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Transcriber’s Notes

Obvious typographical errors have been silently corrected. Variations
in hyphenation have been standardised but all other spelling and
punctuation remains unchanged.

Footnotes are placed at the end of chapter.

Italics are represented thus _italic_, and superscripts thus ^.

The periods of the satellites of Uranus have been added to the table as
specified in a subsequent note.

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[Illustration:
LONDON STEREOSCOPIC CO.      PHOTOMEZZOTYPE.

STANMORE OBSERVATORY.

INSIDE VIEW.]




                            TELESCOPIC WORK

                                  FOR

                          STARLIGHT EVENINGS.

                                  BY

                     WILLIAM F. DENNING, F.R.A.S.
      (FORMERLY PRESIDENT OF THE LIVERPOOL ASTRONOMICAL SOCIETY).

  “To ask or search I blame thee not, for heaven
  Is as the book of God before thee set,
  Wherein to read his wondrous works.”
                                     MILTON.

                                LONDON:
           TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET.
                                 1891.

                       [_All rights reserved._]


                    [Illustration: ALERE FLAMMAM.]

                    PRINTED BY TAYLOR AND FRANCIS,
                     RED LION COURT, FLEET STREET.




PREFACE.


It having been suggested by some kind friends that a series of articles
on “Telescopes and Telescopic Work,” which I wrote for the ‘Journal of
the Liverpool Astronomical Society’ in 1887-8, should be reprinted, I
have undertaken the revision and rearrangement of the papers alluded
to. Certain other contributions on “Large and Small Telescopes,”
“Planetary Observations,” and kindred subjects, which I furnished to
‘The Observatory’ and other scientific serials from time to time, have
also been included, and the material so much altered and extended that
it may be regarded as virtually new matter. The work has outgrown my
original intention, but it proved so engrossing that it was found
difficult to ensure greater brevity.

The combination of different papers has possibly had the effect of
rendering the book more popular in some parts than in others. This
is not altogether unintentional, for the aim has been to make the
work intelligible to general readers, while also containing facts
and figures useful to amateur astronomers. It is merely intended as a
contribution to popular astronomy, and asserts no rivalry with existing
works, many of which are essentially different in plan. If any excuse
were, however, needed for the issue of this volume it might be found
in the rapid progress of astronomy, which requires that new or revised
works should be published at short intervals in order to represent
existing knowledge.

The methods explained are approximate, and technical points have
been avoided with the view to engage the interest of beginners who
may find it the stepping-stone to more advanced works and to more
precise methods. The object will be realized if observers derive any
encouragement from its descriptions or value from its references,
and the author sincerely hopes that not a few of his readers will
experience the same degree of pleasure in observation as he has done
during many years.

No matter how humble the observer, or how paltry the telescope,
astronomy is capable of furnishing an endless store of delight to its
adherents. Its influences are elevating, and many of its features
possess the charms of novelty as well as mystery. Whoever contemplates
the heavens with the right spirit reaps both pleasure and profit, and
many amateurs find a welcome relaxation to the cares of business in
the companionship of their telescopes on “starlight evenings.”

The title chosen is not, perhaps, a comprehensive one, but it covers
most of the ground, and no apology need be offered for dealing with one
or two important objects not strictly within its scope.

For many of the illustrations I must express my indebtedness to the
Editors of the ‘Observatory’ to the Council of the R.A.S., to the
proprietors of ‘Nature,’ to Messrs. Browning, Calver, Cooke & Sons,
Elger, Gore, Horne Thornthwaite and Wood, Klein, and other friends.

The markings on Venus and Jupiter as represented on pages 150 and 180
have come out much darker than was intended, but these illustrations
may have some value as showing the position and form of the features
delineated. It is difficult to reproduce delicate planetary markings
in precisely the same characters as they are displayed in a good
telescope. The apparent orbits of the satellites of the planets,
delineated in figs. 41, 44, &c., are liable to changes depending on
their variable position relatively to the Earth, and the diagrams are
merely intended to give a good idea of these satellite systems.

  W. F. D.

  Bishopston, Bristol,
  1891.


Plates I. and II. are views of the Observatory and Instruments recently
erected by Mr. Klein at Stanmore, Middlesex, lat. 51° 36′ 57″ N.,
long. 0° 18′ 22″ W. The height above sea-level is 262 feet. The
telescope is a 20-inch reflector by Calver, of 92 inches focus; the
tube is, however, 152 inches long so as to cut off all extraneous
rays. It is mounted equatoreally, and is provided with a finder of
6 inches aperture—one of Tulley’s famous instruments a century ago.
The large telescope is fixed on a pillar of masonry 37 feet high, and
weighing 115 tons. Mr. Klein proposes to devote the resources of his
establishment to astronomical photography, and it has been provided
with all the best appliances for this purpose. The observatory is
connected by telephone with Mr. Klein’s private residence, and the
timepieces and recording instruments are all electrically connected
with a centre of observation in his study.




CONTENTS.


