Young Folks' Library, Volume XI (of 20) by Various

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The name of diffraction is given to all the modifications which the
luminous rays undergo when they come in contact with the surface of
bodies. Light, under these circumstances, is subject to a sort of
deviation, at the same time becoming decomposed, whence result those
curious appearances in the shadows of objects which were observed for
the first time by Grimaldi and Newton.

The most interesting phenomena of diffraction are those presented by
_gratings_, as are technically denominated the systems of linear and
very narrow openings situated parallel to one another and at very
small intervals. A system of this kind may be realized by tracing with
a diamond, for instance, on a pane of glass equidistant lines very
close together. As the light would be able to pass in the interstices
between the strokes, whereas it would be stopped in the points
corresponding to those where the glass was not smooth, there is, in
reality, an effect produced as if there were a series of openings very
near to each other. A hundred strokes, about 1/25th of an inch in
length, may thus be drawn without difficulty. The light is then
decomposed in spectra, each overlapping the other. It is a phenomenon
of this kind which is seen when we look into the light with the eye
half closed; the eyelashes in this case, acting as a net-work or
grating. These net-works may also be produced by reflection, and it is
to this circumstance that are due the brilliant colors observed when a
pencil of luminous rays is reflected on a metallic surface regularly
striated.

To the phenomena of gratings must be attributed, too, the colors,
often so brilliant, to be seen in mother-of-pearl. This substance is
of a laminated structure; so much so, that in carving it the different
folds are often cut in such a way as to form a regular net-work upon
the surface. It is, again, to a phenomenon of this sort that are due
the rainbow hues seen in the feathers of certain birds, and sometimes
in spiders' webs. The latter, although very fine, are not simple, for
they are composed of a large number of pieces joined together by a
viscous substance, and thus constitute a kind of net-work.

If the sun is near the horizon, and the shadow of the observer falls
upon the grass, upon a field of corn, or other surface covered with
dew, there is visible an aureola, the light of which is especially
bright about the head, but which diminishes from below the middle of
the body. This light is due to the reflection of light by the moist
stubble and the drops of dew. It is brighter about the head, because
the blades that are near where the shadow of the head falls expose to
it all that part of them which is lighted up, whereas those farther
off expose not only the part which is lighted up, but other parts
which are not, and this diminishes the brightness in proportion as
their distance from the head increases. The phenomenon is seen
whenever there is simultaneously mist and sun. This fact is easily
verified upon a mountain. As soon as the shadow of a mountaineer is
projected upon a mist, his head gives rise to a shadow surrounded by a
luminous aureola.

[Illustration: FOG-BOW SEEN FROM THE MATTERHORN.]

_The Illustrated London News_ of July 8, 1871, illustrates one of
these apparitions, "The Fog-Bow, seen from the Matterhorn," observed
by E. Whymper in this celebrated region of the Alps. The observation
was taken just after the catastrophe of July 14, 1865; and by a
curious coincidence, two immense white aerial crosses projected into
the interior of the external arc. These two crosses were no doubt
formed by the intersection of circles, the remaining parts of which
were invisible. The apparition was of a grand and solemn character,
further increased by the silence of the fathomless abyss into which
the four ill-fated tourists had just been precipitated.

[Illustration]

Other optical appearances of an analogous kind are manifested under
different conditions. Thus, for instance, if any one, turning his back
to the sun, looks into water, he will perceive the shadow of his head,
but always very much deformed. At the same time he will see starting
from this very shadow what seem to be luminous bodies, which dart
their rays in all directions with inconceivable rapidity, and to a
great distance. These luminous appearances--these aureola rays--have,
in addition to the darting movement, a rapid rotary movement around
the head.

[Illustration]

THE PLANET VENUS

BY AGNES M. CLERKE.

I.

HESPERUS AND PHOSPHOR.

[Illustration]

The radiant planet that hangs on the skirts of dusk and dawn

"like a jewel in an Ethiop's ear,"

has been known and sung by poets in all ages. Its supremacy over the
remainder of the starry host is recognized in the name given it by the
Arabs, those nomad watchers of the skies, for while they term the moon
"El Azhar," "the Brighter One," and the sun and moon together "El
Azharan," "the Brighter Pair," they call Venus "Ez Zahra," the bright
or shining one _par excellence_, in which sense the same word is used
to describe a flower. This "Flower of Night" is supposed to be no
other than the white rose into which Adonis was changed by Venus in
the fable which is the basis of all early Asiatic mythology. The
morning and evening star is thus the celestial symbol of that union
between earth and heaven in the vivifying processes of nature,
typified in the love of the goddess for a mortal.

