_LIGHT._
"What soul was his, when, from the naked top
Of some bold headland, he beheld the sun
Rise up and bathe the world in light! He looked--
Ocean and earth, the solid frame of earth
And ocean's liquid mass, beneath him lay
In gladness and deep joy."
We live in a mighty ocean whose waves are ever rushing hither and
thither, always according to law, with velocity inconceivable, almost
immeasurable. These waves lave the shore of that island of space which
is our home, travelling to it from remotest regions, and making known to
us all that we know of what lies outside our small abode. We call these
waves, or rather their effects, by the name of Light. We recognise in
light--
"offspring of Heav'n's first-born
And of th' Eternal co-eternal beam"--
the antecedent of all else that exists in the universe; or, as Sir John
Herschel said, "the superior in point of rank and conception to all
other products or results of creative power in the physical world. It is
light which alone can give, and does give us, the assurance of a uniform
and all-pervading energy--a mechanism almost beyond conception, complex,
minute, and powerful, by which that influence, or rather that movement,
is propagated. Our evidence of the existence of gravitation fails us
beyond the region of the double stars, or leaves at best only a
presumption amounting to moral conviction in its favour. But the
argument for a unity of design and action afforded by light stands
unweakened by distance, and is co-extensive with the universe itself."
What, then, is light? What is that mysterious movement of some essence
pervading all space, whereby, from remotest depths, news is brought to
us, after journeys lasting many years, though space is traversed at a
rate exceeding more than ten million times the velocity of the swiftest
express train?
Light is in reality the result of undulations in what is called the
ether of space, a perfectly transparent, almost perfectly elastic
medium, occupying not only void space, but flowing as freely through the
densest solids as the summer breeze flows through the forest trees. The
waves of light cannot in this way pass through solid or liquid, or even
aerial bodies, but either they are sooner or later brought to rest, or
else they are more or less gradually deflected; just as the waves which
traverse the ocean come to their end, or are deflected, when they meet
the shore or shallows near the shore.
All light, however, has its real origin, not in the ethereal ocean
itself, but in the movements of the minute particles of which all forms
of matter known to us are composed. A tiny atom, far too small to be
perceived with a microscope, even though one should be made ten thousand
times more powerful than any yet constructed, when set in rapid
vibration, raises minute waves in the ethereal ocean, just as a small
body, vibrating on the surface of a sheet of water, would generate waves
there. And as the water-waves would travel radially away from the place
of their birth, so do the light-waves generated by the vibrations of one
of the atoms composing a luminous body radiate forth in all directions
through the ethereal ocean until, encountering some obstacle, they are
sent (reduced in size) in a new direction.
In some luminous bodies there are atoms vibrating in many different
periods (all very small) so as to cause light-waves of many different
kinds to proceed from the body. In other cases the atoms all vibrate at
one rate, or at two or three or some definite number of rates, so that
only light-waves of certain kinds proceed from the body. But in all
cases these light-waves only cause us to see the body when they flow in
through the pupil of the eye, and falling upon the retina (or the
choroid membrane, or whatever part of the eye it may be which finally
receives the waves), convey to the optic nerve, and thence to the brain,
the information that such and such a body, so coloured, so shaped, so
moving, exists towards that direction from which the light-waves seem to
come. The body so _seen_, as we call it, may be the original source of
light, or may be a body on which light has been reflected to us.
It is in this way that we receive information from light-waves. It will
be conceived how minute they must be, how perfectly they must retain
their separate character, multitudinous though they are, in traversing
the ether (even when that ether is clogged by the gross matter of our
ordinary air), if we remember how through the tiny eye-pupil we often
receive light-waves telling us of all the details, all the varieties of
colour and brightness, all the movements in a rich landscape.
Even more startling are the thoughts suggested by a view of the starlit
heavens. From hundreds of suns at once the light-waves which have
traversed varying but all enormous distances pour in upon the small
circle of the eye-pupil, waves of many kinds coming in together from
each sun. The waves which thus reach the eye from one bright star have
been but a few years upon their journey; all that time they have been
traversing an ocean swept in every part by untold millions of other
waves, and yet they arrive as perfect in order and regularity as rollers
which have traversed a wide sea pour in upon a level shore. From another
star, as bright as the first, they have been years in travelling; from
some among the fainter stars, hundreds, perhaps thousands of years. Yet
still they flow on, each order of waves in perfect uniformity as when
they first left their parent sun.
