THE TELESCOPE
Had anyone told Ptolemy that his earth-centered system of sun, moon, and
stars would ultimately be overthrown, not by philosophy but by the
overwhelming evidence furnished by a little optical instrument which so
aided the human eye that it could actually see systems of bodies in
revolution round each other in the sky, he would no doubt have
vehemently denied that any such thing was possible. To be sure, it took
fourteen centuries to bring this about, and the discovery even then was
without much doubt due to accident.
Through all this long period when astronomy may be said to have merely
existed, practically without any forward step or development, its
devotees were unequipped with the sort of instruments which were
requisite to make the advance possible. There were astrolabes and
armillary spheres, with crudely divided circles, and the excellent work
done with them only shows the genius of many of the early astronomers
who had nothing better to work with. Regarding star-places made with
instruments fixed in the meridian, Bessel, often called the father of
practical astronomy, used to say that, even if you provided a bad
observer with the best of instruments, a genius could surpass him with a
gun barrel and a cart wheel.
Before the days of telescopes, that is, prior to the seventeenth
century, it was not known whether any of the planets except the earth
had a moon or not; consequently the masses of these planets were but
very imperfectly ascertained; the phases of Mercury and Venus were
merely conjectured; what were the actual dimensions of the planets could
only be guessed at; the approximate distances of sun, moon, and planets
were little better than guesses; the distances of the stars were wildly
inaccurate; and the positions of the stars on the celestial sphere, and
of sun, moon, and planets among them were far removed from modern
standards of precision--all because the telescope had not yet become
available as an optical adjunct to increase the power of the human eye
and enable it to see as if distances were in considerable measure
annihilated.
Galileo almost universally is said to have been the inventor of the
telescope, but intimate research into the question would appear to give
the honor of that original invention to another, in another country.
What Galileo deserves the highest praise for, however, is the
reinvention independently of an "optick tube" by which he could bring
distant objects apparently much nearer to him; and being an astronomer,
he was by universal acknowledgment first of all men to turn a telescope
on the heavenly bodies. This was in the year 1609, and his first
discovery was the phase of Venus, his second the four Medicean moons or
satellites of Jupiter, discoveries which at that epoch were of the
highest significance in establishing the truth of the Copernican system
beyond the shadow of doubt.
But the first telescopes of which we have record were made, so far as
can now be ascertained, in Holland very early in the 17th century.
Metius, a professor of mathematics, and Jansen and Lipperhey, who were
opticians in Middelburg--all three are entitled to consideration as
claimants of the original invention of the telescope. But that such an
instrument was pretty well known would appear to be shown by his
government's refusal of a patent to Lipperhey in 1608; while the
officials recognizing the value of such an instrument for purposes of
war, got him to construct several telescopes and ordered him to keep the
invention a secret.
Within a year Galileo heard that an instrument was in use in Holland by
which it was possible to see distant objects as if near at hand. Skilled
in optics as he was, the reinvention was a task neither long nor
difficult for him. One of his first instruments magnified but three
times; still it made a great sensation in Venice where he exhibited the
little tube to the authorities of that city, in which he first invented
it.
Galileo's telescope was of the simplest type, with but two lenses; the
one a double convex lens with which an image of the distant object is
formed, the other a double concave lens, much smaller which was the
eye-lens for examining the image. It is this simple form of Galilean
telescope that is still used in opera glasses and field glasses, because
of the shorter tube necessary.
Galileo carried on the construction of telescopes, all the time
improving their quality and enlarging their power until he built one
that magnified thirty times. What the diameter of the object glass was
we do not know, perhaps two inches or possibly a little more. Glass of a
quality good enough to make a telescope of cannot have been abundant or
even obtainable except with great difficulty in those early days.
Other discoveries by this first of celestial observers were the spots on
the sun, the larger mountains of the moon, the separate stars of which
the Milky Way is composed, and, greatest wonder of all, the anomalous
"handles" (_ansae_, he called them) of Saturn, which we now know as the
planet's ring, the most wonderful of all the bodies in the sky.