The ancient Greeks, on the other hand, not unnaturally took the star,
which they saw alternately emerging from the effulgence of the rising
and setting sun, in the east and in the west, for two distinct bodies,
and named it differently according to the time of its appearance. The
evening star they called Hesperus, and from its place on the western
horizon, fabled an earthly hero of that name, the son of Atlas, who
from the slopes of that mountain on the verge of the known world used
to observe the stars until eventually carried off by a mighty wind,
and so translated to the skies. These divine honors were earned by his
piety, wisdom, and justice as a ruler of men, and his name long shed a
shimmering glory over those Hesperidean regions of the earth, where
the real and unreal touched hands in the mystical twilight of the
unknown.

But the morning star shone with a different significance as the herald
of the day, the torchbearer who lights the way for radiant Aurora on
her triumphal progress through the skies. Hence he was called
Eosphorus, or Phosphorus, the bearer of the dawn, translated into
Latin as Lucifer, the Light-bearer. The son of Eos, or Aurora, and the
Titan Astraeus, he was of the same parentage as the other multitude of
the starry host, to whom a similar origin was ascribed, and from whom
in Greek mythology he was evidently believed to differ only in the
superior order of his brightness. Homer, who mentions the planet in
the following passage:

"But when the star of Lucifer appeared,
The harbinger of light, whom following close,
Spreads o'er the sea the saffron-robed morn."

(LORD DERBY'S "Iliad.")

recognizes no distinction between those celestial nomads, the planets,
"wandering stars," as the Arabs call them, which visibly change their
position relatively to the other stars, and the latter, whose places
on the sphere are apparently fixed and immutable. In this he and his
compatriots were far behind the ancient Egyptians, who probably
derived their knowledge from still earlier speculators in Asia, for
they not only observed the movements of some at least of the planets,
but believed that Mercury and Venus revolved as satellites round the
sun, which in its turn circled round our lesser world. Pythagoras is
said to have been the first to identify Hesperus with Phosphor, as the

"Silver planet both of eve and morn,"

and by Plato the same fact is recognized. The other planets, all of
which had, according to him, been originally named in Egypt and Syria,
have each its descriptive title in his nomenclature. Thus the
innermost, "the Star of Mercury," is called Stilbon, "the Sparkler,"
Mars, Pyroeis, "the Fiery One," while Jupiter, the planet of the
slowest course but one, is designated as Phaeton, and Saturn, the
tardiest of all, Phaenon. These names were in later times abandoned in
favor of those of the divinities to whom they were respectively
dedicated, unalterably associated now with the days of the week, over
which they have been selected to preside.

The Copernican theory, which once and forever "brushed the cobwebs out
of the sky," by clearing away the mists of pre-existing error, first
completely explained the varying positions of the Shepherd's star,
irradiating the first or last watch of night, according to her
alternate function as the follower or precursor of the sun. As she
travels on a path nearer to him by more than twenty-five and a half
million miles than that of the earth, she is seen by us on each side
of him in turn after passing behind or in front of him. The points at
which her orbit expands most widely to our eyes--an effect of course
entirely due to perspective, as her distance from the sun is not then
actually increased--are called her eastern and western elongations;
that at which she passes by the sun on the hither side her inferior,
and on the farther side her superior conjunction. At both conjunctions
she is lost to our view, since she accompanies the sun so closely as
to be lost in his beams, rising and setting at the same time, and
travelling with him in his path through the heavens during the day.
When at inferior conjunction, or between us and the sun, she turns her
dark hemisphere to us like the new moon, and would consequently be
invisible in any case, but when in the opposite position, shows us her
illuminated face, and is literally a day star, invisible only because
effaced by the solar splendor. It is as she gradually separates from
him, after leaving this latter position, circling over that half of
her orbit which lies to the east of him, that she begins to come into
view as an evening star, following him at a greater and greater
distance, and consequently setting later, until she attains her
greatest eastern elongation, divided from the sun about 45 deg. of his
visible circuit through the heavens, and consequently remaining above
the horizon for some four hours after him. From this point she again
appears to draw nearer to him until she passes on his hither side in
inferior conjunction, from which she emerges on the opposite side to
the westward, and begins to shine as a morning star, preceding him on
his track, at a gradually increasing distance, until attaining her
greatest westward elongation, and finally completing her cycle by
returning to superior conjunction once more in a period of about five
hundred and eighty-four days.

