THE GROWTH OF OBSERVATION
The earliest astronomical observations must have been made in the Dawn
of Historic Time by the men who tended their flocks upon the great
plains. As they watched the clear night sky they no doubt soon noticed
that, with the exception of the moon and those brilliant wandering
objects known to us as the planets, the individual stars in the heaven
remained apparently fixed with reference to each other. These seemingly
changeless points of light came in time to be regarded as sign-posts to
guide the wanderer across the trackless desert, or the voyager upon the
wide sea.
Just as when looking into the red coals of a fire, or when watching the
clouds, our imagination conjures up strange and grotesque forms, so did
the men of old see in the grouping of the stars the outlines of weird
and curious shapes. Fed with mythological lore, they imagined these to
be rough representations of ancient heroes and fabled beasts, whom they
supposed to have been elevated to the heavens as a reward for great
deeds done upon the earth. We know these groupings of stars to-day under
the name of the Constellations. Looking up at them we find it extremely
difficult to fit in the majority with the figures which the ancients
believed them to represent. Nevertheless, astronomy has accepted the
arrangement, for want of a better method of fixing the leading stars in
the memory.
Our early ancestors lived the greater part of their lives in the open
air, and so came to pay more attention in general to the heavenly orbs
than we do. Their clock and their calendar was, so to speak, in the
celestial vault. They regulated their hours, their days, and their
nights by the changing positions of the sun, the moon, and the stars;
and recognised the periods of seed-time and harvest, of calm and stormy
weather, by the rising or setting of certain well-known constellations.
Students of the classics will recall many allusions to this, especially
in the Odes of Horace.
As time went on and civilisation progressed, men soon devised measuring
instruments, by means of which they could note the positions of the
celestial bodies in the sky with respect to each other; and, from
observations thus made, they constructed charts of the stars. The
earliest complete survey of this kind, of which we have a record, is the
great Catalogue of stars which was made, in the second century B.C., by
the celebrated Greek astronomer, Hipparchus, and in which he is said to
have noted down about 1080 stars.
It is unnecessary to follow in detail the tedious progress of
astronomical discovery prior to the advent of the telescope. Certain it
is that, as time went on, the measuring instruments to which we have
alluded had become greatly improved; but, had they even been perfect,
they would have been utterly inadequate to reveal those minute
displacements, from which we have learned the actual distance of the
nearest of the celestial orbs. From the early times, therefore, until
the mediæval period of our own era, astronomy grew up upon a faulty
basis, for the earth ever seemed so much the largest body in the
universe, that it continued from century to century to be regarded as
the very centre of things.
To the Arabians is due the credit of having kept alive the study of the
stars during the dark ages of European history. They erected some fine
observatories, notably in Spain and in the neighbourhood of Bagdad.
Following them, some of the Oriental peoples embraced the science in
earnest; Ulugh Beigh, grandson of the famous Tamerlane, founding, for
instance, a great observatory at Samarcand in Central Asia. The Mongol
emperors of India also established large astronomical instruments in the
chief cities of their empire. When the revival of learning took place in
the West, the Europeans came to the front once more in science, and
rapidly forged ahead of those who had so assiduously kept alight the
lamp of knowledge through the long centuries.
The dethronement of the older theories by the Copernican system, in
which the earth was relegated to its true place, was fortunately soon
followed by an invention of immense import, the invention of the
Telescope. It is to this instrument, indeed, that we are indebted for
our knowledge of the actual scale of the celestial distances. It
penetrated the depths of space; it brought the distant orbs so near,
that men could note the detail on the planets, or measure the small
changes in their positions in the sky which resulted from the movement
of our own globe.
It was in the year 1609 that the telescope was first constructed. A
year or so previous to this a spectacle-maker of Middleburgh in Holland,
one Hans Lippershey, had, it appears, hit upon the fact that distant
objects, when viewed through certain glass lenses suitably arranged,
looked nearer.[8] News of this discovery reached the ears of Galileo
Galilei, of Florence, the foremost philosopher of the day, and he at
once applied his great scientific attainments to the construction of an
instrument based upon this principle. The result was what was called an
"optick tube," which magnified distant objects some few times. It was
not much larger than what we nowadays contemptuously refer to as a
"spy-glass," yet its employment upon the leading celestial objects
instantly sent astronomical science onward with a bound. In rapid
succession Galileo announced world-moving discoveries; large spots upon
the face of the sun; crater-like mountains upon the moon; four
subordinate bodies, or satellites, circling around the planet Jupiter;
and a strange appearance in connection with Saturn, which later
telescopic observers found to be a broad flat ring encircling that
planet. And more important still, the magnified image of Venus showed
itself in the telescope at certain periods in crescent and other forms;
a result which Copernicus is said to have announced should of necessity
follow if his system were the true one.