But even this is not all. Among the waves which reach the eye many, nay,
most, are so small that ordinary vision cannot perceive their action.
Take, however, a telescope, and so gather them together as to intensify
this action, and they are rendered perceptible, just as the unnoticed
heaving of ocean becomes a manifest wave-motion when it reaches a
regularly narrowing inlet. Thus, from stars so remote that their light
has required thousands, or, even in some cases, perhaps, hundreds of
thousands of years in reaching us, the light-waves flow steadily in upon
us. So small are these waves, that the breadth of from forty to sixty
thousand of them would occupy but a single inch. Through every point in
space waves from all the hundred millions of stars are at all times
simultaneously rushing at the rate of one hundred and eighty-five
thousand miles in every second of time: yet they travel on altogether
undisturbed, and each tells its story as distinctly as though the ether
had conveyed no other message, and that message but for a short
distance.
It would be difficult to say which thought, considered in its real
significance, is more striking,--the thought of what is done for us by
light regarded as a terrestrial phenomenon, or the thought of what light
is doing, and has done, in presenting to us a view of the starlit
heavens.
When the sun rises in splendour above the eastern horizon, tinting the
sky with varied colours, lighting up the clouds which till then have
been but dark patches on the heavens, bringing out the colours of hill
and dale, rock and river, fields and woods, the heart gladdens at the
spectacle. A pleasing melancholy falls on us as the light fades away at
eventide, tint after tint vanishing, until at length the gloom of night
enshrouds all. The full splendour of mid-day, the chastened splendour of
a moonlit night, and the glory of the heavens when "all the stars shine,
and the shepherd gladdens in his heart," stir the soul in like manner;
and it might seem to many that to analyse these glories, to explain
their scientific meaning, would be to rob the mind of the pleasure it
had before found in such scenes. Many would be disposed to think that a
purer enjoyment is expressed by Augustine than any student of science
could find in the wonders of light, in those words in which he
expresses his sense of the loveliness of fair forms and brilliant
colours. "For light, queen of the colours," he says, "bathing all I can
look upon, from morning till evening, let me go where I will, will still
keep gliding by me in unnumbered guises, and soothe me whilst I am busy
at other things, and am thinking nothing of her." But the sensuous
pleasure afforded by light is enhanced, while a purer and higher
enjoyment is superadded, when the real meaning of the display is
understood. As the astronomer sees in the sun a more glorious object
than the sun of the poet, recognising in imagination not only the
visible splendour of that orb, but the mighty energy with which it is
swaying the motions of a scheme of circling worlds, the wondrous
activities at work throughout its entire frame, the inconceivable tumult
which must prevail in that seemingly silent globe, so the glories of
light, rightly understood, are far more impressive than as they appeal
simply to the senses.
[Illustration: Fig. 1.--Sunrise on the Righi.]
Consider, for instance, the real meaning of sunrise. The orb seemingly
rising above the horizon, but, in reality, at rest, is the source of all
the glory which is spreading over the fair face of earth. The atoms of
that remote body, vibrating with intensest activity, send forth in all
directions ethereal waves, and of these relatively but a very few, about
one in two thousand millions, fall upon our earth, producing the
phenomena of sunlight. They have been little more than eight minutes on
the road, but in that short time they have traversed more than
90,000,000 of miles. Were they to fall directly upon our earth, we
should see few of the splendours which attend the uprising of the sun.
The deep air clothing our earth receives the onward rushing waves, and
reflects them in all directions. To use Biot's simile, "The air is a
sort of brilliant veil, which multiplies and diversifies the sunlight by
an infinity of repercussions." Nor is the wonder of the scene, or its
effect in filling the mind with solemn and poetic thoughts,
diminished--on the contrary, it is enhanced--by the recollection that
the gradually growing glory of day is brought about by the slow turning
of the mighty earth,--
"that spinning sleeps
On her soft axle, as she paces even,
And bears us soft with the smooth air along."
