THE STORY OF THE HEAVENS pdf

THE STORY OF THE HEAVENS

PLATE I.
THE PLANET SATURN,
IN 1872.

THE
Story of the Heavens
SIR ROBERT STAWELL BALL, LL.D. D.Sc.
Author of "Star-Land"
FELLOW OF THE ROYAL SOCIETY OF LONDON, HONORARY FELLOW OF
THE ROYAL SOCIETY OF
EDINBURGH, FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY,
SCIENTIFIC ADVISER TO THE
COMMISSIONERS OF IRISH LIGHTS, LOWNDEAN PROFESSOR OF
ASTRONOMY AND
GEOMETRY IN THE UNIVERSITY OF CAMBRIDGE, AND FORMERLY
ROYAL ASTRONOMER OF IRELAND


WITH TWENTY-FOUR COLOURED PLATES AND NUMEROUS
ILLUSTRATIONS


NEW AND REVISED EDITION


CASSELL and COMPANY, Limited
LONDON, PARIS, NEW YORK & MELBOURNE
1900

ALL RIGHTS RESERVED



PREFACE TO ORIGINAL EDITION.
I have to acknowledge the kind aid which I have received in the preparation of this
book.
Mr. Nasmyth has permitted me to use some of the beautiful drawings of the Moon,
which have appeared in the well-known work published by him in conjunction with
Mr. Carpenter. To this source I am indebted for Plates VII., VIII., IX., X., and Figs.
28, 29, 30.
Professor Pickering has allowed me to copy some of the drawings made at Harvard
College Observatory by Mr. Trouvelot, and I have availed myself of his kindness for
Plates I., IV., XII., XV.
I am indebted to Professor Langley for Plate II., to Mr. De la Rue for Plates III. and
XIV., to Mr. T.E. Key for Plate XVII., to Professor Schiaparelli for Plate XVIII., to
the late Professor C. Piazzi Smyth for Fig. 100, to Mr. Chambers for Fig. 7, which has
been borrowed from his "Handbook of Descriptive Astronomy," to Dr. Stoney for Fig.
78, and to Dr. Copeland and Dr. Dreyer for Fig. 72. I have to acknowledge the
valuable assistance derived from Professor Newcomb's "Popular Astronomy," and
Professor Young's "Sun." In revising the volume I have had the kind aid of the Rev.
Maxwell Close.
I have also to thank Dr. Copeland and Mr. Steele for their kindness in reading through
the entire proofs; while I have also occasionally availed myself of the help of Mr.
Cathcart.
ROBERT S. BALL.
Observatory, Dunsink, Co. Dublin.
12th May, 1886.

NOTE TO THIS EDITION.

I have taken the opportunity in the present edition to revise the work in accordance
with the recent progress of astronomy. I am indebted to the Royal Astronomical
Society for the permission to reproduce some photographs from their published series,
and to Mr. Henry F. Griffiths, for beautiful drawings of Jupiter, from which Plate XI.
was prepared.
ROBERT S. BALL.
Cambridge,
1st May, 1900.

CONTENTS.
page
Introduction 1
chapter
I.
The Astronomical
Observatory
9
II. The Sun 29
III. The Moon 70
IV. The Solar System 107
V. The Law of Gravitation 122
VI. The Planet of Romance 150
VII. Mercury 155
VIII. Venus 167
IX. The Earth 192
X. Mars 208
XI. The Minor Planets 229
XII. Jupiter 245
XIII. Saturn 268
XIV. Uranus 298

XV. Neptune 315
XVI. Comets 336
XVII. Shooting Stars 372
XVIII. The Starry Heavens 409
XIX. The Distant Suns 425
XX. Double Stars 434
XXI. The Distances of the Stars 441
XXII. Star Clusters and Nebulæ 461
XXIII.
The Physical Nature of the
Stars
477
XXIV.
The Precession and Nutation
of the Earth's Axis
492
XXV. The Aberration of Light 503
XXVI.
The
Astronomical
Significance of Heat
513
XXVII.