The discoveries made with the telescope produced, as time went on, a
great alteration in the notions of men with regard to the universe at
large. It must have been, indeed, a revelation to find that those points
of light which they called the planets, were, after all, globes of a
size comparable with the earth, and peopled perchance with sentient
beings. Even to us, who have been accustomed since our early youth to
such an idea, it still requires a certain stretch of imagination to
enlarge, say, the Bright Star of Eve, into a body similar in size to our
earth. The reader will perhaps recollect Tennyson's allusion to this in
_Locksley Hall, Sixty Years After_:--
"Hesper--Venus--were we native to that splendour or in Mars,
We should see the Globe we groan in, fairest of their evening stars.
"Could we dream of wars and carnage, craft and madness, lust and spite,
Roaring London, raving Paris, in that point of peaceful light?"
The form of instrument as devised by Galileo is called the Refracting
Telescope, or "Refractor." As we know it to-day it is the same in
principle as his "optick tube," but it is not quite the same in
construction. The early _object-glass_, or large glass at the end, was a
single convex lens (see Fig. 8, p. 113, "Galilean"); the modern one is,
on the other hand, composed of two lenses fitted together. The attempts
to construct large telescopes of the Galilean type met in course of time
with a great difficulty. The magnified image of the object observed was
not quite pure; its edges, indeed, were fringed with rainbow-like
colours. This defect was found to be aggravated with increase in the
size of object-glasses. A method was, however, discovered of
diminishing this colouration, or _chromatic aberration_ as it is called
from the Greek word [chrôma] (_chroma_), which means colour, viz. by
making telescopes of great length and only a few inches in width. But
the remedy was, in a way, worse than the disease; for telescopes thus
became of such huge proportions as to be too unwieldy for use. Attempts
were made to evade this unwieldiness by constructing them with skeleton
tubes (see Plate II., p. 110), or, indeed, even without tubes at all;
the object-glass in the tubeless or "aerial" telescope being fixed at
the top of a high post, and the _eye-piece_, that small lens or
combination of lenses, which the eye looks directly into, being kept in
line with it by means of a string and manoeuvred about near the ground
(Plate III., p. 112). The idea of a telescope without a tube may appear
a contradiction in terms; but it is not really so, for the tube adds
nothing to the magnifying power of the instrument, and is, in fact, no
more than a mere device for keeping the object-glass and eye-piece in a
straight line, and for preventing the observer from being hindered by
stray lights in his neighbourhood. It goes without saying, of course,
that the image of a celestial object will be more clear and defined when
examined in the darkness of a tube.
The ancients, though they knew nothing of telescopes, had, however,
found out the merit of a tube in this respect; for they employed simple
tubes, blackened on the inside, in order to obtain a clearer view of
distant objects. It is said that Julius Cæsar, before crossing the
Channel, surveyed the opposite coast of Britain through a tube of this
kind.
[Illustration: PLATE II. GREAT TELESCOPE OF HEVELIUS
This instrument, 150 feet in length, with a _skeleton_ tube, was
constructed by the celebrated seventeenth century astronomer, Hevelius
of Danzig. From an illustration in the _Machina Celestis_.
(Page 110)]
A few of the most famous of the immensely long telescopes above alluded
to are worthy of mention. One of these, 123 feet in length, was
presented to the Royal Society of London by the Dutch astronomer
Huyghens. Hevelius of Danzig constructed a skeleton one of 150 feet in
length (see Plate II., p. 110). Bradley used a tubeless one 212 feet
long to measure the diameter of Venus in 1722; while one of 600 feet is
said to have been constructed, but to have proved quite unworkable!
Such difficulties, however, produced their natural result. They set men
at work to devise another kind of telescope. In the new form, called the
Reflecting Telescope, or "Reflector," the light coming from the object
under observation was _reflected_ into the eye-piece from the surface of
a highly polished concave metallic mirror, or _speculum_, as it was
called. It is to Sir Isaac Newton that the world is indebted for the
reflecting telescope in its best form. That philosopher had set himself
to investigate the causes of the rainbow-like, or prismatic colours
which for a long time had been such a source of annoyance to telescopic
observers; and he pointed out that, as the colours were produced in the
passage of the rays of light _through_ the glass, they would be entirely
absent if the light were reflected from the _surface_ of a mirror
instead.
