CHAPTER IV.

ON ORRERIES OR PLANETARIUMS.

History of such machines--Sphere of Archimedes and Posidonius--Dr. Long’s _Uranium_--Wheel-work of the common Planetarium--Figure representing this machine--Problems which may be performed by it

_page_ 517-527.

DR. HENDERSON’S PLANETARIUM.

Section of its wheel-work--Number of teeth in the wheels and pinions which move the different planets--Extreme accuracy of these movements. _page_ 527-538.

ON THE VARIOUS OPINIONS WHICH WERE ORIGINALLY FORMED OF SATURN’S RING, ILLUSTRATED WITH 13 VIEWS.

When and by whom its true figure was discovered.

_page_ 538-543.

ON THE SUPPOSED DIVISION OF THE EXTERIOR RING OF SATURN.

Kater’s, Short’s, Quetelet’s and Decuppis’s observations

_page_ 543-547.

APPENDIX.

1. DESCRIPTION OF THE EARL OF ROSSE’S LARGEST TELESCOPE.

Composition of the speculum, and the process of casting it--Mode of grinding and polishing it--Manner in which it is filled up--Expenses incurred in its construction--Results of observations which have been made with it--Two views representing this instrument and the buildings connected with it--Sir J. South’s remarks and anticipations

_page_ 548-562.

2. HINTS TO AMATEURS IN ASTRONOMY RESPECTING THE CONSTRUCTION OF TELESCOPES.

_page_ 563.

LIST OF ENGRAVINGS.

_Figure_ _Page_

1. Representation of the diminution of the intensity of light. 22

2. Illustrative of the refraction of light. 43

3. Representing the angles of incidence and refraction. 44

4. The refraction of the atmosphere. 51

5. Various forms of lenses. 65

6, 7, 8. Parallel, converging, and diverging rays. 66

9, 10, 11. Passage of parallel, diverging, and converging rays through convex lenses. 67

12. Passage of parallel rays through concave lenses. 69

13. Images formed by convex lenses. 71

14. Angle of incidence and reflection. 83

15. Images as reflected from a plane mirror. 84

16. Illustrative of reflections from a plane mirror. 85

17. Shewing how the image in a plane mirror is twice the length of the object. 86

18. Reflection from _concave_ mirrors. 87

19. Reflection from _convex_ mirrors. 89

20. Parallel rays as reflected from concave mirrors. 91

21. Diverging rays as reflected from concave mirrors. 91

22. Images formed before concave mirrors. 93

23. Images formed behind concave mirrors. 96

24. Illustrating the magnifying power of concave mirrors. 97

25. Inverted images formed in the front of concave mirrors. 98

26. Illustrative of deceptions produced by concave mirrors. 100

27, 28. Experiment with a bottle half filled with water. 101

29. Effect of extraordinary refraction on ships at sea. 109

30. Experiment for illustrating the causes of uncommon refraction. 117

31. Prismatic spectrum. 127

32. Different foci of coloured rays in convex lenses. 129

33. Experiment to show the different foci of red and violet rays. 129

34. Illustrative of the prismatic colours. 136

35. Explanatory of refraction and reflection from drops of rain. 147

36. Explanatory of the rainbow. 149

37. Images of objects formed in a dark chamber. 187

38. The revolving Camera Obscura. 194

39, 40. The portable Camera Obscura. 195, 196

40, 41, 42. Illustrative of the angle of vision, and the apparent magnitude of objects. 206, 207, 208

43. The Galilean telescope. 215

44. The astronomical telescope. 218

45, 46. The aerial refracting telescope. 226

47. The common refracting telescope. 228

48, 49, 50. Manner in which the rays of light are refracted in telescopes. 231

51. Telescope with a single lens. 234

52. Illustrative of spherical aberration 236

53. Illustrative of the principle of achromatic telescopes. 241

54, 55. Double and treble achromatic object-glass. 242

57. Common stand for achromatic telescopes. 260

58. Equatorial stand for achromatic telescopes. 262

59. Dollond’s stand for achromatic telescopes. 264

60. Blair’s fluid achromatic object-glass. 271

61. Barlow’s fluid telescope. 274

62, 63, 64, 65, 66. Various forms of reflecting telescopes. 288

67. Gregorian reflecting telescope. 293

69. The aerial reflector. 313

70. Front view of the aerial reflector. 314

71. Construction of large reflecting telescope 322

72. Reflecting telescope with a single mirror 332

73. Huygenian eye-piece. 336

74. Ramsden’s eye-piece. 339

75, 76. Combination of lenses for achromatic eye-pieces. 340

77, 78. Diagonal eye-pieces. 344, 345

79. Terrestrial eye-piece with four lenses. 349

80. Eye-piece of an old Dutch achromatic telescope. 356

81. Pancratic eye-piece. 359

82. Manner of exhibiting the solar spots. 384

84. Mode of measuring distances from one station. 430

85. Cavallo’s micrometer. 446

86. The equatorial telescope, or portable observatory. 455

87. Figure to illustrate the principle of the quadrant. 491

88. The astronomical quadrant. 493

89. The astronomical circle. 496

90. The transit instrument. 502

91. Plan of a private observatory. 511

92. Rotatory dome for an observatory. 513

93. Wheel-work of a planetarium. 521

94. Perspective view of a planetarium. 522

95. Apparatus for exhibiting the retrograde motions of the planets. 525

96. Section of the wheel-work of Dr. Henderson’s planetarium. 528

97. Thirteen views of the supposed form of Saturn’s ring. 539

98. Earl of Rosse’s Great Telescope. 559

99. Section of the machinery connected with the telescope. 560

100. Perspective view of the author’s observatory--to front the title.

THE

PRACTICAL ASTRONOMER.