Digitized by the Internet Archive in 2014 https://archive.org/details/practicalastronoOOdick THE AUTHOR'S OBSERVATORY. THE PRACTICAL ASTRONOMER, COMPRISING ILLUSTRATIONS OF LIGHT AND COLOURS PRACTICAL DESCRIPTIONS OF ALL KINDS OF TELESCOPES THE USE OF THE EQUATORIAL- TRANSIT — CIRCULAR, AND OTHER ASTRONOMICAL INSTRUMENTS, A PARTICULAR ACCOUNT OF THE EARL OF ROSSE'S LARGE TELESCOPES, 'AND OTHER TOPICS CONNECTED WITH ASTRONOMY. BY THOMAS DICK, LL.D. AUTHOR OF THE " CHRISTIAN PHILOSOPHER," "CELESTIAL SCENERY." "THE SIDEREAL HEAVENS," &C. &C. Illustrate totti) <3nz fjuntiretf lEngratnngs. SEELEY, BURNSIDE, AND SEELEY, FLEET-STREET, LONDON. MDCCCXLV. PRINTED BY L. SEELBY. PREFACE. The following work was announced several years ago in the preface to the volume on " The Sidereal Heavens ; " since which time numerous enquiries have been made after it by correspondents in England, the West Indies, and America. It was nearly ready for publication three years ago ; but circumstances over which the Author had no controul, prevented its appearance at that period. This delay, however, has enabled him to introduce de- scriptions of certain instruments and inventions which were partly unknown at the time to which he refers. The title u Practical Astronomer " has been fixed upon, as the shortest that could be selected, although the volume does not comprise a variety of topics and discussions generally comprehended in this department of astronomy. The work is intended for the informa- tion of general readers, especially for those who have acquired a relish for astronomical pursuits, and who wish to become acquainted with the instruments by which celestial observations are made, and to apply their mechanical skill to the construction of some of those which they may wish to possess. With this view the Author has entered into a variety of minute details, in reference to the construction and practical application of all kinds VI PREFACE. of telescopes, &c. which are not to be found in general treatises on Optics and Astronomy. As Light is the foundation of astronomical science, and of all the instruments used for celestial observation, a brief description is given of the general properties of light — of the laws by which it is refracted and reflected when passing through different mediums — and of the effects it produces in the system of nature — in order to prepare the way for a clear understanding of the princi- ples on which optical instruments are constructed, and the effects they produce. As this, as well as every other physical subject, forms a part of the arrangements of the Creator throughout the material system — the Author has occasionally taken an opportunity of directing the attention of the reader to the Wisdom and Beneficence of the Great First Cause, and of introducing those moral reflections which naturally flow from the subject. The present is the ninth volume which the Author has presented to the public, and he indulges the hope that it will meet with the same favourable reception which his former publications have uniformly experienced. It was originally intended to conclude the volume with a few remarks on the utility of astronomical studies, and their moral and religious tendency, but this has been prevented, for the present, in consequence of the work having swelled to a greater size than was anticipated. Should he again appear before the public as an author, the subject of discussion and illustration will have a more direct bearing than the present on the great objects of religion and a future world. Broughty Ferry, near Dundee, August, 1845. CONTENTS. PART I. ON LIGHT. INTRODUCTION. Necessity of light to the knowledge and happiness of all sentient beings — Its beautiful and enlivening effects — An emblem of the Deity — Provision made for its universal diffusion - page 1 —7. CHAPTER I. General Properties of Light. Interesting nature of this study — Different hypotheses which have been formed respecting the nature of light —It radiates in straight lines — Moves with amazing velocity — Flows in all directions from luminous bodies — Duration of its impressions on the eye — Supposed to have a certain degree of force or momentum — Experiments in relation to this point — Its intensity diminished in proportion to the square of the distance— Its reflection from opake bodies renders objects visible — Intensity of reflected light — Subject to the law of attrac- tion — Forms a constituent part of certain bodies — Solar phosphoric and the phenomena they exhibit — Produces certain effects on planets and flowers, exemplified in a variety of instances — Supposed to have an influence on the propagation of sound - page 8 — 37 vii CONTENTS. Reflections on the nature of light, and the multifarious effects it pro- duces throughout the universe — A representation of the Divinity — Wisdom and Goodness of God displayed in its formation page 37 — 40. CHAPTER II. On the Refraction of Light. Nature of refraction — Illustrated by experiments — Angle of refrac- tion—Familiar experiments illustrative of refraction — Refraction explains the causes of many curious and interesting phenomena — Its effect on the heavenly bodies— On the twilight — Illustrated