"The eyes of the Lord are in every place."--_Proverbs_ xv. 3.
[Page 42]
"Man, having one kind of an eye given him by his Maker, proceeds
to construct two other kinds. He makes one that magnifies invisible
objects thousands of times, so that a dull razor-edge appears as
thick as three fingers, until the amazing beauty of color and form
in infinitesimal objects is entrancingly apparent, and he knows that
God's care of least things is infinite. Then he makes the other kind
four or six feet in diameter, and penetrates the immensities of space
thousands of times beyond where his natural eye can pierce, until he
sees that God's immensities of worlds are infinite also."--BISHOP
FOSTER.
[Page 43]
III.
_THE TELESCOPE._
Frequent allusion has been made in the previous chapter to discovered
results. It is necessary to understand more clearly the process by
which such results have been obtained. Some astronomical instruments
are of the simplest character, some most delicate and complex.
When a man smokes a piece of glass, in order to see an eclipse
of the sun, he makes a simple instrument. Ferguson, lying on his
back and slipping beads on a string at a certain distance above
his eye, measured the relative distances of the stars. The use
of more complex instruments commenced when Galileo applied the
telescope to the heavens. He cannot be said to have invented the
telescope, but he certainly constructed his own without a pattern,
and used it to good purpose. It consists of a lens, O B (Fig. 13),
which acts as a multiple prism to bend all the rays to one point
at R. Place the eye there, and it receives as much light as if it
were as large as the lens O B. The rays, however, are convergent,
and the point difficult to [Page 44] find. Hence there is placed at
R a concave lens, passing through which the rays emerge in parallel
lines, and are received by the eye. Opera-glasses are made upon
precisely this principle to-day, because they can be made
conveniently short.
[Illustration: Fig. 13.--Refracting Telescope.]
If, instead of a concave lens at R, converting the converging rays
into parallel ones, we place a convex or magnifying lens, the minute
image is enlarged as much as an object seems diminished when the
telescope is reversed. This is the grand principle of the refracting
telescope. Difficulties innumerable arise as we attempt to enlarge
the instruments. These have been overcome, one after another, until
it is now felt that the best modern telescope, with an object lens
of twenty-six inches, has fully reached the limit of optical power.
_The Reflecting Telescope_.
This is the only kind of instrument differing radically from the
refracting one already described. It receives the light in a concave
mirror, M (Fig. 14), which reflects it to the focus F, producing the
same result as the lens of the refracting telescope. Here a mirror
may be placed obliquely, reflecting the image at right angles to the
eye, outside the tube, in which case it is called the Newtonian
telescope; or a mirror at R may be placed perpendicularly, and send
the rays through [Page 45] an opening in the mirror at M. This form
is called the Gregorian telescope. Or the mirror M may be slightly
inclined to the coming rays, so as to bring the point F entirely
outside the tube, in which case it is called the Herschelian
telescope. In either case the image may be magnified, as in the
refracting telescope.
[Illustration: Fig. 14.--Reflecting Telescope.]
Reflecting telescopes are made of all sizes, up to the Cyclopean
eye of the one constructed by Lord Rosse, which is six feet in
diameter. The form of instrument to be preferred depends on the
use to which it is to be put. The loss of light in passing through
glass lenses is about two-tenths. The loss by reflection is often
one-half. In view of this peculiarity and many others, it is held
that a twenty-six-inch refractor is fully equal to any six-foot
reflector.
The mounting of large telescopes demands the highest engineering
ability. The whole instrument, with its vast weight of a twenty-six-inch
glass lens, with its accompanying tube and appurtenances, must be
pointed as nicely as a rifle, and held as steadily as the axis
of the globe. To give it the required steadiness, the foundation
on which it is placed is sunk deep in the earth, far from rail or
other roads, and no part of the observatory is allowed to touch
this support. When a star is once found, the earth swiftly rotates
the telescope away from it, and it passes out of the field. To
avoid this, clock-work is so arranged that the great telescope
follows the star by the hour, if required. It will take a star at
its eastern rising, and hold it constantly in view while it climbs
to the meridian and sinks in the west (Fig. 15). The reflector
demands still more difficult engineering. That of Lord Rosse has
a metallic mirror [Page 46] weighing six tons, a tube forty feet
long, which, with its appurtenances, weighs seven tons more. It
moves between two walls only 10° east and west. The new Paris
reflector (Fig. 16) has a much wider range of movement.