The reflecting telescope, however, had in turn certain drawbacks of its
own. A mirror, for instance, can plainly never be polished to such a
high degree as to reflect as much light as a piece of transparent glass
will let through. Further, the position of the eye-piece is by no means
so convenient. It cannot, of course, be pointed directly towards the
mirror, for the observer would then have to place his head right in the
way of the light coming from the celestial object, and would thus, of
course, cut it off. In order to obviate this difficulty, the following
device was employed by Newton in his telescope, of which he constructed
his first example in 1668. A small, flat mirror was fixed by thin wires
in the centre of the tube of the telescope, and near to its open end. It
was set slant-wise, so that it reflected the rays of light directly into
the eye-piece, which was screwed into a hole at the side of the tube
(see Fig. 8, p. 113, "Newtonian").
Although the Newtonian form of telescope had the immense advantage of
doing away with the prismatic colours, yet it wasted a great deal of
light; for the objection in this respect with regard to loss of light by
reflection from the large mirror applied, of course, to the small mirror
also. In addition, the position of the "flat," as the small mirror is
called, had the further effect of excluding from the great mirror a
certain proportion of light. But the reflector had the advantage, on the
other hand, of costing less to make than the refractor, as it was not
necessary to procure flawless glass for the purpose. A disc of a certain
metallic composition, an alloy of copper and tin, known in consequence
as _speculum metal_, had merely to be cast; and this had to be ground
and polished _upon one side only_, whereas a lens has to be thus treated
_upon both its sides_. It was, therefore, possible to make a much larger
instrument at a great deal less labour and expense.
[Illustration: PLATE III. A TUBELESS, OR "AERIAL" TELESCOPE
From an illustration in the _Opera Varia_ of Christian Huyghens.
(Page 110)]
[Illustration: FIG. 8.--The various types of Telescope. All the above
telescopes are _pointed_ in the same direction; that is to say, the rays
of light from the object are coming from the left-hand side.]
We have given the Newtonian form as an example of the principle of the
reflecting telescope. A somewhat similar instrument had, however, been
projected, though not actually constructed, by James Gregory a few years
earlier than Newton's, _i.e._ in 1663. In this form of reflector, known
as the "Gregorian" telescope, a hole was made in the big concave mirror;
and a small mirror, also concave, which faced it at a certain distance,
received the reflected rays, and reflected them back again through the
hole in question into the eye-piece, which was fixed just behind (see
Fig. 8, p. 113, "Gregorian"). The Gregorian had thus the sentimental
advantage of being _pointed directly at the object_. The hole in the big
mirror did not cause any loss of light, for the central portion in which
it was made was anyway unable to receive light through the small mirror
being directly in front of it. An adaptation of the Gregorian was the
"Cassegrainian" telescope, devised by Cassegrain in 1672, which differed
from it chiefly in the small mirror being convex instead of concave (see
Fig. 8, p. 113, "Cassegrainian"). These _direct-view_ forms of the
reflecting telescope were much in vogue about the middle of the
eighteenth century, when many beautiful examples of Gregorians were made
by the famous optician, James Short, of Edinburgh.
An adaptation of the Newtonian type of telescope is known as the
"Herschelian," from being the kind favoured by Sir William Herschel. It
is, however, only suitable in immense instruments, such as Herschel was
in the habit of employing. In this form the object-glass is set at a
slight slant, so that the light coming from the object is reflected
straight into the eye-piece, which is fixed facing it in the side of the
tube (see Fig. 8, p. 113, "Herschelian"). This telescope has an
advantage over the other forms of reflector through the saving of light
consequent on doing away with the _second_ reflection. There is,
however, the objection that the slant of the object-glass is productive
of some distortion in the appearance of the object observed; but this
slant is of necessity slight when the length of the telescope is very
great.
The principle of this type of telescope had been described to the
French Academy of Sciences as early as 1728 by Le Maire, but no one
availed himself of the idea until 1776, when Herschel tried it. At
first, however, he rejected it; but in 1786 he seems to have found that
it suited the huge instruments which he was then making. Herschel's
largest telescope, constructed in 1789, was about four feet in diameter
and forty feet in length. It is generally spoken of as the "Forty-foot
Telescope," though all other instruments have been known by their
_diameters_, rather than by their lengths.