by iigures -------- page 41— 53. EXTRAORDINARY CASES OF REFRACTION IN RELATION TO TERRESTRIAL OBJECTS. Extraordinary appearance of the coast of France from Hastings- Appearance of a ship seen by Captain Colby, beyond the coast of Caithness — Scoresby's view of his father's ship when beyond the horizon— Phenomenon near the Himalaya mountains — Bell Rock light-house — Summary statement of the diversified effects of re- fraction—Reflections on the beneficent and diversified effects pro- duced by the law of refraction —It increases the length of the day, particularly in the polar regions — Is the cause of that splendour which appears in the objects around us— Quantity of refraction in respect to terrestrial objects, and its utility — Its effects may be more diversified in other worlds - page 53 — 63. CHAPTER III. On the Refraction of Light through Spherical Transparent Substances, or Lenses. Refraction the foundation of optical instruments— Various forms of lenses — Parallel, converging, and diverging rays — Illustrated by dia- grams — Concave lenses, their effects, and how to find their focal distances— Images formed by convex lenses — Illustrated by expe- riments—Principles in relation to images formed by lenses — Their magnifying powers, &c. - page 63—75. reflections deduced from the preceding subject. Property of the rays of light in forming images of objects— Wonder- ful results and discoveries which have flowed from this property — CONTENTS. ix in relation to our knowledge of the scenery of the heavens and the minute parts of nature — and of our views of the attributes of Deity - - page 75 — 80. CHAPTER IV. On the Reflection of Light. Nature of reflection — Plane, convex, and concave speculums— Angle of reflection — Reflection of objects from plane mirrors, illustrated by figures — Reflection by Convex and Concave mirrors — Properties of convex mirrors, and the purposes to which they are applied. Pro- perties of concave speculums, and their utility — Of the images formed by concave speculums — Illustrated by a variety of figures and experiments— Their power of magnifying and burning- Amusing deceptions produced by — Resemblance between the pro- perties of convex lenses, and concave mirrors— Quantity of light reflected by polished surfaces - page 81 — 106. UNCOMMON APPEARANCES OF NATURE PRODUCED BY THE COMBINED INFLUENCE OF REFLECTION AND REFRACTION. Fata Morgana — The Mirage — Inverted images of ships seen in the horizon — Appearance of Dover castle at Ramsgate — Spectre of the Brocken — Scenes in the Highlands of Scotland — Large cross seen . at Migne in France — Dr. Wollaston's illustrations of such phe- nomena — Utility of science in dissipating superstitious fears page 106—118. REMARKS AND REFLECTIONS IN REFERENCE TO THE PHENOMENA DESCRIBED ABOVE. Light, the beauty of the universe, and a symbol of the Divinity — In other worlds it may produce an infinite variety of sublime scenery page 118-122. CHAPTER V. Sect. 1. — On the Colours of Light. Colours, the beauty of nature — Opinions which were formerly enter- tained respecting their cause — Sir I. Newton's experiments with the Prism — Colours and phenomena produced by the prism — Im- perfection of optic lenses — Various illustrations — Differently coloured rays have not the same illuminating power — Heating and X CONTENTS. chemical properties of some of the rays of the solar spectrum- property of communicating the Magnetic power — Fraunhofer, and his discoveries in reference to the spectrum — Experiments on white and coloured light - - - - - page 123-137. Sect. 2. — on the colours of natural objects. Colours not in the objects themselves, but in the light which falls upon them — Illustrations of this position — Atmosphere the source of a variety of colours — Various natural phenomena, in relation to colour, explained - page 137 — 143. Sect. 3. — phenomena of the rainbow. Rainbow described — Experiments to illustrate its cause — Descriptions of its various phenomena, and optical explanations of their causes — Rainbows exhibiting complete circles — Their appearance in different countries — Summary view of the principal facts respecting the rainbow — Lunar rainbows — Scriptural allusions to the rainbow — Whether there was any rainbow before the deluge, page 144 — 157. Sect. 4.— reflections on the beauty and utility of colours. Beauty and variety derived from colours in the scenery of nature — Colours produced by the atmosphere in different countries — What would be the aspect of nature, in heaven and on earth, were there only one colour — How it would affect the common intercourse and employments of society — Wisdom and Beneficence of the Creator displayed in the diversity of colours — Throughout all the systems of the universe, a diversity of colours prevails — This subject has a tendency to inspire us with gratitude - - page 158— 168. PART II. ON TELESCOPES. CHAPTER I. History of the Invention of Telescopes. The telescope a noble instrument— Effects it produces— Whether known to the ancients— Friar Bacon's ideas respecting telescopes— CONTENTS. xi First constructed in Holland — The invention claimed by different persons — Galileo's account of the construction of his telescope — Discoveries which he made with this instrument — How his discove- ries were received by the learned — Specimens of learned nonsense brought forward by pretended philosophers — Supposed length of Galileo's telescope — Various claimants to the invention of this instrument ----- - - page 169 — 183. CHAPTER II. of the Camera Obscura. Appearance of objects in a camera obscura — The dark chamber — This instrument serves to explain the nature of a refracting tele- scope — Particulars to be attended to, in exhibiting objects with the Camera — It illustrates the nature of vision — Revolving camera obscura — Portable camera - page 184 — 196. THE DAGUERREOTYPE. An important discovery for fixing the images produced by the camera — Description of the Daguerreotype process — Preparation of the plate, fixing the impression, &c. — Preparation of photogenic paper — Beneficial effects which this art may produce — Representa- tions of objects in the heavens, &c. - - page 196 — 205. CHAPTER III. On the Optical Angle, and the Apparent Magnitude of Objects. Various illustrations of the apparent magnitude of objects — Fallacies in relation to apparent magnitudes — Apparent magnitudes in the heavens — Difference between absolute and apparent magnitudes page 206—213. CHAPTER IV. On the Different Kinds of Refracting Telescopes. Sect. 1. — the galilean telescope. Construction and peculiar properties of this instrument, page 214 — 217. xii CONTENTS. Sect. 2.— the common astronomical refracting telescope. Description of its nature and construction — How its magnifying power is determined. Table of the linear aperture, magnifying powers, &c., of astronomical telescopes from 1 to 1*20 feet in length — Summary view of the properties of this telescope page 218 — 224 Sect. 3. — the aerial telescope. This telescope is used without a tube — Description of the apparatus connected with it, illustrated with figures — Huygens' Hartsocker's and Cassini's large telescopes - page 224 -228. Sect. 4. — the common refracting telescope for terrestrial OBJECTS. Arrangement of its lenses — Magnifying power — Manner in which the rays of light are refracted through the telescopes now described page 228—231. Sect. 5. — telescope formed by a single lens. Various experiments in relation to this point — Experiments with a lens 26 focal distance, and 11^ inches diameter page 232 — 235. Sect. 6. — the achromatic telescope. Imperfections of common refracting telescopes — Dollond's discovery — Newton's error — Explanation of the principle of achromatic teles- copes — Combination of lenses — Difficulties in the construction of such instruments — Difficulty in procuring large disks of flint glass — Guinaud's experiments - page 235 — 248. notices of some large achromatic telescopes on the continent, and in great britain. The Dorpat telescope — Sir J. South' s telescope — Captain Smyth's — Rev. Dr. Pearson's— Mr. Lawson's— Mr. Cooper's— Mr, Bridges', &c, — Achromatics in Cambridge and Paris observatories. pages 248—254. ACHROMATIC TELESCOPES OF A MODERATE SIZE, WITH THEIR PRICES, AS SOLD BY LONDON OPTICIANS. The 2\ feet Achromatic — The 3h feet — The powers applied to it — CONTENTS . xiii and the views it gives of the heavenly bodies — The 5 feet achro- matic — Stands for telescopes, illustrated by engravings page 254—264. PROPORTIONS OR CURVATURE OF THE LENSES WHICH FORM AN ACHROMATIC OBJECT-GLASS. Various tables and explanations - page 265 — 269, ACHROMATIC TELESCOPES COMPOSED OF FLUID LENSES. Blair's fluid telescope, with an account of its performance — Barlow's large refracting telescope with a fluid concave lens— Its construc- tion, and the effect it produces on double stars, &c. — Rogers' achro- matic telescope on a new plan — Wilson's telescope, &c. page 269—283. CHAPTER V. On Reflecting Telescopes. Sect. 1. — history of the invention, and a general description of the construction of these instruments. Gregory's Reflector— Newtonian Reflector — Cassegrainian Reflector — Magnifying powers of reflectors — Short's Reflectors — Their powers and prices — General remarks on Gregorian reflectors — Apertures and magnifying powers of Newtonian telescopes — Prices of Re- flecting telescopes ------ page 284 — 301 , Sect. 2. —the herschelian telescope. Description of Sir W. Herschel's 40 feet telescope,with its machinery, apparatus, and the discoveries made by it — Sir J. Herschel's 20 feet reflector - - - - - page 301 — 308. Sect. 3, — ramage's large reflecting telescope page 308—311. Sect. 4. — the aerial reflector — constructed by the author. Construction of this telescope, and the manner of using it — Illustrated xiv CONTENTS. by figures— Its properties and advantages — Tube not necessary in reflecting telescopes — How a large reflector might be constructed without a tube — How the form of a telescope may be used for viewing perspectives page 311 — 325. Sect. 4. — earl of rosse's reflecting telescopes. His mode of forming a large speculum, &c, see also, Appendix page 325—328. Sect. 5.