  CHAPTER I.
                                                                    Page

  THE TELESCOPE, ITS INTENTION AND THE DEVELOPMENT OF ITS POWERS       1


  CHAPTER II.

  RELATIVE MERITS OF LARGE AND SMALL TELESCOPES                       20


  CHAPTER III.

  NOTES ON TELESCOPES AND THEIR ACCESSORIES                           38


  CHAPTER IV.

  NOTES ON TELESCOPIC WORK                                            66


  CHAPTER V.

  THE SUN                                                             87


  CHAPTER VI.

  THE MOON                                                           113


  CHAPTER VII.

  MERCURY                                                            137


  CHAPTER VIII.

  VENUS                                                              145


  CHAPTER IX.

  MARS                                                               155


  CHAPTER X.

  THE PLANETOIDS                                                     167


  CHAPTER XI.

  JUPITER                                                            170


  CHAPTER XII.

  SATURN                                                             195


  CHAPTER XIII.

  URANUS AND NEPTUNE                                                 215


  CHAPTER XIV.

  COMETS AND COMET-SEEKING                                           227


  CHAPTER XV.

  METEORS AND METEORIC OBSERVATIONS                                  260


  CHAPTER XVI.

  THE STARS                                                          286


  CHAPTER XVII.

  NEBULÆ AND CLUSTERS OF STARS                                       324


  NOTES AND ADDITIONS                                                347

  INDEX                                                              353




ILLUSTRATIONS.


  PLATE I. Interior of Mr. Klein’s Observatory            _Frontispiece_

       II. View of Mr. Klein’s Grounds and Observatory   _To face_ p. 82

  FIG.                                                              PAGE

   1. The Galilean Telescope                                           7

   2. Royal Observatory, Greenwich, in Flamsteed’s time                8

   3. Sir Isaac Newton                                                10

   4. Gregorian Telescope                                             10

   5. Cassegrainian Telescope                                         11

   6. Newtonian Telescope                                             11

   7. Common Refracting-Telescope                                     12

   8. Le Mairean or Herschelian Telescope                             13

   9. 10-inch Reflecting-Telescope on a German Equatoreal, by Calver  17

  10. Lord Rosse’s 6-foot Reflecting-Telescope                        22

  11. Refracting-Telescope, by Browning                               32

  12. “The Popular Reflector,” by Calver                              40

  13. 3-inch Refracting-Telescope, by Newton & Co.                    41

  14. Huygens’s Negative Eyepiece                                     46

  15. Ramsden’s Positive Eyepiece                                     47

  16. Berthon’s Dynamometer                                           50

  17. Cooke and Sons’ Educational Telescope                           52

  18. Refracting-Telescope on a German Equatoreal                     67

  19. The Author’s Telescope: a 10-inch With-Browning Reflector       77

  20. Sun-spot of June 19, 1889                                       95

  21. Solar Eclipses visible in England, 1891 to 1922                 98

  22. Total Solar Eclipse of August 19, 1887                          98

  23. Belts of Sun-spots, visible Oct. 29, 1868                      104

  24. Shadows cast by Faculæ                                         109

  25. Light-spots and streaks on Plato, 1879-82. (A. Stanley
        Williams.)                                                   126