The Tides 531
Appendix 558

LIST OF PLATES.
PLATE
I. The Planet Saturn Frontispiece

II. A Typical Sun-spot
To face
page
9
A. The Sun " " 44
III.
Spots and Faculæ on the
Sun
" " 37
IV.
Solar Prominences or
Flames
" " 57
V. The Solar Corona " " 62
VI.
Chart of the Moon's
Surface
" " 81
B. Portion of the Moon " " 88
VII.
The Lunar
Crater
Triesnecker
" " 93
VIII. A Normal Lunar Crater " " 97
IX. The Lunar Crater Plato " " 102
X. The Lunar Crater Tycho " " 106
XI. The Planet Jupiter " " 254
XII. Coggia's Comet " " 340
C. Comet A., 1892, 1 Swift " " 358

XIII.
Spectra of the Sun and of
three Stars
" " 47
D.
The Milky Way, near
Messier II.
" " 462
XIV.
The Great Nebula in
Orion
" " 466
XV.
The Great Nebula in
Andromeda
" " 468
E. Nebulæ in the Pleiades " " 472
F. ω Centauri " " 474
XVI.
Nebulæ observed with
Lord Rosse's Telescope
" " 476
XVII. The Comet of 1882 " " 357
XVIII.

Schiaparelli's Map of
Mars
" " 221

LIST OF ILLUSTRATIONS.

FIG. PAGE
1. Principle of the Refracting Telescope 11
2.
Dome of the South Equatorial at Dunsink
Observatory, Co. Dublin
12
3. Section of the Dome of Dunsink Observatory 13
4. The Telescope at Yerkes Observatory, Chicago 15
5. Principle of Herschel's Reflecting Telescope 16
6.
South Front of the Yerkes Observatory,
Chicago
17
7. Lord Rosse's Telescope 18
8. Meridian Circle 20
9. The Great Bear 27
10. Comparative Sizes of the Earth and the Sun 30
11. The Sun, photographed September 22, 1870 33
12. Photograph of the Solar Surface 35
13. An ordinary Sun-spot 36
14. Scheiner's Observations on Sun-spots 38
15.
Zones on the Sun's Surface in which Spots
appear
39
16. Texture of the Sun and a small Spot 43
17. The Prism 45
18. Dispersion of Light by the Prism 46
19. Prominences seen in Total Eclipses 53
20. View of the Corona in a Total Eclipse 62

21.
View of Corona during Eclipse of January 22,
1898
63
22. The Zodiacal Light in 1874 69
23. Comparative Sizes of the Earth and the Moon 73
24. The Moon's Path around the Sun 76
25. The Phases of the Moon 76
26. The Earth's Shadow and Penumbra 78
27. Key to Chart of the Moon (Plate VI.) 81
28. Lunar Volcano in Activity: Nasmyth's Theory 97
29. Lunar Volcano: Subsequent Feeble Activity 97
30.
Lunar Volcano: Formation of the Level Floor
by Lava
98
31. Orbits of the Four Interior Planets 115
32. The Earth's Movement 116
33. Orbits of the Four Giant Planets 117
34. Apparent Size of the Sun from various Planets 118
35. Comparative Sizes of the Planets 119
36. Illustration of the Moon's Motion 130
37. Drawing an Ellipse 137
38. Varying Velocity of Elliptic Motion 140
39. Equal Areas in Equal Times 141
40. Transit of the Planet of Romance 153
41.
Variations in Phase and apparent Size of
Mercury
160

42. Mercury as a Crescent 161
43. Venus, May 29, 1889 170
44. Different Aspects of Venus in the Telescope 171
45. Venus on the Sun at the Transit of 1874 177
46.
Paths of Venus across the Sun in the Transits of
1874 and 1882
179
47.
A Transit of Venus, as seen from Two
Localities
183
48. Orbits of the Earth and of Mars 210
49. Apparent Movements of Mars in 1877 212
50. Relative Sizes of Mars and the Earth 216
51,
52.
Drawings of Mars 217
53.
Elevations and Depressions on the Terminator
of Mars
217
54. The Southern Polar Cap on Mars 217
55.
The Zone of Minor Planets between Mars and
Jupiter
234
56. Relative Dimensions of Jupiter and the Earth 246
57–
60.

