick in water seems bent. Ships below the horizon are
sometimes seen above, because of the different density of the layers
of air.
Thus light coming from the interstellar spaces, and entering our
atmosphere, is bent down more and more by its increasing density.
The effect is greatest when the sun or star is near the horizon,
none at all in the zenith. This brings the object into view before
it is risen. Allowance for this displacement is made in all delicate
astronomical observations.
[Illustration: Fig. 12.--Atmospherical Refraction.]
Notice on the floor the shadow of the window-frames. The glass
of almost every window is so bent as to turn the sunlight aside
enough to obliterate some of the shadows or increase their thickness.