— reflecting telescopes with glass specula. Various experiments on this subject, with their results, page 329—331. Sect. 6. — a reflecting telescope with a single mirror and no eye-piece. Experiments illustrative of this construction - page 332 — 334. ON THE EYE-PIECES OF TELESCOPES. ASTRONOMICAL EYE-PIECES. Huygenian eye-piece — Ramsden's eye-piece — Aberration of lenses — Celestial eye-pieces with variable powers. Diagonal eye-pieces — Various forms of them described — Various aspects in which objects may be viewed by them page 335—347. TERRESTRIAL EYE-PIECES. Eye-pieces with four lenses — Proportions of the focal lengths of these lenses — Dimensions and powers of several eye-pieces stated. page 347—353. DESCRIPTION OF AN EYE-PIECE, &C, OF AN OLD DUTCH ACHROMATIC TELESCOPE. This telescope supposed to have been invented in Holland before Dol- lond's discovery was known — Peculiarity of its eye-piece page 354 — 357. DESCRIPTION OF THE PANCRATIC EYE-TUBE. - page 357 — 360. CHAPTER VI. Miscellaneous Remarks in Relation to Telescopes. 1. Adjustments requisite to be attended to in the use of telescopes CONTENTS. XV — 2. State of the atmosphere most proper for observing terrestrial, and celestial objects — Average number of hours in the year fit for celestial observations. — 3. On the magnifjang powers requisite for observing the phenomena of the different planets — Comets — Double stars, &c. — Illustrated at large from p. 369 — 380. — 4. Mode of exhibiting the solar spots — Eye-pieces best adapted for this pur- pose — How they may be exhibited to a large company — Mode in which their dimensions may be determined. — 5. On the space-pene- trating power of telescopes — Herschel's observations on space-pene- trating powers — Comparison of achromatic and Gregorian reflectors. — 6. On choosing telescopes, and ascertaining their properties — Various modes of ascertaining the goodness of telescopes — General remarks and cautions on this point — A circumstance which requires to be attended to in using achromatics. — 7. On the mode of deter- mining the magnifying power of telescopes — Various experiments in relation to this point. — 8. On cleaning the lenses of telescopes. page 361—407. ON M EG AL ASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS. Mode of adapting a telescope for this purpose— objects to which they may be applied. ------ page 407—411. REFLECTIONS ON LIGHT AND VISION, AND ON THE NATURE AND UTILITY OF TELESCOPES. Wonderful and mysterious nature of light — The organ of vision, and its expansive range — Wonderful nature of the telescope, and the objects it has disclosed to view — No boundaries should be set to the discoveries of science and the improvement of art — The telescope is a machine which virtually transports us to the distant regions of space — It enlarges our views of the sublime scenes of creation — It has tended to amplify our conceptions of the empire and the attri- butes of the Deity — Various uses of this instrument in relation to science and common life. . page 411—431. CHAPTER VII. On the Method of Grinding and Polishing Optical Lenses and Specula. 1. Directions for grinding lenses for eye-glasses, microscopes, &c. — 2. Method of casting and grinding the specula of reflecting tele- scopes — Compositions for speculum metal — To try the figure of the metal — To adjust the eye-hole of Gregorian reflectors — To center the specula — To center lenses. - page 432 — 442. xvi CONTENTS. PART III. ON VARIOUS ASTRONOMICAL INSTRUMENTS. CHAPTER I. On Micrometers. Various descriptions of micrometers — Cavallo^s micrometer described — To ascertain the value of its divisions — Practical uses of this micrometer — Problems which may be solved by it — Tables for facilitating its use. ----- page 443 — 452. CHAPTER II. On the Equatorial Telescope, or Portable Observatory. History of equatorials — Description of one of the simplest construc- tion of these instruments — To adjust the equatorial for observation — To adjust the line of sight — Description of the nonius — To find the meridian line by one observation — Manner of observing stars and planets in the day-time. - page 453 — 464. observations, by the author, on the fixed stars and PLANETS, MADE IN THE DAY-TIME, BY THE EQUATORIAL. Object of these observations — stars of the first and second magnitudes — General deductions from these observations. - page 464 — 469. OBSERVATIONS ON THE PLANETS IN THE DAY-TIME. Series of observations on Venus^ when near the sun — Seen at the time of her superior conjunction in 1843 — Conclusions deduced from these observations — phenomena observed during these observations — Remarkable phenomenon during an eclipse of the sun. page 469—480. CONTENTS. xvii OBSERVATIONS ON JUPITER AND OTHER General conclusions, &c. UTILITY OF CELESTIAL DAY OBSERVATIONS. - ON THE ASTRONOMICAL QUADRANT. THE ASTRONOMICAL CIRCLE. the transit instrument. - - - page 502 — 50.5. CHAPTER III. On Observatories. Leading features of a spot adapted for celestial observations — Public and private observatories — Greenwich observatory — Instruments with which an observatory should be furnished — The Author's private observatory — Revolving domes for observatories — Cautions to be attended to in celestial observations. - page 506 — 516. CHAPTER IV. On Orreries or Planetariums. History of such machines — Sphere of Archimedes and Possidonius — Dr. Long's Uranium — Wheel- work of the common Planetarium — Figure representing this machine — Problems which may be per- formed 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. planets. page 480—485. page 485—491. page 492-496. page 496 — 502. 