William F. Denning.

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TELESCOPIC WORK ***




Produced by Chris Curnow, Les Galloway and the Online
Distributed Proofreading Team at http://www.pgdp.net (This
file was produced from images generously made available
by The Internet Archive)





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.

The layout of several tables has been modified to maintain clarity
within wdth restrictions.




[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 ‘_Opus Majus_’:—“Greater things may
be performed by refracted light, for, from the foregoing principles,
follows easily that the greatest objects may be seen very small, the
remote very near, and _vice versâ_. For we can give transparent bodies
such form and position with respect to the eye and the object that the
rays are refracted and bent to where we like, so that we, under any
angle, see the objects near or far, and in that manner we can, at a
great distance, read the smallest letters, and we can count atoms and
sand-grains, on account of the greatness of the angle under which they
are seen.”

Fracastor, in a work published at Venice in 1538, states:—“If we look
through two eye-lenses, placed the one upon the other, everything
will appear larger and nearer.” He also says:—“There are made certain
eye-lenses of such a thickness that if the moon or any other celestial
body is viewed through them they appear to be so near that their
distance does not exceed that of the steeples of public buildings.”

In other writings will also be found intimations as to the important
action of lenses; and it is hardly accountable that a matter so
valuable in its bearings was allowed to remain without practical
issues. The progressive tendency and the faculty of invention must
indeed have been in an incipient stage, and contrasts strongly with the
singular avidity with which ideas are seized upon and realized in our
own day.

Many important discoveries have resulted from pure accident; and it
has been stated that the first _bonâ fide_ telescope had its origin in
the following incident:—The children of a spectacle-maker, Zachariah
Jansen, of Middleberg, in Zealand, were playing with some lenses, and
it chanced that they arranged two of them in such manner that, to their
astonishment, the weathercock of an adjoining church appeared much
enlarged and more distinct. Having mentioned the curious fact to their
father, he immediately turned it to account, and, by fixing two lenses
on a board, produced the first telescope!

This view of the case is, however, a very doubtful one, and the
invention may with far greater probability be attributed to Hans
Lippersheim in 1608. Galilei has little claim to be considered in this
relation; for he admitted that in 1609 the news reached him that a
Dutchman had devised an appliance capable of showing distant objects
with remarkable clearness. He thereupon set to work and experimented
with so much aptitude on the principles involved that he very soon
produced a telescope for himself. With this instrument he detected
the four satellites of Jupiter in 1610, and other successes shortly
followed. Being naturally gratified with the improvements he had
effected in its construction, and with the wonderful discoveries he had
made by its use, we can almost excuse the enthusiasm which prompted him
to attribute the invention to his own ingenuity. But while according
him the honour due to his sagacity in devoting this instrument to
such excellent work, we must not overlook the fact that his claim to
priority cannot be justified. Indeed, that Galilei had usurped the
title of inventor is mentioned in letters which passed between the
scientific men of that time. Fuccari, writing to Kepler, says:—“Galileo
wants to be considered the inventor of the telescope, though he, as
well as I and others, first saw the telescope which a certain Dutchman
first brought with him to Venice, and although he has only improved it
very little.”

In a critical article by Dr. Doberck[2], in which this letter is quoted
and the whole question reviewed with considerable care, it is stated
that Hans Lippersheim (also known as Jan Lapprey), who was born in
Wesel, but afterwards settled at Middleberg, in the Netherlands, as
a spectacle-maker, was really the first to make a telescope, and the
following facts are quoted in confirmation:—“He solicited the States,
as early as the 2nd October, 1608, for a patent for thirty years,
or an annual pension for life, for the instrument he had invented,
promising then only to construct such instruments for the Government.
After inviting the inventor to improve the instrument and alter it so
that they could look through it with both eyes at the same time, the
States determined, on the 4th October, that from every province one
deputy should be elected to try the apparatus and make terms with him
concerning the price. This committee declared on the 6th October that
it found the invention useful for the country, and had offered the
inventor 900 florins for the instrument. He had at first asked 3000
florins for three instruments of rock-crystal. He was then ordered
to deliver the instrument within a certain time, and the patent was
promised him on condition that he kept the invention secret. Lapprey
delivered the instrument in due time. He had arranged it for both
eyes, and it was found satisfactory; but they forced him, against the
agreement, to deliver two other telescopes for the same money, and
refused the patent because it was evident that already several others
had learned about the invention.”

The material from which the glasses were figured appears to have been
quartz; and efforts were made to keep the invention a profound secret,
as it was thought it would prove valuable for “strategetical purposes.”
The cost of these primitive binoculars was about £75 each.

It is singular that, after being allowed to rest so long, the idea of
telescopic construction should have been carried into effect by several
persons almost simultaneously, and that doubts and disputes arose as
to precedence. The probable explanation is that to one individual
only priority was really due, but that, owing to the delays, the
secret could not be altogether concealed from two or three others
who recognized the importance of the discovery and at once entered
into competition with the original inventor. Each of these fashioned
his instrument in a slightly different manner, though the principle
was similar in all; and having in a great measure to rely upon his



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 1 of 32)