Venus is thus Hesperus or Vesper, the evening star, when following the
sun as she passes from beyond him in superior conjunction to inferior
conjunction where she is nearest to the earth. As she again leaves him
behind in her course from this point to the opposite one of superior
conjunction, she appears in her second aspect as Phosphorus or
Lucifer, "the sun of morning," and herald of the day, shining as

"The fair star
That gems the glittering coronet of morn."

II.

THE PHASES OF VENUS.

But the changes in the aspect of Venus due to her varying positions in
her orbit are not confined to those which cause her to oscillate with
a pendulum movement eastward and westward from the sun. The discovery
that she undergoes phases exactly like those of the moon, followed
that of the existence of Jupiter's satellites as the second great
result achieved by the use of the telescope in the hands of Galileo.
The fact that the planets were intrinsically dark bodies revolving
round the sun, and reflecting its light, as he and Copernicus had
maintained, thus received a further ocular demonstration. The
Florentine astronomer describes in a letter to a friend how the
planet, after emerging from superior conjunction as a morning star,
gradually loses her rotundity on the side remote from the luminary,
changing first to a half sphere and then to a waning crescent; until,
after passing through the stage of absolute extinction when
intervening between us and the sun, she re-appears as a morning star,
and undergoes the same series of transformations in inverse order. The
revelation was indeed so novel and unexpected, that when the slight
deformation of the planet's shape was first detected by him, he did
not venture to announce it in plain terms but veiled it, according to
the prevailing fashion of the time, under a Latin anagram. His
celebrated sentence--

"Haec immatura a me jam frustra leguntur."

("Those incomplete observations are as yet read by me in vain.")

forms, by transposing the letters, the more definite statement,

"Cynthiae figuras aemulatur Mater Amorum."

("The mother of the loves imitates the aspects of Diana.")

that is to say, Venus vies with the phases of the moon. The discovery
was an important one from its bearing on popular superstition ascribing
to the planets special influences on human affairs, for since they were
thus shown to transmit to us only borrowed light, belief in their
beneficent or malefic powers over man's destinies received a rude shock.

[Illustration: THE PHASES OP VENUS.]

Galileo's announcement, published in September, 1610, when only a
slight flattening of the planet's disk was visible, received absolute
confirmation in the ensuing months, as she completed her full
half-circle of change on February 24th of the following year, and
consequently exhibited herself to him in all her varying aspects. It
was the first time they had been looked upon by a human eye, since its
unaided powers do not enable it to discern them, although one
exception to this rule is said to have existed. This was the case of
the Swiss mathematician Gauss, who, when a child, on being shown the
crescent star through the telescope, exclaimed to his mother that it
"was turned wrong"; the inference being that he recognized the
reversal of the image in the field of the glass. If it were indeed so,
he deserves to rank with the Siberian savage, who described the
eclipses, or Jupiter's satellites; or the shoemaker of Breslau, who
could see and declare the positions of those minute orbs.

The phases exhibited to us by Venus are due to her moving in an orbit
within that of the earth, at one side of which she is between us and
the sun, while at the other this position is exactly reversed. We may
compare her to a performer in a great celestial circus, lit by a
central chandelier, and ourselves to spectators in an external ring,
from which we see her at one time facing us with the light full on
her, at the opposite point in complete shadow, and at the intermediate
ones in varying degrees of illumination according to our changing
views of her. The same illustration may serve to show why Venus is
brightest, not when full, since she is then beyond the sun, and at the
farthest possible point from us, but when she approaches us at
inferior conjunction, more nearly by over one hundred and thirty
million miles, and still shows us a crescent of her illuminated
surface, before passing into the last phase of total obscuration. When
actually nearest to us she is absolutely invisible, being then, like
the new moon, between us and the sun. Her varying degrees of
brilliancy, even when in the same phase, are thus accounted for by her
alternate retreat from and advance towards us as she circles round the
sun. Completing, as she does, her revolution in about seven months and
a half, she would of course go through the whole series of her
metamorphoses in that time, were the earth, from which we observe
them, a fixed point. Their protraction instead, over a term of five
hundred and eighty-four days, or more than nineteen months, is due to
the simultaneous motion of the earth in the same direction, over her
larger orbit in a longer period, causing the same relative position of
the sister planet to recur only as often as she overtakes her in her
career. Thus the hour and minute hands of a watch, moving at different
rates of speed after meeting on the dial plate at twelve o'clock, will
not again come together until five minutes past one, when the swifter
paced of the two will have completed a revolution and a twelfth. But
were we to retard the motion of the latter, reducing it to only twice
that of its companion, they would always meet at the figure twelve, as
it would exactly complete two circuits while the hour hand was
performing one. Venus thus overtakes and passes the earth once in five
hundred and eighty-four days, or nearly two and a half of her own
years, constituting what is called her synodic period of apparent
revolution as seen from this globe. She thus presents to us all the
phases undergone by our own satellite during a lunar month, passing
from new to full, and _vice versa_, through the various intervening
gradations of form.