  26. Petavius and Wrottesley at Sunset. (T. Gwyn Elger.)            129

  27. Birt, Birt A, and the Straight Wall. (T. Gwyn Elger.)          130

  28. Aristarchus and Herodotus at Sunrise. (T. Gwyn Elger.)         132

  29. Mercury as a Morning Star                                      143

  30. Venus as an Evening Star                                       150

  31. Mars, 1886, April 13, 9^h 50^m                                 157

  32. Orbits of the Satellites of Mars                               159

  33. Jupiter, as drawn by Dawes and others                          178

  34. Jupiter, 1886, April 9, 10^h 12^m                              180

  35. Occultation of Jupiter, Aug. 7, 1889                           186

  36. Jupiter and Satellites seen in a small glass                   187

  37. Shadows of Jupiter’s Satellites II. and III.                   192

  38. Saturn as observed by Cassini in August 1676                   198

  39. Saturn, 1885, Dec. 23, 7^h 54^m                                201

  40. Saturn as observed by F. Terby, February 1887                  203

  41. Apparent orbits of the Five Inner Satellites of Saturn         212

  42. Transit of the Shadow of Titan                                 213

  43. Uranus and his belts                                           218

  44. Apparent orbits of the Satellites of Uranus                    221

  45. Apparent orbit of the Satellite of Neptune                     224

  46. Mars, Saturn, and Regulus in same field, Sept. 20, 1889        226

  47. Comet 1862 III. (Aug. 19, 1862)                                237

  48. Sawerthal’s Comet, 1888 I. (March 25, Brooks)                  237

  49. Brooks’s Double Comet, Sept. 17, 1889                          239

  50. Pons’s Comet (1812). Telescopic view, 1884, Jan. 6             242

  51. Ditto. Ditto, 1884, Jan. 21                                    242

  52. Radiation of Meteors. (Shower of early Perseids, 1878)         263

  53. Double Meteor. Curved Meteor. Fireball                         265

  54. Meteorite found in Chili in 1866                               265

  55. Meteorite which fell at Orgueil in 1864                        265

  56. Fireball of Nov. 23, 1877, 8^h 24^m (J. Plant.)                269

  57. Flight of Telescopic Meteors seen by W. R. Brooks              272

  58. Meteor of Dec. 28, 1888, 6^h 17^m                              277

  59. Large Meteor and streak seen at Jask                           278

  60. The Constellation Orion                                        289

  61. Diagram illustrating the Measurement of Angles of Position     291

  62. Double Stars                                                   301

  63. Trapezium in Orion as seen with the 36-inch refractor          319

  64. Nebulæ and a Star-cluster                                      336

  65. Nebula within a semicircle of stars                            342




                            TELESCOPIC WORK

                                  FOR

                          STARLIGHT EVENINGS.




CHAPTER I.

_THE TELESCOPE, ITS INVENTION AND THE DEVELOPMENT OF ITS POWERS._


The instrument which has so vastly extended our knowledge of the
Universe, which has enabled us to acquire observations of remarkable
precision, and supplied the materials for many sublime speculations in
Astronomy, was invented early in the seventeenth century. Apart from
its special application as a means of exploring the heavens with a
capacity that is truly marvellous, it is a construction which has also
been utilized in certain other departments with signal success. It
provided mankind with a medium through which to penetrate far beyond
the reach of natural vision, and to grasp objects and phenomena which
had either eluded detection altogether or had only been seen in dim and
uncertain characters. It has also proved a very efficient instrument
for various minor purposes of instruction and recreation. The invention
of the telescope formed a new era in astronomy; and though, with a
few exceptions, men were slow at first in availing themselves of its
far-seeing resources, scepticism was soon swept aside and its value
became widely acknowledged.

But though the telescope was destined to effect work of the utmost
import, and to reach a very high degree of excellence in after
times, the result was achieved gradually. Step by step its powers
were enlarged and its qualities perfected, and thus the stream of
astronomical discovery has been enabled to flow on, stimulated by every
increase in its capacity.

There is some question as to whom may be justly credited with
the discovery of its principles of construction. Huygens, in his
‘Dioptrics,’ remarks:—“I should have no hesitation in placing above all
the rest of mankind the individual who, solely by his own reflections,
without the aid of any fortuitous circumstances, should have achieved
the invention of the telescope.” There is reason to conclude, however,
that its discovery resulted from accident rather than from theory. It
is commonly supposed that Galileo Galilei is entitled to precedence;
but there is strong evidence to show that he had been anticipated.
In any case it must be admitted that Galilei[1] had priority in
successfully utilizing its resources as a means of observational
discovery; for he it was who, first of all men, saw Jupiter’s
satellites, the crescent form of Venus, the mountains and craters on
the Moon, and announced them to an incredible world.

It has been supposed, and not without some basis of probability, that a
similar instrument to the telescope had been employed by the ancients;
for certain statements contained in old historical records would
suggest that the Greek philosophers had some means of extending their
knowledge further than that permitted by the naked eye. Democritus
remarked that the Galaxy or “Milky Way” was nothing but an assemblage
of minute stars; and it has been asked, How could he have derived this
information but by instrumental aid? It is very probable he gained the
knowledge by inferences having their source in close observation; for
anyone who attentively studies the face of the sky must be naturally
led to conclude that the appearance of the “Milky Way” is induced by
immense and irregular clusterings of small stars. In certain regions
of the heavens there are clear indications of this: the eye is enabled
to glimpse some of the individual star-points, and to observe how they
blend and associate with the denser aggregations which give rise to the
milky whiteness of the Galaxy.

Refracting lenses, or “burning-glasses,” were known at a very early
period. A lens, roughly figured into a convex shape and obviously
intended for magnifying objects, has been recovered from the ruins
of Herculaneum, buried in the ejections from Vesuvius in the year 79
A.D. Pliny and others refer to lenses that burnt by refraction, and
describe globules of glass or crystal which, when exposed in the sun,
transmit sufficient heat to ignite combustible material. The ancients
undoubtedly used tubes in the conduct of their observations, but no
lenses seem to have been employed with them, and their only utility
consisted in the fact of their shutting out the extraneous rays of
light. But spectacles were certainly known at an early period. Concave
emeralds are said to have been employed by Nero in witnessing the
combats of the gladiators, and they appear to have been the same in
effect as the spectacles worn by short-sighted people in our own times.
But the ancients supposed that the emerald possessed inherent qualities
specially helpful to vision, rather than that its utility resulted
simply from its concavity of figure. In the 13th century spectacles
were more generally worn, and the theory of their construction
understood.

It is remarkable that the telescope did not come into use until so
long afterwards. Vague references were made to such an instrument, or
rather suggestions as to the possibilities of its construction, which
show that, although the principle had perhaps been conceived, the idea
was not successfully put into practice. Roger Bacon, who flourished in
the 13th century, wrote in his