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. xviii CONTENTS. ON THE SUPPOSED DIVISION OF THE EXTERIOR RING OF SATURN. Rater'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 — Ex- penses 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. . . . . . . . . §4 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. . . . . . . . . . . gg 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 XX LIST OF ENGRAVINGS. Figure ■ p age 39, 40. The portable Camera Obscura 195, 196 40*, 41, 42. Illustrative of the angle of vision, and the apparent mag- nitude 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 tele- scopes. 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. 3 84 84. Mode of measuring distances from one station 430 85. Cavallo's micrometer. .. .. 44 6 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. 50 2 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. 52 2 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. . . 5 39 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. PART I. ON LIGHT. I N TRODUCTION. Light is that invisible etherial matter which renders objects perceptible by the visual organs. It appears to be distributed throughout the im- mensity of the universe, and is essentially requisite to the enjoyment of every rank of perceptive existence. It is by the agency of this mysterious substance, that we become acquainted with the beauties and sublimities of the universe, and the wonderful operations of the Almighty Creator. Without its universal influence, an impenetrable veil would be thrown over the distant scenes of creation ; the sun, the moon, the planets, and the starry orbs, would be shrouded in the deepest darkness, and the variegated surface of the globe on which we dwell, would be almost unnoticed and unknown. Creation would disappear, a mys- teriou s gloom would surround the mind of every B 2 THE PRACTICAL ASTRONOMER. intelligence, all around would appear a dismal waste, and an undistinguished chaos. To what- ever quarter we might turn, no form nor comeli- ness would be seen, and scarcely a trace of the perfections and agency of an All Wise and Almighty Being could be perceived throughout the universal gloom. In short, without the influ- ence of light, no world could be inhabited, no animated being could subsist in the manner it now does, no knowledge could be acquired of the -works of God, and happiness, even in the lowest degree, could scarcely be enjoyed by any organized intelligence. We have never yet known what it is to live in a world deprived of this delightful visitant ; for in the darkest night we enjoy a share of its bene- ficial agency, and even in the deepest dungeon its influence is not altogether unfelt.* The blind, indeed, do not directly enjoy the advantages of light, but its influence is reflected upon them, and their knowledge is promoted through the medium of those who enjoy the use of their visual organs. Were all the inhabitants of the world deprived of their eye-sight, neither knowledge nor happiness, such as we now possess, could possibly be enjoyed. There is nothing which so strikingly displays the beneficial and enlivening effects of light, as the dawn of a mild morning after a night of darkness and tempest. All appears gloom and desolation, in our terrestrial abode, till a faint light begins to whiten the eastern horizon. Every succeeding * Those unfortunate individuals who have been confined in the darkest dungeons have declared, that though on their first entrance, no object could be perceived, perhaps for a day or two, yet, in the course of time, as the pupils of their eyes expanded, they could readily perceive mice, rats, and other animals that infested their cells, and likewise the walls of their apartments ; which shows that, even in such situations, light is present, and produces a certain degree of influence. INTRODUCTION. s moment brings along with it something new and enlivening. The crescent of light towards the east, now expands its dimensions and rises "up- wards towards the cope of heaven ; and objects, which a little before were immersed in the deepest gloom, begin to be clearly distinguished. At length the sun arises, and all nature is animated by his appearance ; the magnificent scene of creation, which a little before was involved in ob- scurity, opens gradually to view, and every object around excites sentiments of wonder, delight, and adoration. The radiance which emanates from this luminary, displays before us a w 7 orld strewed with blessings and embellished with the most beautiful attire. It unveils the lofty mountains and the forests with which they are crowned — the fruitful fields with the crops that cover them — the meadows, with the rivers which water and refresh them — the plains adorned with verdure, the placid lake and the expansive ocean. It removes the curtain of darkness from the abodes of men, and shows us the cities, towns and vil- lages, the lofty domes, the glittering spires, and the