The Occultation of Jupiter 255
61. Jupiter and his Four Satellites 258
62. Disappearances of Jupiter's Satellites 259
63. Mode of Measuring the Velocity of Light 264
64. Saturn 270
65. Relative Sizes of Saturn and the Earth 273
66.
Method of Measuring the Rotation of Saturn's
Rings
288
67.
Method of Measuring the Rotation of Saturn's
Rings
289
68. Transit of Titan and its Shadow 295
69. Parabolic Path of a Comet 339
70. Orbit of Encke's Comet 346
71. Tail of a Comet directed from the Sun 363
72. Bredichin's Theory of Comets' Tails 366
73. Tails of the Comet of 1858 367
74. The Comet of 1744 368
75. The Path of the Fireball of November 6, 1869 375
76. The Orbit of a Shoal of Meteors 378
77. Radiant Point of Shooting Stars 381
78. The History of the Leonids 385
79. Section of the Chaco Meteorite 398
80. The Great Bear and Pole Star 410
81. The Great Bear and Cassiopeia 411
82. The Great Square of Pegasus 413
83. Perseus and its Neighbouring Stars 415

84. The Pleiades 416
85. Orion, Sirius, and Neighbouring Stars 417
86. Castor and Pollux 418
87. The Great Bear and the Lion 419
88. Boötes and the Crown 420
89. Virgo and Neighbouring Constellations 421
90. The Constellation of Lyra 422
91. Vega, the Swan, and the Eagle 423
92. The Orbit of Sirius 426
93. The Parallactic Ellipse 444
94. 61 Cygni and the Comparison Stars 447
95. Parallax in Declination of 61 Cygni 450
96. Globular Cluster in Hercules 463
97. Position of the Great Nebula in Orion 466
98. The Multiple Star θ Orionis 467
99. The Nebula N.G.C. 1499 471
100. Star-Map, showing Precessional Movement 493
101. Illustration of the Motion of Precession 495

[Pg 1]
THE
Story of the Heavens.
"The Story of the Heavens" is the title of our book. We have indeed a wondrous story
to narrate; and could we tell it adequately it would prove of boundless interest and of
exquisite beauty. It leads to the contemplation of grand phenomena in nature and great
achievements of human genius.
Let us enumerate a few of the questions which will be naturally asked by one who
seeks to learn something of those glorious bodies which adorn our skies: What is the
Sun—how hot, how big, and how distant? Whence comes its heat? What is the Moon?
What are its landscapes like? How does our satellite move? How is it related to the

earth? Are the planets globes like that on which we live? How large are they, and how
far off? What do we know of the satellites of Jupiter and of the rings of Saturn? How
was Uranus discovered? What was the intellectual triumph which brought the planet
Neptune to light? Then, as to the other bodies of our system, what are we to say of
those mysterious objects, the comets? Can we discover the laws of their seemingly
capricious movements? Do we know anything of their nature and of the marvellous
tails with which they are often decorated? What can be told about the shooting-stars
which so often dash into our atmosphere and perish in a streak of splendour? What is
the nature of those constellations of bright stars which have been recognised from all
antiquity, and of the host of smaller stars which our telescopes disclose? Can it be true
that these countless orbs are really majestic suns, sunk to an appalling[Pg 2] depth in
the abyss of unfathomable space? What have we to tell of the different varieties of
stars—of coloured stars, of variable stars, of double stars, of multiple stars, of stars
that seem to move, and of stars that seem at rest? What of those glorious objects, the
great star clusters? What of the Milky Way? And, lastly, what can we learn of the
marvellous nebulæ which our telescopes disclose, poised at an immeasurable
distance? Such are a few of the questions which occur when we ponder on the
mysteries of the heavens.
The history of Astronomy is, in one respect, only too like many other histories. The
earliest part of it is completely and hopelessly lost. The stars had been studied, and
some great astronomical discoveries had been made, untold ages before those to
which our earliest historical records extend. For example, the observation of the
apparent movement of the sun, and the discrimination between the planets and the
fixed stars, are both to be classed among the discoveries of prehistoric ages. Nor is it
to be said that these achievements related to matters of an obvious character. Ancient
astronomy may seem very elementary to those of the present day who have been
familiar from childhood with the great truths of nature, but, in the infancy of science,
the men who made such discoveries as we have mentioned must have been sagacious
philosophers.
Of all the phenomena of astronomy the first and the most obvious is that of the rising

