Telescopic Work for Starlight Evenings

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 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 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.”

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