palaces and temples with which the landscape is adorned. The flowers expand their buds and put forth their colours, the birds awake to melody, man goes forth to his labour, the sounds of human voices are heard, and all appears life and activity, as if a new world had emerged from the darkness of Chaos. The whole of this splendid scene, which light produces, may be considered as a new creation, no less grand and beneficent than the first creation, when the command was issued, " Let there be light, and light was." The aurora and the rising sun cause the earth and all the objects which adorn its surface, to arise out of that profound B 2 4 THE PRACTICAL ASTRONOMER. darkness and apparent desolation which deprived us of the view of them, as if they had been no more. It may be affirmed, in full accordance with truth, that the efflux of light in the dawn of the morning, after a dark and cloudy night, is even more magnificent and exhilarating than at the first moment of its creation. At that period, there were no spectators on earth to admire its glorious effects ; and no objects, such as we now behold, to be embellished with its radiance. The earth was a shapeless chaos, where no beauty or order could be perceived; the mountains had not reared their heads ; the seas were not collected into their channels ; no rivers rolled through the valleys, no verdure adorned the plains ; the atmosphere was not raised on high to reflect the radiance, and no animated beings existed to diversify and enliven the scene. But now, when the dawning of the morning scatters the darkness of the night, it opens to view a scene of beauty and magnificence. The heavens are adorned with azure, the clouds are tinged with the most lively colours, the moun- tains and plains are clothed with verdure, and the whole of this lower creation stands forth arrayed with diversified scenes of beneficence and grandeur, while the contemplative eye looks round and wonders. Such, then, are the important and beneficent effects of that light which every moment diffuses its blessings around us. It may justly be consi- dered as one of the most essential substances con- nected with the system of the material universe, and which gives efficiency to all the other princi- ples and arrangements of nature. Hence we are informed, in the sacred history, that light was the first production of the Almighty Creator, and the first born of created beings ; for without it the INTRODUCTION. 5 universe would have presented nothing but an immense blank to all sentient existences. Hence, likewise, the Divine Being is metaphorically re- presented under the idea of light, as being the source of knowledge and felicity to all subordinate intelligences: " God is light, and in Him is no darkness at all ;" and he is exhibited as " dwelling in light unapproachable and full of glory, whom no man hath seen or can see." In allusion to these circumstances, Milton, in his Paradise Lost, introduces the following beautiful apostrophe : — 6 Hail holy light ! offspring of heaven first born, Or of the eternal co-eternal beam ! May I express thee unblam'd ? since God is light, And never but in unapproached light Dwelt from eternity; dwelt then in thee, Bright effluence of bright essence increate, Before the sun Before the heavens thou wert, and at the voice Of God, as with a mantle, did'st invest The rising world of waters dark and deep Won from the void and formless infinite.' As light is an element of so much importance and utility in the system of nature, so we find that arrangements have been made for its universal diffusion throughout all the worlds in the universe. The sun is one of the principal sources of light to this earth on which we dwell, and to all the other planetary bodies. And, in order that it may be equally distributed over every portion of the surfaces of these globes, to suit the exigencies of their inhabitants, they are endowed with a motion of rotation, by which every part of their surfaces is alternately turned towards the source of light ; and when one hemisphere is deprived of the direct influence of the solar rays, its inhabi- tants derive a portion of light from luminaries in more distant regions, and have their views directed to other suns and systems dispersed, in 6 THE PRACTICAL ASTRONOMER. countless numbers, throughout the remote spaces of the universe. Around several of the planets, satellites, or moons, have been arranged for the purpose of throwing light on their surfaces in the absence of the sun, while at the same time the primary planets themselves reflect an effulgence of light upon their satellites. All the stars which our unassisted vision can discern in the midnight sky, and the millions more which the telescope alone enables us to descry, must be considered as so many fountains of light, not merely to illumi- nate the voids of immensity, but to irradiate with their beams surrounding worlds with which they are more immediately connected, and to diffuse a general lustre throughout the amplitudes of infi- nite space. And, therefore, we have every reason to believe, that, could we fly, for thousands of years, with the swiftness of a seraph, through the spaces of immensity, we should