and the setting of the sun. We may assume that in the dawn of human intelligence
these daily occurrences would form one of the first problems to engage the attention
of those whose thoughts rose above the animal anxieties of everyday existence. A sun
sets and disappears in the west. The following morning a sun rises in the east, moves
across the heavens, and it too disappears in the west; the same appearances recur
every day. To us it is obvious that the sun, which appears each day, is the same sun;
but this would not seem reasonable to one who thought his senses showed him that the
earth was a flat plain of indefinite extent, and that around the inhabited regions on all
sides extended, to vast distances, either desert wastes or trackless oceans. How could
that same sun, which plunged into the ocean at a fabulous distance in the west,[Pg 3]
reappear the next morning at an equally great distance in the east? The old mythology
asserted that after the sun had dipped in the western ocean at sunset (the Iberians, and
other ancient nations, actually imagined that they could hear the hissing of the waters
when the glowing globe was plunged therein), it was seized by Vulcan and placed in a
golden goblet. This strange craft with its astonishing cargo navigated the ocean by a
northerly course, so as to reach the east again in time for sunrise the following
morning. Among the earlier physicists of old it was believed that in some manner the
sun was conveyed by night across the northern regions, and that darkness was due to
lofty mountains, which screened off the sunbeams during the voyage.
In the course of time it was thought more rational to suppose that the sun actually
pursued his course below the solid earth during the course of the night. The early
astronomers had, moreover, learned to recognise the fixed stars. It was noticed that,
like the sun, many of these stars rose and set in consequence of the diurnal movement,
while the moon obviously followed a similar law. Philosophers thus taught that the
various heavenly bodies were in the habit of actually passing beneath the solid earth.
By the acknowledgment that the whole contents of the heavens performed these
movements, an important step in comprehending the constitution of the universe had
been decidedly taken. It was clear that the earth could not be a plane extending to an
indefinitely great distance. It was also obvious that there must be a finite depth to the
earth below our feet. Nay, more, it became certain that whatever the shape of the earth

might be, it was at all events something detached from all other bodies, and poised
without visible support in space. When this discovery was first announced it must
have appeared a very startling truth. It was so difficult to realise that the solid earth on
which we stand reposed on nothing! What was to keep it from falling? How could it
be sustained without tangible support, like the legendary coffin of Mahomet? But
difficult as it may have been to receive this doctrine, yet its necessary truth in due
time[Pg 4] commanded assent, and the science of Astronomy began to exist. The
changes of the seasons and the recurrence of seed-time and harvest must, from the
earliest times, have been associated with certain changes in the position of the sun. In
the summer at mid-day the sun rises high in the heavens, in the winter it is always
low. Our luminary, therefore, performs an annual movement up and down in the
heavens, as well as a diurnal movement of rising and setting. But there is a third
species of change in the sun's position, which is not quite so obvious, though it is still
capable of being detected by a few careful observations, if combined with a
philosophical habit of reflection. The very earliest observers of the stars can hardly
have failed to notice that the constellations visible at night varied with the season of
the year. For instance, the brilliant figure of Orion, though so well seen on winter
nights, is absent from the summer skies, and the place it occupied is then taken by
quite different groups of stars. The same may be said of other constellations. Each
season of the year can thus be characterised by the sidereal objects that are
conspicuous by night. Indeed, in ancient days, the time for commencing the cycle of
agricultural occupations was sometimes indicated by the position of the constellations
in the evening.
By reflecting on these facts the early astronomers were enabled to demonstrate the
apparent annual movement of the sun. There could be no rational explanation of the
changes in the constellations with the seasons, except by supposing that the place of
the sun was altering, so as to make a complete circuit of the heavens in the course of
the year. This movement of the sun is otherwise confirmed by looking at the west after
sunset, and watching the stars. As the season progresses, it may be noticed each
evening that the constellations seem to sink lower and lower towards the west, until at

