THROUGH THE TELESCOPE
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[Illustration:
PLATE I.
The 40-inch Refractor of the Yerkes Observatory.]
THROUGH
THE TELESCOPE
BY
JAMES BAIKIE, F.R.A.S.
WITH 32 FULL-PAGE ILLUSTRATIONS FROM PHOTOGRAPHS
AND 26 SMALLER FIGURES IN THE TEXT
[Illustration]
LONDON
ADAM AND CHARLES BLACK
1906
TO
C. N. B. AND H. E. B.
PREFACE
The main object of the following chapters is to give a brief and
simple description of the most important and interesting facts
concerning the heavenly bodies, and to suggest to the general reader
how much of the ground thus covered lies open to his personal survey
on very easy conditions. Many people who are more or less interested
in astronomy are deterred from making practical acquaintance with the
wonders of the heavens by the idea that these are only disclosed to
the possessors of large and costly instruments. In reality there is
probably no science which offers to those whose opportunities and
means of observation are restricted greater stores of knowledge and
pleasure than astronomy; and the possibility of that quickening of
interest which can only be gained by practical study is, in these
days, denied to very few indeed.
Accordingly, I have endeavoured, while recounting the great triumphs
of astronomical discovery, to give some practical help to those who
are inclined to the study of the heavens, but do not know how to
begin. My excuse for venturing on such a task must be that, in the
course of nearly twenty years of observation with telescopes of all
sorts and sizes, I have made most of the mistakes against which others
need to be warned.
The book has no pretensions to being a complete manual; it is merely
descriptive of things seen and learned. Nor has it any claim to
originality. On the contrary, one of its chief purposes has been to
gather into short compass the results of the work of others. I have
therefore to acknowledge my indebtedness to other writers, and notably
to Miss Agnes Clerke, Professor Young, Professor Newcomb, the late
Rev. T. W. Webb, and Mr. W. F. Denning. I have also found much help in
the _Monthly Notices_ and _Memoirs_ of the Royal Astronomical
Society, and the _Journal_ and _Memoirs_ of the British Astronomical
Association.
The illustrations have been mainly chosen with the view of
representing to the general reader some of the results of the best
modern observers and instruments; but I have ventured to reproduce a
few specimens of more commonplace work done with small telescopes. I
desire to offer my cordial thanks to those who have so kindly granted
me permission to reproduce illustrations from their published works,
or have lent photographs or drawings for reproduction--to Miss Agnes
Clerke for Plates XXV.-XXVIII. and XXX.-XXXII. inclusive; to
Mrs. Maunder for Plate VIII.; to M. Loewy, Director of the Paris
Observatory, for Plates XI.-XIV. and Plate XVII.; to Professor E. B.
Frost, Director of the Yerkes Observatory, for Plates I., VII., XV.,
and XVI.; to M. Deslandres, of the Meudon Observatory, for Plate IX.,
and the gift of several of his own solar memoirs; to the Astronomer
Royal for England, Sir W. Mahony Christie, for Plate V.; to Mr. H.
MacEwen for the drawings of Venus, Plate X.; to the Rev. T. E. R.
Phillips for those of Mars and Jupiter, Plates XX. and XXII.; to
Professor Barnard for that of Saturn, Plate XXIV., reproduced by
permission from the _Monthly Notices_ of the Royal Astronomical
Society; to Mr. W. E. Wilson for Plates XXIX. and XXXII.; to Mr. John
Murray for Plates XVIII. and XIX.; to the proprietors of _Knowledge_
for Plate VI.; to Mr. Denning and Messrs. Taylor and Francis for Plate
III. and Figs. 6 and 20; to the British Astronomical Association for
the chart of Mars, Plate XXI., reproduced from the _Memoirs_; and to
Messrs. T. Cooke and Sons for Plate II. For those who wish to see for
themselves some of the wonders and beauties of the starry heavens
the two Appendices furnish a few specimens chosen from an innumerable
company; while readers who have no desire to engage in practical work
are invited to skip Chapters I. and II.