never approach a region of absolute darkness, but should find our- selves, every moment encompassed with the ema- nations of light, and cheered with its benign influ- ences. That Almighty Being who inhabiteth immensity and " dwells in light inaccessible," evidently appears to have diffused light over the remotest spaces of his creation, and to have thrown a radiance upon all the provinces of his wide and eternal empire, so that every intellectual being, wherever existing, may feel its beneficent effects, and be enabled, through its agency, to trace his wonderful operations, and the glorious attributes with which he is invested. As the science of astronomy depends solely on the influence of light upon the organ of vision, which is the most noble and extensive of all our senses ; and as the construction of telescopes and other astronomical instruments is founded upon INTRODUCTION. 7 our knowledge of the nature of light and the laws by which it operates — it is essentially requisite, before proceeding to a description of such instru- ments, to take a cursory view of its nature and properties, in so far as they have been ascertained, and the effects it produces when obstructed by certain bodies, or when passing through different mediums. 8 THE PRACTICAL ASTRONOMER. CHAPTER I. GENERAL PROPERTIES OF LIGHT. It is not my intention to discuss the subject of light in minute detail — a subject which is of con- siderable extent, and which would require a separate treatise to illustrate it in all its aspects and bearings. All that I propose is to offer a few illustrations of its general properties, and the laws by which it is refracted and reflected, so as to prepare the way for explaining the nature and construction of telescopes, and other optical instru- ments. There is no branch of natural science more deserving of our study and investigation than that which relates to light — whether we consider its beautiful and extensive effects — the magnificence and grandeur of the objects it unfolds to view — the numerous and diversified phenomena it exhi- bits — the optical instruments which a knowledge of its properties has enabled us to construct — or the daily advantages we derive, as social beings, from its universal diffusion. If air, which serves as the medium of sound, and the vehicle of speech, enables us to carry on an interchange of thought and affection with our fellow -men 5 how much GENERAL PROPERTIES OF LIGHT. 9 more extensively is that intercourse increased by light, which presents the images of our friends and other objects as it were immediately before us, in all their interesting forms and aspects — the speaking eye— the rosy cheeks — the benevolent smile, and the intellectual forehead ! The eye, more susceptible of multifarious impressions than the other senses, ' takes in at once the landscape of the world,' and enables us to distinguish, in a moment, the shapes and forms of all its objects, their relative positions, the colours that adorn them, their diversified aspect, and the motions by which they are transported from one portion of space to another. Light, through the medium of the eye, not only unfolds to us the persons of others, in all their minute modifications and pecu- liarities, but exhibits us to ourselves. It presents to our own vision a faithful portrait of our peculiar features behind reflecting substances, without which property we should remain entirely ignorant of those traits of countenance which characterize us in the eyes of others. But, what is the nature of this substance we call light, which thus unfolds to us the scenes of creation ? On this subject two leading opinions have prevailed in the philosophical world. One of those opinions is, that the whole sphere of the universe is filled with a subtle matter, which receives from luminous bodies an agitation which is incessantly continued, and which, by its vibra- tory motion, enables us to perceive luminous bodies. According to this opinion, light may be considered as analogous to sound, which is conveyed to the ear by the vibratory motions of the air. This was the hypothesis of Descartes, which was adopted, with some modifications, by the cele- brated Euler, Huygens, Franklin, and other philo- B 5 10 THE PRACTICAL ASTRONOMER. sophers, and has been admitted by several scien- tific gentlemen of the present day. The other opinion is, that light consists of the emission or emanation of the particles of luminous bodies, thrown out incessantly on all sides, in consequence of the continued agitation it experiences. This is the hypothesis of the illustrious Newton, and has been most generally adopted by British philosophers. To the first hypothesis, it is objected that, if true, 6 light would not only spread itself in a direct line, but its motion would be transmitted in every direction like that of sound, and would convey the impression of luminous bodies in the regions of space beyond the obstacles that inter- vene to stop its progress.' No wall or other opaque body could obstruct its course, if it undu- lated in every direction like sound ; and it would be a necessary consequence, that we should have no night, nor any such phenomena as eclipses of the sun or moon, or of the satellites of Jupiter and Saturn. This objection has never been very satisfactorily answered. On the other hand, Euler brings forward the following objections against the Newtonian doctrine of emanation. 