The Story Of The Heavens by Sir Robert Stawell Ball

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Title: The Story of the Heavens

Author: Robert Stawell Ball

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THE STORY OF THE HEAVENS

PLATE I. THE PLANET SATURN, IN 1872.

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

C A S S E L L and C O MPA N Y, 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

"T

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

THE

STORY OF THE HEAVENS.

he 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

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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 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, 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 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

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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 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 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 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

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which all the planets revolve, from the earth to the sun; and he established the 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.)

PLATE II.

A TYPICAL SUN-SPOT.

(AFTER LANGLEY.)

CHAPTER I.

THE ASTRONOMICAL OBSERVATORY.

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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 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 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.

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.

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Fig. 1.—Principle of the
Refracting Telescope.

Fig. 2.—The Dome of the South Equatorial at Dunsink

Observatory Co Dublin.

Fig. 3.—Section of the Dome of Dunsink Observatory.

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

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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 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.

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Fig. 4.—The Telescope at Yerkes Observatory, Chicago.

Fig. 4.—The Telescope at Yerkes Observatory, Chicago.

(From the Astrophysical Journal, Vol. vi., No. 1.)

The power of a refracting telescope—so far as the expression has any definite meaning—is to be measured by the

diameter of its object-glass. There has, indeed, been some honourable rivalry between the various civilised nations as to
which should possess the greatest refracting telescope. Among the notable instruments that have been successfully
completed is that erected in 1881 by Sir Howard Grubb, of Dublin, at the splendid observatory at Vienna. Its
dimensions may be estimated from the fact that the object-glass is two feet and three inches in diameter. Many
ingenious contrivances help to lessen the inconvenience incident to the use of an instrument possessing such vast
proportions. Among them we may here notice the method by which the graduated circles attached to the telescope are
brought within view of the observer. These circles are necessarily situated at parts of the instrument which lie remote
from the eye-piece where the observer is stationed. The delicate marks and figures are, however, easily read from a
distance by a small auxiliary telescope, which, by suitable reflectors, conducts the rays of light from the circles to the eye
of the observer.

Numerous refracting telescopes of exquisite perfection have been produced by Messrs. Alvan Clark, of

Cambridgeport, Boston, Mass. One of their most famous telescopes is the great Lick Refractor now in use on Mount
Hamilton in California. The diameter of this object-glass is thirty-six inches, and its focal length is fifty-six feet two
inches. A still greater effort has recently been made by the same firm in the refractor of forty inches aperture for the
Yerkes Observatory of the University of Chicago. The telescope, which is seventy-five feet in length, is mounted under

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Fig. 5.—Principle of Herschel's

Refracting Telescope.

a revolving dome ninety feet in diameter, and in order to enable the
observer to reach the eye-piece without using very large step-ladders, the
floor of the room can be raised and lowered through a range of twenty-two
feet by electric motors. This is shown in Fig. 4, while the south front of the
Yerkes Observatory is represented in Fig. 6.

Fig. 6.—South Front of the Yerkes Observatory, Chicago.

Fig. 6.—South Front of the Yerkes Observatory, Chicago.

(From the Astrophysical Journal, Vol. vi., No. 1.)

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