CONTENTS
CHAPTER PAGE
I. THE TELESCOPE--HISTORICAL 1
II. THE TELESCOPE--PRACTICAL 14
III. THE SUN 47
IV. THE SUN'S SURROUNDINGS 68
V. MERCURY 81
VI. VENUS 89
VII. THE MOON 100
VIII. MARS 130
IX. THE ASTEROIDS 148
X. JUPITER 154
XI. SATURN 172
XII. URANUS AND NEPTUNE 190
XIII. COMETS AND METEORS 203
XIV. THE STARRY HEAVENS 230
XV. CLUSTERS AND NEBULÆ 256
APPENDIX I.: LIST OF LUNAR FORMATIONS 273
APPENDIX II.: LIST OF OBJECTS FOR THE TELESCOPE 278
INDEX 285
LIST OF ILLUSTRATIONS
PRINTED SEPARATELY FROM THE TEXT
PLATE _To face page_
I. The 40-inch Refractor of the Yerkes Observatory
[_Frontispiece_
II. Six-inch Photo-Visual Refractor, equatorially mounted 30
III. Twenty-inch Reflector, Stanmore Observatory 36
IV. Telescope House and 8-1/2-inch 'With' Reflector 38
V. The Sun, February 3, 1905. Royal Observatory,
Greenwich 48
VI. Photograph of Bridged Sunspot (Janssen). _Knowledge_,
February, 1890 50
VII. Solar Surface with Faculæ. Yerkes Observatory 60
VIII. Coronal Streamers: Eclipse of 1898. From Photographs
by Mrs. Maunder 70
IX. The Chromosphere and Prominences, April 11, 1894.
Photographed by M. H. Deslandres 74
X. Venus. H. MacEwen. Five-inch Refractor 94
XI. The Moon, April 5, 1900. Paris Observatory 102
XII. The Moon, November 13, 1902. Paris Observatory 108
XIII. The Moon, September 12, 1903. Paris Observatory 110
XIV. Region of Maginus: Overlapping Craters. Paris
Observatory 112
XV. Clavius, Tycho, and Mare Nubium. Yerkes Observatory 114
XVI. Region of Theophilus and Altai Mountains. Yerkes
Observatory 116
XVII. Apennines, Alps, and Caucasus. Paris Observatory 118
XVIII. Chart of the Moon. Nasmyth and Carpenter }
} 124
XIX. Key to Chart of Moon. Nasmyth and Carpenter }
XX. Mars: Drawing 1, January 30, 1899--12 hours.
Drawing 2, April 22, 1903--10 hours 134
XXI. Chart of Mars. Memoirs of the British Astronomical
Association, Vol. XI., Part III., Plate VI. 138
XXII. Jupiter, January 6, 1906--8 hours 20 minutes.
Instrument, 9-1/4-inch Reflector 158
XXIII. Jupiter, February 17, 1906. J. Baikie,
18-inch Reflector 166
XXIV. Saturn, July 2, 1894. E. E. Barnard,
36-inch Equatorial 172
XXV. Great Comet. Photographed May 5, 1901, with the
13-inch Astrographic Refractor of the Royal
Observatory, Cape of Good Hope 210
XXVI. Photographs of Swift's Comet. By Professor E. E.
Barnard 220
XXVII. Region of the Milky Way in Sagittarius, showing
a Double Black Aperture. Photographed by Professor
E. E. Barnard 232
XXVIII. Irregular Star Clusters. Photographed by E. E.
Barnard 256
XXIX. Cluster M. 13 Herculis. Photographed by Mr. W. E.
Wilson 258
XXX. Photograph of the Orion Nebula (W. H. Pickering) 262
XXXI. Photographs of Spiral Nebulæ. By Dr. Max Wolf 264
XXXII. Photograph of Whirlpool Nebula (M. 51). Taken by
Mr. W. E. Wilson, March 6, 1897 266
LIST OF ILLUSTRATIONS
PRINTED IN THE TEXT
FIG. PAGE
1. Principle of Galilean Telescope 3
2. Principle of Common Refractor 3
3. Dorpat Refractor 7
4. Thirty-inch Refractor, Pulkowa Observatory 9
5. Principle of Newtonian Reflector 11
6. Lord Rosse's Telescope 12
7. Herschel's 4-foot Reflector 13
8. Star--Correct and Incorrect Adjustment 21
9. Small Telescope on Pillar and Claw Stand 26
10. Telescope on Tripod, with Finder and Slow Motions 27
11. Equatorial Mounting for Small Telescope 29
12. Eight-inch Refractor on Equatorial Mounting 32
13. Four-foot Reflector, equatorially mounted 36
14. Drawing of Sunspot 52
15. " " 53
16. " " 56
17. " " 57
18. " " 58
19. Eclipses of the Sun and Moon 69
20. Mercury as a Morning Star. W. F. Denning, 10-inch Reflector 84
21. The Tides 101
22. Lunar Craters 105
23. " " 118
24. Mars 146
25. Jupiter 157
26. Saturn 183
THROUGH THE TELESCOPE
CHAPTER I
THE TELESCOPE--HISTORICAL
The claim of priority in the invention of this wonderful instrument,