length they become invisible from the brightness of the sky. The disappearance is
explained by the supposition that the sun appears to be continually ascending from the
west to meet the stars. This motion is, of course, not to be confounded with the
ordinary diurnal rising and setting, in which all the heavenly bodies participate. It is to
be understood[Pg 5] that besides being affected by the common motion our luminary
has a slow independent movement in the opposite direction; so that though the sun
and a star may set at the same time to-day, yet since by to-morrow the sun will have
moved a little towards the east, it follows that the star must then set a few minutes
before the sun.[1]
The patient observations of the early astronomers enabled the sun's track through the
heavens to be ascertained, and it was found that in its circuit amid the stars and
constellations our luminary invariably followed the same path. This is called the
ecliptic, and the constellations through which it passes form a belt around the heavens
known as the zodiac. It was anciently divided into twelve equal portions or "signs," so
that the stages on the sun's great journey could be conveniently indicated. The
duration of the year, or the period required by the sun to run its course around the
heavens, seems to have been first ascertained by astronomers whose names are
unknown. The skill of the early Oriental geometers was further evidenced by their
determination of the position of the ecliptic with regard to the celestial equator, and by
their success in the measurement of the angle between these two important circles on
the heavens.
The principal features of the motion of the moon have also been noticed with
intelligence at an antiquity more remote than history. The attentive observer perceives
the important truth that the moon does not occupy a fixed position in the heavens.
During the course of a single night the fact that the moon has moved from west to east
across the heavens can be perceived by noting its position relatively to adjacent stars.
It is indeed probable that the motion of the moon was a discovery prior to that of the
annual motion of the sun, inasmuch as it is the immediate consequence of a simple
observation, and involves but little exercise of any intellectual power. In prehistoric
times also, the time of revolution of the moon had been ascertained, and the phases of

our satellite had been correctly attributed to the varying aspect[Pg 6] under which the
sun-illuminated side is turned towards the earth.
But we are far from having exhausted the list of great discoveries which have come
down from unknown antiquity. Correct explanations had been given of the striking
phenomenon of a lunar eclipse, in which the brilliant surface is plunged temporarily
into darkness, and also of the still more imposing spectacle of a solar eclipse, in which
the sun itself undergoes a partial or even a total obscuration. Then, too, the acuteness
of the early astronomers had detected the five wandering stars or planets: they had
traced the movements of Mercury and Venus, Mars, Jupiter, and Saturn. They had
observed with awe the various configurations of these planets: and just as the sun, and
in a lesser degree the moon, were intimately associated with the affairs of daily life, so
in the imagination of these early investigators the movements of the planets were
thought to be pregnant with human weal or human woe. At length a certain order was
perceived to govern the apparently capricious movements of the planets. It was found
that they obeyed certain laws. The cultivation of the science of geometry went hand in
hand with the study of astronomy: and as we emerge from the dim prehistoric ages
into the historical period, we find that the theory of the phenomena of the heavens
possessed already some degree of coherence.
Ptolemy, following Pythagoras, Plato, and Aristotle, acknowledged that the earth's
figure was globular, and he demonstrated it by the same arguments that we employ at
the present day. He also discerned how this mighty globe was isolated in space. He
admitted that the diurnal movement of the heavens could be accounted for by the
revolution of the earth upon its axis, but unfortunately he assigned reasons for the
deliberate rejection of this view. The earth, according to him, was a fixed body; it
possessed neither rotation round an axis nor translation through space, but remained
constantly at rest in what he supposed to be the centre of the universe. According to
Ptolemy's theory the sun and the moon moved in circular orbits around the earth in the
centre. The [Pg 7]explanation of the movements of the planets he found to be more
complicated, because it was necessary to account for the fact that a planet sometimes
advanced and that it sometimes retrograded. The ancient geometers refused to believe

that any movement, except revolution in a circle, was possible for a celestial body:
accordingly a contrivance was devised by which each planet was supposed to revolve
in a circle, of which the centre described another circle around the earth.
Although the Ptolemaic doctrine is now known to be framed on quite an extravagant
estimate of the importance of the earth in the scheme of the heavens, yet it must be
admitted that the apparent movements of the celestial bodies can be thus accounted for
with considerable accuracy. This theory is described in the great work known as the
"Almagest," which was written in the second century of our era, and was regarded for
fourteen centuries as the final authority on all questions of astronomy.
Such was the system of Astronomy which prevailed during the Middle Ages, and was
only discredited at an epoch nearly simultaneous with that of the discovery of the New
World by Columbus. The true arrangement of the solar system was then expounded by
Copernicus in the great work to which he devoted his life. The first principle
established by these labours showed the diurnal movement of the heavens to be due to
the rotation of the earth on its axis. Copernicus pointed out the fundamental difference
between real motions and apparent motions; he proved that the appearances presented
in the daily rising and setting of the sun and the stars could be accounted for by the
supposition that the earth rotated, just as satisfactorily as by the more cumbrous
supposition of Ptolemy. He showed, moreover, that the latter supposition must
attribute an almost infinite velocity to the stars, so that the rotation of the entire
universe around the earth was clearly a preposterous supposition. The second great
principle, which has conferred immortal glory on Copernicus, assigned to the earth its
true position in the universe. Copernicus transferred the centre, about which all the
planets revolve, from the earth to the sun; and he established the[Pg 8] somewhat
humiliating truth, that our earth is merely a planet pursuing a track between the paths
of Venus and of Mars, and subordinated like all the other planets to the supreme sway
of the Sun.
This great revolution swept from astronomy those distorted views of the earth's
importance which arose, perhaps not unnaturally, from the fact that we happen to be
domiciled on that particular planet. The achievements of Copernicus were soon to be