[Illustration: Fig. 15.--Cambridge Equatorial.]
[Illustration: Fig. 16.--New Paris Reflector.]
_The Spectroscope._
A spectrum is a collection of the colors which are dispersed by
a prism from any given light. If it is sunlight, it is a solar
spectrum; if the source of light is a [Page 49] star, candle,
glowing metal, or gas, it is the spectrum of a star, candle, glowing
metal, or gas. An instrument to see these spectra is called a
spectroscope. Considering the infinite variety of light, and its
easy modification and absorption, we should expect an immense number
of spectra. A mere prism disperses the light so imperfectly that
different orders of vibrations, perceived as colors, are mingled. No
eye can tell where one commences or ends. Such a spectrum is said to
be impure. What we want is that each point in the spectrum should be
made of rays of the same number of vibrations. As we can let only a
small beam of light pass through the prism, in studying celestial
objects with a telescope and spectroscope we must, in every
instance, contract the aperture of the instrument until we get only
a small beam of light. In order to have the colors thoroughly
dispersed, the best instruments pass the beam of light through a
series of prisms called a battery, each one spreading farther the
colors which the previous ones had spread. In Fig. 17 the ray is
seen entering through the telescope A, which renders the rays
parallel, and passing [Page 50] through the prisms out to telescope
B, where the spectrum can be examined on the retina of the eye for a
screen. In order to still farther disperse the rays, some batteries
receive the ray from the last prism at O upon an oblique mirror,
send it up a little to another, which delivers it again to the prism
to make its journey back again through them all, and come out to be
examined just above where it entered the first prism.
[Illustration: Fig. 17.--Spectroscope, with Battery of Prisms.]
Attached to the examining telescope is a diamond-ruled scale of glass,
enabling us to fix the position of any line with great exactness.
[Illustration: Fig. 18.--Spectra of glowing Hydrogen and the Sun.]
In Fig. 18 is seen, in the lower part, a spectrum of the sun, with
about a score of its thousands of lines made evident. In the upper
part is seen the spectrum of bright lines given by glowing hydrogen
gas. These lines are given by no other known gas; they are its
autograph. It is readily observed that they precisely correspond
with certain dark lines in the solar spectrum. Hence we easily
know that a glowing gas gives the same bright lines that it absorbs
from the light of another source passing through it--that is, glowing
gas gives out the same rays of light that it absorbs when it is
not glowing.
The subject becomes clearer by a study of the chromolithic plate.
No. 1 represents the solar spectrum, with a few of its lines on an
accurately graduated scale. [Page 51] No.3 shows the bright line of
glowing sodium, and, corresponding to a dark line in the solar
spectrum, shows the presence of salt in that body. No. 2 shows that
potassium has some violet rays, but not all; and there being no dark
line to correspond in the solar spectrum, we infer its absence from
the sun. No.6 shows the numerous lines and bands of barium--several
red, orange, yellow, and four are very bright green ones. The lines
given by any volatilized substances are always in the same place on
the scale.
A patient study of these signs of substances reveals, richer results
than a study of the cuniform characters engraved on Assyrian slabs;
for one is the handwriting of men, the other the handwriting of
God.
One of the most difficult and delicate problems solved by the
spectroscope is the approach or departure of a light-giving body
in the line of sight. Stand before a locomotive a mile away, you
cannot tell whether it approaches or recedes, yet it will dash by
in a minute. How can the movements of the stars be comprehended
when they are at such an immeasurable distance?
It can best be illustrated by music. The note C of the G clef is
made by two hundred and fifty-seven vibrations of air per second.