which has so enlarged our ideas of the scale and variety of
the universe, has been warmly asserted on behalf of a number of
individuals. Holland maintains the rights of Jansen, Lippershey, and
Metius; while our own country produces evidence that Roger Bacon
had, in the thirteenth century, 'arrived at theoretical proof of the
possibility of constructing a telescope and a microscope' and that
Leonard Digges 'had a method of discovering, by perspective glasses
set at due angles, all objects pretty far distant that the sun shone
on, which lay in the country round about.'
All these claims, however, whether well or ill founded, are very
little to the point. The man to whom the human race owes a debt of
gratitude in connection with any great invention is not necessarily he
who, perhaps by mere accident, may stumble on the principle of it, but
he who takes up the raw material of the invention and shows the full
powers and possibilities which are latent in it. In the present
case there is one such man to whom, beyond all question, we owe the
telescope as a practical astronomical instrument, and that man is
Galileo Galilei. He himself admits that it was only after hearing, in
1609, that a Dutchman had succeeded in making such an instrument,
that he set himself to investigate the matter, and produced telescopes
ranging from one magnifying but three diameters up to the one with a
power of thirty-three with which he made his famous discoveries; but
this fact cannot deprive the great Italian of the credit which is
undoubtedly his due. Others may have anticipated him in theory, or
even to a small extent in practice, but Galileo first gave to the
world the telescope as an instrument of real value in research.
The telescope with which he made his great discoveries was constructed
on a principle which, except in the case of binoculars, is now
discarded. It consisted of a double convex lens converging the rays
of light from a distant object, and of a double concave lens,
intercepting the convergent rays before they reach a focus, and
rendering them parallel again (Fig. 1). His largest instrument, as
already mentioned, had a power of only thirty-three diameters, and the
field of view was very small. A more powerful one can now be obtained
for a few shillings, or constructed, one might almost say, for a
few pence; yet, as Proctor has observed: 'If we regard the absolute
importance of the discoveries effected by different telescopes,
few, perhaps, will rank higher than the little tube now lying in the
Tribune of Galileo at Florence.'
[Illustration: FIG. 1.--PRINCIPLE OF GALILEAN TELESCOPE.]
Galileo's first discoveries with this instrument were made in 1610,
and it was not till nearly half a century later that any great
improvement in telescopic construction was effected. In the middle of
the seventeenth century Scheiner and Huygens made telescopes on the
principle, suggested by Kepler, of using two double convex lenses
instead of a convex and a concave, and the modern refracting telescope
is still constructed on essentially the same principle, though, of
course, with many minor modifications (Fig. 2).
[Illustration: FIG. 2.--PRINCIPLE OF COMMON REFRACTOR.]
The latter part of the seventeenth century witnessed the introduction
of telescopes on this principle of the most amazing length, the
increase in length being designed to minimize the imperfections which
a simple lens exhibits both in definition and in colour. Huygens
constructed one such telescope of 123 feet focal length, which he
presented to the Royal Society of London; Cassini, at Paris, used
instruments of 100 and 136 feet; while Bradley, in 1722, measured the
diameter of Venus with a glass whose focal length was 212-1/4 feet.