To return to the refracting telescope. A solution of the colour
difficulty was arrived at in 1729 (two years after Newton's death) by an
Essex gentleman named Chester Moor Hall. He discovered that by making a
double object-glass, composed of an outer convex lens and an inner
concave lens, made respectively of different kinds of glass, _i.e._
_crown_ glass and _flint_ glass, the troublesome colour effects could
be, _to a very great extent_, removed. Hall's investigations appear to
have been rather of an academic nature; and, although he is believed to
have constructed a small telescope upon these lines, yet he seems to
have kept the matter so much to himself that it was not until the year
1758 that the first example of the new instrument was given to the
world. This was done by John Dollond, founder of the well-known optical
firm of Dollond, of Ludgate Hill, London, who had, quite independently,
re-discovered the principle.
This "Achromatic" telescope, or telescope "free from colour effects," is
the kind ordinarily in use at present, whether for astronomical or for
terrestrial purposes (see Fig. 8, p. 113, "Achromatic"). The expense of
making large instruments of this type is very great, for, in the
object-glass alone, no less than _four_ surfaces have to be ground and
polished to the required curves; and, usually, the two lenses of which
it is composed have to fit quite close together.
With the object of evading the expense referred to, and of securing
_complete_ freedom from colour effects, telescopes have even been made,
the object-glasses of which were composed of various transparent liquids
placed between thin lenses; but leakages, and currents set up within
them by changes of temperature, have defeated the ingenuity of those who
devised these substitutes.
The solution of the colour difficulty by means of Dollond's achromatic
refractor has not, however, ousted the reflecting telescope in its best,
or Newtonian form, for which great concave mirrors made of glass,
covered with a thin coating of silver and highly polished, have been
used since about 1870 instead of metal mirrors. They are very much
lighter in weight and cheaper to make than the old specula; and though
the silvering, needless to say, deteriorates with time, it can be
renewed at a comparatively trifling cost. Also these mirrors reflect
much more light, and give a clearer view, than did the old metallic
ones.
When an object is viewed through the type of astronomical telescope
ordinarily in use, it is seen _upside down_. This is, however, a matter
of very small moment in dealing with celestial objects; for, as they are
usually round, it is really not of much consequence which part we regard
as top and which as bottom. Such an inversion would, of course, be most
inconvenient when viewing terrestrial objects. In order to observe the
latter we therefore employ what is called a terrestrial telescope, which
is merely a refractor with some extra lenses added in the eye portion
for the purpose of turning the inverted image the right way up again.
These extra lenses, needless to say, absorb a certain amount of light;
wherefore it is better in astronomical observation to save light by
doing away with them, and putting up with the slight inconvenience of
seeing the object inverted.
This inversion of images by the astronomical telescope must be specially
borne in mind with regard to the photographs of the moon in Chapter XVI.
In the year 1825 the largest achromatic refractor in existence was one
of nine and a half inches in diameter constructed by Fraunhofer for the
Observatory of Dorpat in Russia. The largest refractors in the world
to-day are in the United States, _i.e._ the forty-inch of the Yerkes
Observatory (see Plate IV., p. 118), and the thirty-six inch of the
Lick. The object-glasses of these and of the thirty-inch telescope of
the Observatory of Pulkowa, in Russia, were made by the great optical
house of Alvan Clark & Sons, of Cambridge, Massachusetts, U.S.A. The
tubes and other portions of the Yerkes and Lick telescopes were,
however, constructed by the Warner and Swasey Co., of Cleveland, Ohio.
The largest reflector, and so the largest telescope in the world, is
still the six-foot erected by the late Lord Rosse at Parsonstown in
Ireland, and completed in the year 1845. It is about fifty-six feet in
length. Next come two of five feet, with mirrors of silver on glass;
one of them made by the late Dr. Common, of Ealing, and the other by the
American astronomer, Professor G.W. Ritchey. The latter of these is
installed in the Solar Observatory belonging to Carnegie Institution of
Washington, which is situated on Mount Wilson in California. The former
is now at the Harvard College Observatory, and is considered by
Professor Moulton to be probably the most efficient reflector in use at
present. Another large reflector is the three-foot made by Dr. Common.
It came into the possession of Mr. Crossley of Halifax, who presented it
to the Lick Observatory, where it is now known as the "Crossley
Reflector."