Twice as many vibrations per second would give us the note C an octave
above. Sound travels at the rate of three hundred and sixty-four
yards per second. If the source of these two hundred and fifty-seven
vibrations could approach us at three hundred and sixty-four yards
per second, it is obvious that twice as many waves would be put
into a given space, and we should hear the upper C when only waves
enough were made for the lower C. The same [Page 52] result would
appear if we carried our ear toward the sound fast enough to take up
twice as many valves as though we stood still. This is apparent to
every observer in a railway train. The whistle of an approaching
locomotive gives one tone; it passes, and we instantly detect
another. Let two trains, running at a speed of thirty-six yards a
second, approach each other. Let the whistle of one sound the note
E, three hundred and twenty-three vibrations per second. It will be
heard on the other as the note G, three hundred and eighty-eight
vibrations per second; for the speed of each train crowds the
vibrations into one-tenth less room, adding 32+ vibrations per
second, making three hundred and eighty-eight in all. The trains
pass. The vibrations are put into one-tenth more space by the
whistle making them, and the other train allows only nine-tenths of
what there are to overtake the ear. Each subtracts 32+ vibrations
from three hundred and twenty-three, leaving only two hundred and
fifty-eight, which is the note C. Yet the note E was constantly
uttered.
[Illustration: 1. Solar Spectrum. 2. Spectrum of Potassium. 3.
Spectrum of Sodium. 4. Spectrum of Strontium. 5. Spectrum of Calcium.
6. Spectrum of Barium.]
If a source of light approach or depart, it will have a similar
effect on the light waves. How shall we detect it? If a star approach
us, it puts a greater number of waves into an inch, and shortens their
length. If it recedes, it increases the length of the wave--puts
a less number into an inch. If a body giving only the number of
vibrations we call green were to approach sufficiently fast, it
would crowd in vibrations enough to appear what we call blue, indigo,
or even violet, according to its speed. If it receded sufficiently
fast, it would leave behind it only vibrations enough to fill up
[Page 53] the space with what we call yellow, orange, or red,
according to its speed; yet it would be green, and green only, all
the time. But how detect the change? If red waves are shortened they
become orange in color; and from below the red other rays, too far
apart to be seen by the eye, being shortened, become visible as red,
and we cannot know that anything has taken place. So, if a star
recedes fast enough, violet vibrations being lengthened become
indigo; and from above the violet other rays, too short to be seen,
become lengthened into visible violet, and we can detect no movement
of the colors. The dark lines of the spectrum are the cutting out of
rays of definite wave-lengths. If the color spectrum moves away,
they move with it, and away from their proper place in the ordinary
spectrum. If, then, we find them toward the red end, the star is
receding; if toward the violet end, it is approaching. Turn the
instrument on the centre of the sun. The dark lines take their
appropriate place, and are recognized on the ruled scale. Turn it on
one edge, that is approaching us one and a quarter miles a second by
the revolution of the sun on its axis, the spectral lines move
toward the violet end; turn the spectroscope toward the other edge
of the sun, it is receding from us one and a quarter miles a second
by reason of the axial revolution, and the spectral lines move
toward the red end. Turn it near the spots, and it reveals the
mighty up-rush in one place and the down-rush in another of one
hundred miles a second. We speak of it as an easy matter, but it is
a problem of the greatest delicacy, almost defying the mind of man
to read the movements of matter.
It should be recognized that Professor Young, of [Page 54]
Princeton, is the most successful operator in this recent realm of
science. He already proposes to correct the former estimate of the
sun's axial revolutions, derived from observing its spots, by the
surer process of observing accelerated and retarded light.
Within a very few years this wonderful instrument, the spectroscope,
has made amazing discoveries. In chemistry it reveals substances
never known before; in analysis it is delicate to the detection of
the millionth of a grain. It is the most deft handmaid of chemistry,
the arts, of medical science, and astronomy. It tells the chemical
constitution of the sun, the movements taking place, the nature of
comets, and nebulæ. By the spectroscope we know that the atmospheres
of Venus and Mars are like our own; that those of Jupiter and Saturn
are very unlike; it tells us which stars approach and which recede,
and just how one star differeth from another in glory and substance.
In the near future we shall have the brilliant and diversely colored
flowers of the sky as well classified into orders and species as
are the flowers of the earth.
[Page 55]
IV.