The phases of Venus are amongst the most beautiful subjects for
observation in a moderate telescope, as her silver bow, gradually
brightening in the evening dusk, or fading in the dawn,

"On a bed of daffodil sky,"

is, after the two greater luminaries that rule the day and night, the
most brilliant object in the heavens.

III.

THE SILVER CROWN.

The parallel between Venus and

"That orbed maiden with fire laden,
Whom mortals call the moon,"

is carried a stage further. Most of us are familiar with the spectacle
in which the Ancient Egyptians saw symbolized Horus on the lap of
Isis, but which we more prosaically term "the old moon in the new
moon's arms." The strongly illuminated half circle next the sun is
then seen embracing with its horns a dusky sphere, contrasting with it
as tarnished silver does with the newly burnished metal. The same
phenomenon is occasionally, though very rarely, exhibited by Venus,
while close to the sun at inferior injunction, when the shadowy form
of the full orb is seen to shine dimly within her crescent with what
is termed "the ashen light." More wonderful still, this "glimmering
sphere" is then crowned, as with a silver halo, by a thin luminous
arch, forming a secondary sickle facing the one nearest the sun, and
doubtless due to the refraction of his rays round the globe of the
planet, through the upper regions of her twilight atmosphere. This
spectacle was first observed by the Jesuit Ricciolo, an opponent of
the Copernican theory, on January 9th, 1643. He describes the planet
as ruddy near the sun, yellowish in the middle, and of greenish blue
on the side remote from the sun; while he also noted the bow of light
limiting the dark hemisphere. Scarcely daring to trust his own
eyesight, he ascribed these appearances, although he recorded them, to
illusory reflection in the telescope.

[Illustration: VENUS AT HER GREATEST BRILLIANCY.]

They were again seen, however, by Derham about 1715, and six years
later by Kirch, in Berlin, who has the following entry in his diary
for Saturday, June 29, 1721:--"I found Venus in a region where the sky
was not very clear. The planet was narrow, and I seemed to see its
dark side, though this is almost incredible. The diameter of Venus was
65", and its sickle seemed to tremble in the atmospheric vapors."
Again, on March 8th, 1726, he records a similar observation. "We
observed Venus with the twenty-six foot telescope. I perceived her
dark side, and its edge seemed to describe a smaller circle than that
of the light side, as is the case of the moon." This effect is due to
irradiation, that is to say, to the glare from a bright surface,
giving a deceptive enlargement to its apparent area. He again saw the
dark side of the planet in October, 1759, as did Harding at Goettingen,
with Herschel's ten-foot reflector, on January 24th, 1806. This latter
observer saw it on this occasion stand out against the background of
the sky as of a pale ashen green, while on February 28th following, it
seemed to him of a pale reddish gray, like the color of the eclipsed
moon.

That the latter body should send to us from her nocturnal shadows
sufficient light to be visible is easily explicable, since she is then
flooded with earth-light reflected on her from a surface thirteen and
one-half times greater than her own, and probably casting on her an
illumination transcending our full moonlight in the same proportion.
But the secondary light of Venus admits of no such explanation, since
earth-light on her surface, diminished by 1/12000th part compared to
what it is on that of the moon, would be quite insufficient to render
her visible to our eyes. The phenomenon was therefore adduced as an
argument for the habitability of the planets by Gruithuisen, of the
Munich Observatory, who, writing early in this century, suggested that
the ashen light of Venus might be due to general illuminations in
celebration by her inhabitants of some periodically recurring
festivity, The materials for a flare-up on so grand a scale would, he
thought, exist in abundance, as he conjectured the vegetation of our
planetary neighbor to be more luxuriant than that of our Brazilian
forests. The phosphorescence of the Aphroditean oceans, warm and
teeming with life, as they are held to be by Zollner, was advanced as
an explanatory hypothesis, with scarcely more plausibility, by
Professor Safarik, while others have resorted to the supposition of
atmospheric or electrical luminosity producing on a large scale some
such display as that of the aurora borealis.

Professor Vogel, of Berlin, who himself saw part of the night-side of
Venus, in its semi-obscurity in November, 1871, ascribed its
visibility to a twilight effect caused by a very extensive atmosphere.
The light thus transmitted to us by aerial diffusion and giving the
ashen light, is reflected sunlight, while that sent by the luminous
arc on its edge is direct sunlight, refracted, or bent round to us,
from behind the planet. The silver selvedge of the dawn edging the
dark limb may consequently be the brightest part of the broken nimbus
that then seems to surround her.