followed by the invention of the telescope, that wonderful instrument by which the
modern science of astronomy has been created. To the consideration of this important
subject we shall devote the first chapter of our book.
PLATE II.
A TYPICAL SUN-SPOT.
(AFTER LANGLEY.)

[Pg 9]
CHAPTER I.
THE ASTRONOMICAL OBSERVATORY.
Early Astronomical Observations—The Observatory of Tycho Brahe—The Pupil of
the Eye—Vision of Faint Objects—The Telescope—The Object-Glass—Advantages
of Large Telescopes—The Equatorial—The Observatory—The Power of a
Telescope—Reflecting Telescopes—Lord Rosse's Great Reflector at Parsonstown—
How the mighty Telescope is used—Instruments of Precision—The Meridian
Circle—The Spider Lines—Delicacy of pointing a Telescope—Precautions necessary
in making Observations—The Ideal Instrument and the Practical One—The
Elimination of Error—Greenwich Observatory—The ordinary Opera-Glass as an
Astronomical Instrument—The Great Bear—Counting the Stars in the
Constellation—How to become an Observer.
The earliest rudiments of the Astronomical Observatory are as little known as the
earliest discoveries in astronomy itself. Probably the first application of instrumental
observation to the heavenly bodies consisted in the simple operation of measuring the
shadow of a post cast by the sun at noonday. The variations in the length of this
shadow enabled the primitive astronomers to investigate the apparent movements of
the sun. But even in very early times special astronomical instruments were employed
which possessed sufficient accuracy to add to the amount of astronomical knowledge,
and displayed considerable ingenuity on the part of the designers.
Professor Newcomb[2] thus writes: "The leader was Tycho Brahe, who was born in
1546, three years after the death of Copernicus. His attention was first directed to the

study of astronomy by an eclipse of the sun on August 21st, 1560, which was total in
some parts of Europe. Astonished that such a phenomenon could be predicted, he
devoted himself to a study of the methods of observation and calculation by[Pg 10]
which the prediction was made. In 1576 the King of Denmark founded the celebrated
observatory of Uraniborg, at which Tycho spent twenty years assiduously engaged in
observations of the positions of the heavenly bodies with the best instruments that
could then be made. This was just before the invention of the telescope, so that the
astronomer could not avail himself of that powerful instrument. Consequently, his
observations were superseded by the improved ones of the centuries following, and
their celebrity and importance are principally due to their having afforded Kepler the
means of discovering his celebrated laws of planetary motion."
The direction of the telescope to the skies by Galileo gave a wonderful impulse to the
study of the heavenly bodies. This extraordinary man is prominent in the history of
astronomy, not alone for his connection with this supreme invention, but also for his
achievements in the more abstract parts of astronomy. He was born at Pisa in 1564,
and in 1609 the first telescope used for astronomical observation was constructed.
Galileo died in 1642, the year in which Newton was born. It was Galileo who laid
with solidity the foundations of that science of Dynamics, of which astronomy is the
most splendid illustration; and it was he who, by promulgating the doctrines taught by
Copernicus, incurred the wrath of the Inquisition.
The structure of the human eye in so far as the exquisite adaptation of the pupil is
concerned presents us with an apt illustration of the principle of the telescope. To see
an object, it is necessary that the light from it should enter the eye. The portal through
which the light is admitted is the pupil. In daytime, when the light is brilliant, the iris
decreases the size of the pupil, and thus prevents too much light from entering. At
night, or whenever the light is scarce, the eye often requires to grasp all it can. The
pupil then expands; more and more light is admitted according as the pupil grows
larger. The illumination of the image on the retina is thus effectively controlled in
accordance with the requirements of vision.
A star transmits to us its feeble rays of light, and from[Pg 11] those rays the image is