Since Galileo's time, only three centuries past, the progress in size
and improvement in quality of the telescope have been marvelous. And
this advance would not have been possible except for, first, the
discoveries still kept in large part secret by the makers of optical
glass which have enabled them to make disks of the largest size; second,
the consummate skill of modern opticians in fashioning these disks into
perfect lenses; and third, the progress in the mechanical arts and
engineering, by which telescope tubes of many tons' weight are mounted
or poised so delicately that the thrust of a finger readily swerves them
from one point of the heavens to another.
As the telescope is the most important of all astronomical instruments,
it is necessary to understand its construction and adjustment and how
the astronomer uses it. Telescopes are optical instruments, and nothing
but optical parts would be requisite in making them, if only the optical
conditions of their perfect working could be obtained without other
mechanical accessories.
In original principle, all telescopes are as simple as Galileo's; first,
an object glass to form the image of the distant object; second the
eyepiece usually made of two lenses, but really a microscope, to magnify
that image, and working in the same way that any microscope magnifies an
object close at hand; and third, a tube to hold all the necessary
lenses in the true relative positions.
[Illustration: THE 100-INCH HOOKER TELESCOPE, LARGEST REFLECTOR
IN THE WORLD, ON MT. WILSON. (_Photo, Mt. Wilson Solar
Observatory._)]
[Illustration: THE LARGEST REFRACTOR, THE 40-INCH TELESCOPE AT
YERKES OBSERVATORY. DOME 90 FT. IN DIAMETER. (_Photo, Yerkes
Observatory._)]
[Illustration: THE 150-FT. TOWER AT THE MT. WILSON SOLAR
OBSERVATORY. At the left is a diagram of tower, telescope
and pit. At the upper right is an exterior view of the
tower; below a view looking down into the pit, 75 ft. deep.
(_Photo, Mt. Wilson Solar Observatory._)]
The focal lengths of object glass and eyepiece will determine just what
distance apart the lenses must be in order to give perfect vision. But
it is quite as important that the axes of all the lenses be adjusted
into one and the same straight line, and then held there rigidly and
permanently. Otherwise vision with the telescope will be very imperfect
and wholly unsatisfactory. The distance from the objective, or object
glass to its focal point is called its focal length; and if we divide
this by the focal length of the eyepiece, we shall have the magnifying
power of the telescope. The eyepiece will usually be made of two lenses,
or more, and we use its focal length considered as a single lens, in
getting the magnifying power. A telescope will generally have many
eyepieces of different focal lengths, so that it will have a
corresponding range of magnifying powers. The lowest magnifying power
will be not less than four or five diameters for each inch of aperture
of the objective; otherwise the eye will fail to receive all the light
which falls upon the glass. A 4-inch telescope will therefore have no
eyepiece with a lower magnifying power than about 20 diameters. The
highest magnifying power advantageous for a glass of this size will be
about 250 to 300, the working rule being about 70 diameters to each inch
of aperture, although the theoretical limit is regarded as 100.
The reason for a variety of eyepieces with different magnifying powers
soon becomes apparent on using the telescope. Comets and nebulae call for
very low powers, while double stars and the planetary surfaces require
the higher powers, provided the state of the atmosphere at the moment
will allow it. If there is much quivering and unsteadiness, nothing is
gained by trying the higher powers, because all the waves of
unsteadiness are magnified also in the same proportion, and sharpness of
vision, or fine definition, or "good seeing," as it is called, becomes
impossible. The vibrations and tremors of the atmosphere are the
greatest of all obstacles to astronomical observation, and the search is
always in order for regions of the world, in deserts or on high
mountains, where the quietest atmosphere is to be found.
Quite another power of the telescope is dependent on its objective
solely: its light-gathering power. Light by which we see a star or
planet is admitted to the retina of the eye through an adjustable
aperture called the pupil. In the dark or at night, the pupil expands to
an average diameter of one-fourth of an inch. But the object-glass of a
telescope, by focusing the rays from a star, pours into the eye, almost
as a funnel acts with water, all the light which falls on its larger
surface. And as geometry has settled it for us that areas of surfaces
are proportioned to the squares of their diameters, a two-inch object
glass focuses upon the retina of the eye 64 times as much light as the
unassisted eye would receive. And the great 40-inch objective of the
Yerkes telescope would, theoretically, yield 25,600 times as much light
as the eye alone. But there would be a noticeable percentage of this
lost through absorption by the glasses of the telescope and scattering
by their surfaces.