A similar appearance is observed during transits of Venus, when she
passes directly between us and the actual solar disk. A silver thread
is then seen encircling that side of the planet which has not yet
entered on the face of the sun or "a shadowy nebulous ring," as it was
described by Mr. Macdonnell at Eden, surrounds the whole planetary
disk when two-thirds of it have passed the solar edge. As it moves off
it, the same aureole again becomes visible, testifying to the
existence of an atmosphere of considerable extent exterior to the
sharply outlined surface ordinarily visible. The shimmering haze of
reflected sunlight which perpetually enfolds her is only made apparent
to us under exceptional circumstances which cut off some portion of
her more immediate light, just as we see the motes in the air
illuminated by a candle if we hide the actual flame from our eyes. The
perennial twilight which seems to reign over the nocturnal hemisphere
of Venus may compensate, perhaps, for the want of a satellite to
modify its darkness.

The great prolongation at other times of the horns of her crescent, so
as to embrace almost her entire circumference with a tenuous ring of
light, is doubtless due to the same cause, as their visibility should
otherwise be limited to a half segment of a circle. The regions thus
shining to us are obviously those on which the sun has not yet set,
his appearance above the horizon being prolonged, as in our own case,
by refraction, though to a much larger extent. The magnitude of the
sun's disk as seen from Venus, a third larger than it appears to us,
is also adducted by Mr. Proctor in his posthumous work, "The Old and
the New Astronomy," edited and completed by Mr. A.C. Ranyard, as an
element in extending the illumination of Venus to more than a
hemisphere of her surface. As his diameter there is 44-1/4 deg., a zone
of more than 22 deg. wide outside the sunward hemisphere is he thinks
illuminated by direct though partial sunlight, the orb being
throughout this tract still partially above the horizon.

[Illustration: GEOGRAPHICAL ASPECT OF VENUS.]

[Illustration]

THE STARS

(FROM STARLAND.)

BY SIR ROBERT S. BALL.

[Illustration]

The group of bodies which cluster around our sun forms a little
island, so to speak, in the extent of infinite space. We may
illustrate this by a map in which we shall endeavor to show the stars
placed at their proper relative distances. We first open the compasses
one inch, and thus draw a little circle to represent the path of the
earth. We are not going to put in all the planets. We take Neptune,
the outermost, at once. To draw its path I open the compasses to
thirty inches, and draw a circle with that radius. That will do for
our solar system, though the comets no doubt will roam beyond these
limits. To complete our map we ought of course to put in some stars.
There are a hundred million to choose from, and we shall begin with
the brightest. It is often called the Dog Star, but astronomers know
it better as Sirius. Let us see where it is to be placed on our map.
Sirius is beyond Neptune, so it must be outside somewhere. Indeed, it
is a good deal further off than Neptune; so I try at the edge of the
drawing-board; I have got a method of making a little calculation that
I do not intend to trouble you with, but I can assure you that the
results it leads me to are quite correct; they show me that this board
is not big enough. But could a board which was big enough fit into
this lecture theatre? Here, again, I make my little calculations, and
I find that there would not be room for a board sufficiently great; in
fact, if I put the sun here at one end, with its planets around it?
Sirius would be too near on the same scale if it were at the further
corner. The board would have to go out through the wall of the
theatre, out through London. Indeed, big as London is, it would not be
large enough to contain the drawing-board that I should require. It
would have to stretch about twenty miles from where we are now
assembled. We may therefore dismiss any hope of making a practical map
of our system on this scale if Sirius is to have its proper place. Let
us, then, take some other star. We shall naturally try with the
nearest of all. It is one that we do not know in this part of the
world, but those that live in the southern hemisphere are well
acquainted with it. The name of this star is Alpha Centauri. Even for
this star we should require a drawing three or four miles long if the
distance from the earth to the sun is to be taken as one inch. You see
what an isolated position our sun and his planets occupy. The members
of the family are all close together, and the nearest neighbors are
situated at enormous distances. There is a good reason for this
separation. The stars are very pretty and perfectly harmless to us
where they are at present situated. They might be very troublesome
neighbors if they were very much closer to our system. It is therefore
well they are so far off; they would be constantly making disturbances
in the sun's family if they were near at hand. Sometimes they would be
dragging us into unpleasantly great heat by bringing us too close to
the sun, or producing a coolness by pulling us away from the sun,
which would be quite as disagreeable.

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