formed. Even with the most widely-opened pupil, it may, however, happen that the
image is not bright enough to excite the sensation of vision. Here the telescope comes
to our aid: it catches all the rays in a beam whose original dimensions were far too
great to allow of its admission through the pupil. The action of the lenses concentrates
those rays into a stream slender enough to pass through the small opening. We thus
have the brightness of the image on the retina intensified. It is illuminated with nearly
as much light as would be collected from the same object through a pupil as large as
the great lenses of the telescope.
Fig. 1.—Principle of the Refracting Telescope.
In astronomical observatories we employ telescopes of two entirely different classes.
The more familiar forms are those known as refractors, in which the operation of
condensing the rays of light is conducted by refraction. The character of the refractor
is shown in Fig. 1. The rays from the star fall upon the object-glass at the end of the
telescope, and on passing through they become refracted into a converging beam, so
that all intersect at the focus. Diverging from thence, the rays encounter the eye-piece,
which has the effect of restoring them to parallelism. The large cylindrical beam
which poured down on the object-glass has been thus condensed into a small one,
which can enter the pupil. It should, however, be added that the composite nature of
light requires a more complex form of object-glass than the simple lens here shown. In
a refracting telescope we have to employ what is known as the achromatic
combination, consisting of one lens of flint glass and one of crown glass, adjusted to
suit each other with extreme care.
[Pg 12]
Fig. 2.—The Dome of the
South Equatorial at Dunsink Observatory Co Dublin.
[Pg 13]
Fig. 3.—Section of the
Dome of Dunsink Observatory.
The appearance of an astronomical observatory, designed to accommodate an
instrument of moderate dimensions, is shown in the adjoining figures. The first (Fig.

2) represents the dome erected at Dunsink Observatory for the equatorial telescope,
the object-glass of which was presented to the Board of Trinity College, Dublin, by
the late Sir James South. The main part of the building is a cylindrical wall, on the top
of which reposes a hemispherical roof. In this roof is a shutter, which can be opened
so as to allow the telescope in the interior to obtain a view of the heavens. The dome
is capable of revolving so that the opening may be turned towards that part of the sky
where the object happens to be situated. The next view (Fig. 3) exhibits a section
through the dome, showing the machinery by which the attendant causes it to revolve,
as well as the telescope itself. The eye of the observer is placed at the eye-piece, and
he is represented in the act of turning a handle, which has the power of slowly moving
the telescope, in order to adjust the instrument accurately on the celestial body which
it is desired to observe. The two lenses which together form the object-glass of this
instrument are twelve inches in diameter, and the quality of the telescope mainly
depends on the accuracy with which[Pg 14] these lenses have been wrought. The eye-
piece is a comparatively simple matter. It consists merely of one or two small lenses;
and various eye-pieces can be employed, according to the magnifying power which
may be desired. It is to be observed that for many purposes of astronomy high
magnifying powers are not desirable. There is a limit, too, beyond which the
magnification cannot be carried with advantage. The object-glass can only collect a
certain quantity of light from the star; and if the magnifying power be too great, this
limited amount of light will be thinly dispersed over too large a surface, and the result
will be found unsatisfactory. The unsteadiness of the atmosphere still further limits the
extent to which the image may be advantageously magnified, for every increase of
power increases in the same degree the atmospheric disturbance.
A telescope mounted in the manner here shown is called an equatorial. The
convenience of this peculiar style of supporting the instrument consists in the ease
with which the telescope can be moved so as to follow a star in its apparent journey
across the sky. The necessary movements of the tube are given by clockwork driven
by a weight, so that, once the instrument has been correctly pointed, the star will
remain in the observer's field of view, and the effect of the apparent diurnal movement

will be neutralised. The last refinement in this direction is the application of an
electrical arrangement by which the driving of the instrument is controlled from the
standard clock of the observatory.
[Pg 15]