But if this is true of a scene of terrestrial splendour, how much more
fully may it be said of the glories of the heavens? No poet, if unaware
of the real meaning of modern discoveries respecting the celestial
bodies, can be moved by the starlit depths as the astronomer is, at
least the astronomer whose study of science is not limited to mere
observation and calculation. Hundreds of bright points of light
sparkling, and sometimes varying strangely in colour, form, no doubt, a
beautiful scene; but the scene is not less beautiful, and certainly it
is far more impressive, when we remember that every one of these points
of light is a sun, mighty in attractive energy like ours, its whole
surface glowing with fiery heat, and every particle of its substance
constantly in motion, if not always in the fierce rush of cosmic
hurricanes, yet with the ceaseless vibrations which generate the
ethereal light-waves telling us of the star's existence.
There is one strange thought connected with the motion of light-waves
through the ether of space which has not, I think, received the
attention it deserves.
Every one knows that when we look at the heavens we do not see the
celestial bodies where they are, but where they _were_, and again, not
where they were at any one moment of time, but some where they were a
short time ago, others where they were very long ago. But it is not so
generally known, or remembered by those who do know it, that if light
were not so active as it is the result would be that utterly incorrect
pictures of the celestial depths would continually be presented to us.
As matters actually are no orb in space can appear very far from its
true place. We see the sun, for instance, at any moment, not where he
is, but where he was (or rather towards the direction in which he lay)
about eight minutes before. But as the real velocity of the earth, and
therefore the apparent velocity of the sun, amounts only to about
eighteen miles per second, the sun is only thrown about 9000 miles out
of his true position, which is but about the ninetieth part of his
diameter: so that we see the sun very nearly in his right place. Now it
might seem that a star whose light takes, say, twenty years in reaching
us, must be seen very far from its true place, supposing the star to be
travelling along very quickly; and, in one sense, this is true. If such
a star is moving at the rate of fifty miles per second, athwart the line
of sight, it will be out of place by so considerable a distance as
315,000,000,000 of miles. Yet the star will appear very nearly in its
true position, simply because, at the star's enormous distance from us,
even the great distance just named is reduced to a very small apparent
amount. Such a star would, in fact, be displaced by only about the
thirtieth part of the sun's or moon's apparent diameters, or by about a
fifteenth part of the distance separating the middle star of the Great
Bear's tail from its small companion, sometimes called Jack by the
Middle Horse. Thus the stellar heavens present very truly to us the
positions of the stars; for such athwart motion as I have just imagined
would be very much larger than the motion of far the greater number of
the stars. But we only thus see the heavens truly pictured because of
the enormous velocity with which light travels. If light swept along
only at the rate of a hundred miles in a second (a velocity still far
beyond our powers of conception), there would be no believing what we
should see, for every star, and our own sun, and all the planets, and
even our own companion planet, the moon, would be thrown in appearance
very far from their true positions. If they were all shifted in position
by the same amount and in the same direction the picture would still be
true, in a sense, just as we see a true picture of an object at the
bottom of a clear lake, though the picture is displaced by the
refractive action of the water on the rays of light. But, in the
imagined case, the sun, and moon, and planets, and stars would be
shifted by different amounts and in different ways, simply because they
are moving at different rates and in different directions. The scene
presented to us would have been utterly untrue. Astronomy as a science
could probably have had no existence in such a case. Assuredly it could
have had no existence until students of the heavenly bodies had learned
to accept as the first axiom of their science the doctrine that "Seeing
is not believing."
[Illustration: Fig 2.--Sunset at Sea.]
A strange thought truly, that so active are the orbs peopling space, so
swiftly do they rush onwards upon their orbits, that light, carrying its
message at a rate exceeding six thousand times the velocity of the
swiftest express train, would be utterly unable to give a true account
of the position and movements of the celestial bodies. Fortunately light
gives a true record, because the qualities of the cosmic ether are such
that the message of light is transmitted hundreds of times more swiftly
than the swiftest bodies in the universe travel onwards upon their
orbits around each other or in space.
[Illustration]