Auzout is said to have made glasses of lengths varying from 300 to
600 feet, but, as might have been expected, there is no record of any
useful observations having ever been made with these monstrosities. Of
course, these instruments differed widely from the compact and handy
telescopes with which we are now familiar. They were entirely without
tubes. The object-glass was fastened to a tall pole or to some high
building, and was painfully man[oe]uvred into line with the eye-piece,
which was placed on a support near the ground, by means of an
arrangement of cords. The difficulties of observation with these
unwieldy monsters must have been of the most exasperating type, while
their magnifying power did not exceed that of an ordinary modern
achromatic of, perhaps, 36 inches focal length. Cassini, for instance,
seems never to have gone beyond a power of 150 diameters, which might
be quite usefully employed on a good modern 3-inch refractor in
good air. Yet with such tools he was able to discover four of the
satellites of Saturn and that division in Saturn's ring which still
bears his name. Such facts speak volumes for the quality of the
observer. Those who are the most accustomed to use the almost perfect
products of modern optical skill will have the best conception of,
and the profoundest admiration for, the limitless patience and the
wonderful ability which enabled him to achieve such results with the
very imperfect means at his disposal.
The clumsiness and unmanageableness of these aerial telescopes quickly
reached a point which made it evident that nothing more was to be
expected of them; and attempts were made to find a method of combining
lenses, which might result in an instrument capable of bearing equal
or greater magnifying powers on a much shorter length. The chief
hindrance to the efficiency of the refracting telescope lies in the
fact that the rays of different colours which collectively compose
white light cannot be brought to one focus by any single lens. The red
rays, for example, have a different focal length from the blue, and
so any lens which brings the one set to a focus leaves a fringe of the
other outstanding around any bright object.
In 1729 Mr. Chester Moor Hall discovered a means of conquering this
difficulty, but his results were not followed up, and it was left for
the optician John Dollond to rediscover the principle some twenty-five
years later. By making the object-glass of the telescope double, the
one lens being of crown and the other of flint glass, he succeeded in
obtaining a telescope which gave a virtually colourless image.
This great discovery of the achromatic form of construction at
once revolutionized the art of telescope-making. It was found that
instruments of not more than 5 feet focal length could be constructed,
which infinitely surpassed in efficiency, as well as in handiness,
the cumbrous tools which Cassini had used; and Dollond's 5-foot
achromatics, generally with object-glasses of 3-3/4 inches diameter,
represented for a considerable time the acme of optical excellence.
Since the time of Dollond, the record of the achromatic refractor has
been one of continual, and, latterly, of very rapid progress. For
a time much hindrance was experienced from the fact that it proved
exceedingly difficult to obtain glass discs of any size whose purity
and uniformity were sufficient to enable them to pass the stringent
test of optical performance. In the latter part of the eighteenth
century, a 6-inch glass was considered with feelings of admiration,
somewhat similar to those with which we regard the Yerkes 40-inch
to-day; and when, in 1823, the Dorpat refractor of 9-6/10 inches was
mounted (Fig. 3), the astronomical world seemed to have the idea that
something very like finality had been reached. The Dorpat telescope
proved, however, to be only a milestone on the path of progress.
Before very long it was surpassed by a glass of 12 inches diameter,
which Sir James South obtained from Cauchoix of Paris, and which is
now mounted in the Dunsink Observatory, Dublin. This, in its turn,
had to give place to the fine instruments of 14·9 inches which were
figured by Merz of Munich for the Pulkowa and Cambridge (U.S.A.)
Observatories; and then there came a pause of a few years, which was
broken by Alvan Clark's completion of an 18-1/2-inch, an instrument
which earned its diploma, before ever it left the workshop of its
constructor, by the discovery of the companion to Sirius.
[Illustration: FIG. 3.--DORPAT REFRACTOR.]
The next step was made on our side of the Atlantic, and proved to be
a long and notable one, in a sense definitely marking out the boundary
line of the modern era of giant refractors. This was the completion,
by Thomas Cooke, of York, of a 25-inch instrument for the late Mr.