The first makers of telescopes soon encountered a most discouraging
difficulty, because it seemed to them absolutely insuperable. This is
known as chromatic aberration, or the scattering of light in a
telescope due simply to its color or wave length. When light passes
through a prism, red is refracted the least and violet the most. Through
a lens it is the same, because a lens may be regarded as an indefinite
system of prisms. The image of a star or planet, then, formed by a
single lens cannot be optically perfect; instead it will be a confused
intermingling of images of various colors. With low powers this will not
be very troublesome, but great indistinctness results from the use of
high magnifying powers.
The early makers and users of telescopes in the latter part of the
seventeenth century found that the troublesome effects of chromatic
aberration could be much reduced by increasing the focal length of the
objective. This led to what we term engineering difficulties of a very
serious nature, because the tubes of great length were very awkward in
pointing toward celestial objects, especially near the zenith, where the
air is quietest. And it was next to impossible to hold an object
steadily in the field, even after all the troubles of getting it there
had been successfully overcome.
Bianchini and Cassini, Hevelius and Huygens were among the active
observers of that epoch who built telescopes of extraordinary length, a
hundred feet and upward. One tube is said to have been built 600 feet in
length, but quite certainly it could never have been used. So-called
aerial telescopes were also constructed, in which the objective was
mounted on top of a tower or a pole, and the eyepiece moved along near
the ground. But it is difficult to see how anything but fleeting
glimpses of the heavenly bodies could have been obtained with such
contrivances, even if the lenses had been perfect. Newton indeed, who
was expert in optics, gave up the problem of improving the refracting
telescope, and turned his energies toward the reflector.
In 1733, half a century after Newton and a century and a quarter after
Galileo, Chester More Hall, an Englishman, found by experiment that
chromatic aberration could be nearly eliminated by making the objective
of two lenses instead of one, and the same invention was made
independently by Dollond, an English optician, who took out letters
patent about 1760. So the size of telescopes seemed to be limited only
by the skill of the glassmaker and the size of disks that he might find
it practicable to produce.
What Hall and Dollond did was to make the outer or crown lens of the
objective as before, and place behind it a plano-concave lens of dense
flint glass. This had the effect of neutralizing the chromatic effect,
or color aberration, while at the same time only part of the refractive
effect of the crown lens was destroyed. This ingenious but costly
combination prepared the way for the great refracting telescopes of the
present day, because it solved, or seemed to solve, the important
problem of getting the necessary refraction of light rays without
harmful dispersion or decomposition of them.
Through the 18th century and the first years of the 19th many telescopes
of a size very great for that day were built, and their success seemed
complete. With large increase in the size of the disks, however, a new
trouble arose, quite inherent in the glass itself. The two kinds of
glass, flint and crown, do not decompose white light with uniformity, so
that when the so-called achromatic objective was composed of flint and
crown, there was an effect known as irrationality of dispersion, or
secondary spectrum, which produced a very troublesome residuum of blue
light surrounding the images of bright objects. This is the most serious
defect of all the great refractors of the day, and effectively it limits
their size to about 60 inches of aperture, with present types of flint
and crown. It is expected by present experimenters, however, that
further improvements in optical glass will do much to extend this limit;
so that a refracting telescope of much greater size than any now in
existence will be practicable.
Improvements in mounting telescopes, too, are still possible. Within
recent years, Hartness, of Springfield, Vermont, has erected a new and
ingenious type of turret telescope which protects the observer from wind
and cold while his instrument is outside. It affords exceptional
facilities for rapid and convenient observing, as for variable stars,
and is adaptable to both refractors and reflectors.
The captivating study of the heavens can of course be begun with the
naked eye alone, but very moderate optical assistance is a great help
and stimulates. An opera-glass affords such assistance; a field-glass
does still better, and best of all, for certain purposes, is a modern
prism-binocular.