Although to the house of Clark belongs, as we have seen, the credit of
constructing the object-glasses of the largest refracting telescopes of
our time, it has nevertheless keen competitors in Sir Howard Grubb, of
Dublin, and such well-known firms as Cooke of York and Steinheil of
Munich. In the four-foot reflector, made in 1870 for the Observatory of
Melbourne by the firm of Grubb, the Cassegrainian principle was
employed.
With regard to the various merits of refractors and reflectors much
might be said. Each kind of instrument has, indeed, its special
advantages; though perhaps, on the whole, the most perfect type of
telescope is the achromatic refractor.
[Illustration: PLATE IV. THE GREAT YERKES TELESCOPE
Great telescope at the Yerkes Observatory of the University of Chicago,
Williams Bay, Wisconsin, U.S.A. It was erected in 1896-7, and is the
largest refracting telescope in the world. Diameter of object-glass, 40
inches; length of telescope, about 60 feet. The object-glass was made by
the firm of Alvan Clark and Sons, of Cambridge, Massachusetts; the other
portions of the instrument by the Warner and Swasey Co., of Cleveland,
Ohio.
(Page 117)]
In connection with telescopes certain devices have from time to time
been introduced, but these merely aim at the _convenience_ of the
observer and do not supplant the broad principles upon which are based
the various types of instrument above described. Such, for instance, are
the "Siderostat," and another form of it called the "Coelostat," in
which a plane mirror is made to revolve in a certain manner, so as to
reflect those portions of the sky which are to be observed, into the
tube of a telescope kept fixed. Such too are the "Equatorial Coudé" of
the late M. Loewy, Director of the Paris Observatory, and the
"Sheepshanks Telescope" of the Observatory of Cambridge, in which a
telescope is separated into two portions, the eye-piece portion being
fixed upon a downward slant, and the object-glass portion jointed to it
at an angle and pointed up at the sky. In these two instruments (which,
by the way, differ materially) an arrangement of slanting mirrors in the
tubes directs the journey of the rays of light from the object-glass to
the eye-piece. The observer can thus sit at the eye-end of his telescope
in the warmth and comfort of his room, and observe the stars in the same
unconstrained manner as if he were merely looking down into a
microscope.
Needless to say, devices such as these are subject to the drawback that
the mirrors employed sap a certain proportion of the rays of light. It
will be remembered that we made allusion to loss of light in this way,
when pointing out the advantage in light grasp of the Herschelian form
of telescope, where only _one_ reflection takes place, over the
Newtonian in which there are _two_.
It is an interesting question as to whether telescopes can be made much
larger. The American astronomer, Professor G.E. Hale, concludes that the
limit of refractors is about five feet in diameter, but he thinks that
reflectors as large as nine feet in diameter might now be made. As
regards refractors there are several strong reasons against augmenting
their proportions. First of all comes the great cost. Secondly, since
the lenses are held in position merely round their rims, they will bend
by their weight in the centres if they are made much larger. On the
other hand, attempts to obviate this, by making the lenses thicker,
would cause a decrease in the amount of light let through.
But perhaps the greatest stumbling-block to the construction of larger
telescopes is the fact that the unsteadiness of the air will be
increasingly magnified. And further, the larger the tubes become, the
more difficult will it be to keep the air within them at one constant
temperature throughout their lengths.
It would, indeed, seem as if telescopes are not destined greatly to
increase in size, but that the means of observation will break out in
some new direction, as it has already done in the case of photography
and the spectroscope. The direct use of the eye is gradually giving
place to indirect methods. We are, in fact, now _feeling_ rather than
seeing our way about the universe. Up to the present, for instance, we
have not the slightest proof that life exists elsewhere than upon our
earth. But who shall say that the twentieth century has not that in
store for us, by which the presence of life in other orbs may be
perceived through some form of vibration transmitted across illimitable
space? There is no use speaking of the impossible or the inconceivable.
After the extraordinary revelations of the spectroscope--nay, after the
astounding discovery of Röntgen--the word impossible should be cast
aside, and inconceivability cease to be regarded as any criterion.
[8] The principle upon which the telescope is based appears to have been
known _theoretically_ for a long time previous to this. The monk Roger
Bacon, who lived in the thirteenth century, describes it very clearly;
and several writers of the sixteenth century have also dealt with the
idea. Even Lippershey's claims to a practical solution of the question
were hotly contested at the time by two of his own countrymen, _i.e._ a
certain Jacob Metius, and another spectacle-maker of Middleburgh, named
Jansen.