Newall. It did not retain for long its pride of place. The palm was
speedily taken back to America by Alvan Clark's construction of the
26-inch of the Washington Naval Observatory, with which Professor
Asaph Hall discovered in 1877 the two satellites of Mars. Then came
Grubb's 27-inch for Vienna; the pair of 30-inch instruments, by Clark
and Henry respectively, for Pulkowa (Fig. 4) and Nice; and at last the
instrument which has for a number of years been regarded as the finest
example of optical skill in the world, the 36-inch Clark refractor of
the Lick Observatory, California. Placed at an elevation of over 4,000
feet, and in a climate exceptionally well suited for astronomical
work, this fine instrument has had the advantage of being handled by a
very remarkable succession of brilliant observers, and has, since
its completion, been looked to as a sort of court of final appeal in
disputed questions. But America has not been satisfied even with
such an instrument, and the 40-inch Clark refractor of the Yerkes
Observatory is at present the last word of optical skill so far as
achromatics are concerned (Frontispiece). It is not improbable that it
may also be the last word so far as size goes, for the late Professor
Keeler's report upon its performance implies that in this splendid
telescope the limit of practicable size for object-glasses is being
approached. The star images formed by the great lens show indications
of slight flexure of the glass under its own weight as it is turned
from one part of the sky to another. It would be rash, however, to
say that even this difficulty will not be overcome. So many obstacles,
seemingly insuperable, have vanished before the astronomer's imperious
demand for 'more light,' and so many great telescopes, believed in
their day to represent the absolute culmination of the optical art,
are now mere commoners in the ranks where once they were supreme, that
it may quite conceivably prove that the great Yerkes refractor, like
so many of its predecessors, represents only a stage and not the end
of the journey.
[Illustration: FIG. 4.--30-INCH REFRACTOR, PULKOWA OBSERVATORY.]
Meanwhile, Sir Isaac Newton, considering, wrongly as the sequel
showed, that 'the case of the refractor was desperate,' set about
the attempt to find out whether the reflection of light by means
of suitably-shaped mirrors might not afford a substitute for the
refractor. In this attempt he was successful, and in 1671 presented to
the Royal Society the first specimen, constructed by his own hands, of
that form of reflecting telescope which has since borne his name. The
principle of the Newtonian reflector will be easily grasped from Fig.
5. The rays of light from the object under inspection enter the open
mouth of the instrument, and passing down the tube are converged by
the concave mirror AA towards a focus, before reaching which they
are intercepted by the small flat mirror BB, placed at an angle of
45 degrees to the axis of the tube, and are by it reflected into the
eye-piece E which is placed at the side of the instrument. In this
construction, therefore, the observer actually looks in a direction
at right angles to that of the object which he is viewing, a condition
which seems strange to the uninitiated, but which presents no
difficulties in practice, and is found to have several advantages,
chief among them the fact that there is no breaking of one's neck in
the attempt to observe objects near the zenith, the line of vision
being always horizontal, no matter what may be the altitude of the
object under inspection. Other forms of reflector have been devised,
and go by the names of the Gregorian, the Cassegrain, and the
Herschelian; but the Newtonian has proved itself the superior, and has
practically driven its rivals out of the field, though the Cassegrain
form has been revived in a few instances of late years, and is
particularly suited to certain forms of research.
[Illustration: FIG. 5.--PRINCIPLE OF NEWTONIAN REFLECTOR.]
[Illustration: FIG. 6.--LORD ROSSE'S TELESCOPE.]
At first the mirrors of reflecting telescopes were made of an alloy
known as speculum metal, which consisted of practically 4 parts of
copper to 1 of tin; but during the last half-century this metal has
been entirely superseded by mirrors made of glass ground to the proper
figure, and then polished and silvered on the face by a chemical
process. To the reflecting form of construction belong some of the
largest telescopes in the world, such as the Rosse 6-foot (metal
mirrors), Fig. 6, the Common 5-foot (silver on glass), the Melbourne
4-foot (metal mirrors, Cassegrain form), and the 5-foot constructed by
Mr. Ritchey for the Yerkes Observatory. Probably the most celebrated,
as it was also the first of these monsters, was the 4-foot telescope
of Sir William Herschel, made by himself on the principle which goes
by his name. It was used by him to some extent in the discoveries
which have made his name famous, and nearly everyone who has ever
opened an astronomical book is familiar with the engraving of the huge
40-foot tube, with its cumbrous staging, which Oliver Wendell Holmes
has so quaintly celebrated in 'The Poet at the Breakfast Table' (Fig.
7).
[Illustration: FIG. 7.--HERSCHEL'S 4-FOOT REFLECTOR.]