1 . That, were the sun emitting continually, and in all directions, such floods of luminous matter with a velocity so prodigious, he must speedily be exhausted, or at least, some alteration must, after the lapse of so many ages, be perceptible. 2. That the sun is not the only body that emits rays, but that all the stars have the same quality ; and as every where the rays of the sun must be crossing the rays of the stars, their collision must be vio- lent in the extreme, and that their direction must be changed by such a collision. # * Letters to a German Princess, vol. 1. pp. 68, 69, &c. GENERAL PROPERTIES OF LIGHT. 11 To the first of these objections it is answered — that so vast is the tenuity of light, that it utterly exceeds the power of conception : the most deli- cate instrument having never been certainly put in motion by the impulse of the accumulated sun- beams. It has been calculated that in the space of 385,130,000 Egyptian years, (of 360 days,) the sun would lose only the 1,217)420^ °f his bulk from the continual efflux of his light. And, therefore, if in 385 millions of years the sun's diminution would be so extremely small, it would be altogether insensible during the comparatively short period of five or six thousand years. To the second objection it is replied — that the particles of light are so extremely rare that their distance from one another is incomparably greater than their diameters — that all objections of this kind vanish when we attend to the continuation of the impression upon the retina, and to the small number of luminous particles which are on that account necessary for producing constant vision. For it appears, from the accurate experiments of M. D'Arcy, that the impression of light upon the retina continues eight thirds, and as a particle of light would move through 26,000 miles in that time, constant vision would be maintained by a succession of luminous particles twenty-six thou- sand miles distant from each other. Without attempting to decide on the merits of these two hypotheses, I shall leave the reader to adopt that opinion which he may judge to be attended with the fewest difficulties, and pro- ceed to illustrate some of the properties of light : — 'and in the discussion of this subject, I shall generally adhere to the terms employed by those who have adopted the hypothesis of the emanation of light. 12 THE PRACTICAL ASTRONOMER. 1. Light emanates or radiates from luminous bodies in a straight line. This property is proved by the impossibility of seeing light through bent tubes, or small holes pierced in metallic plates placed one behind another, except the holes be placed in a straight line. If we endeavour to look at the sun or a candle through the bore of a bended pipe, we cannot perceive the object, nor any light proceeding from it, but through a straight pipe the object may be perceived. This is likewise evident from the form of the rays of light that penetrate a dark room, which proceed straight forward in lines proceeding from the luminous body ; and from the form of the shadows which bodies project, which are bounded by right lines passing from the luminous body, and meet- ing the lines which terminate the interposing body. This property may be demonstrated to the eye, by causing light to pass through small holes into a dark room filled with smoke or dust. It is to be understood, however, that in this case, the rays of light are considered as passing through the same medium ; for when they pass from air into water, glass, or other media, they are bent at the point where they enter a different medium, as we shall afterwards have occasion to explain. 2. Light moves with amazing velocity. The ancients believed that it was propagated from the sun and other luminous bodies instantaneously ; but the observations of modern astronomers have demonstrated that this is an erroneous hypothesis, and that light, like other projectiles, occupies a certain time in passing from one part of space to another. Its velocity, however, is prodigious, and exceeds that of any other body with which we are acquainted. It flies across the earth's orbit — a space 190 millions of miles in extent, in the GENERAL PROPERTIES OF LIGHT, 13 course of sixteen and a half minutes, which is at the rate of 192,000 miles every second, and more than a million of times swifter than a cannon ball flying with its greatest velocity. It appears from the discoveries of Dr. Bradley, respecting the aberration of the stars, that light flies from those bodies, with a velocity similar, if not exactly the same ; so that the light of the sun, the planets, the stars, and every luminous body in the universe is propagated with uniform velocity.* But, if the velocity of light be so very great, it may be asked, how does it not strike against all objects with a force equal to its velocity ? If the finest sand were thrown against our bodies with the hun- dredth part of this velocity, each grain would pierce us as certainly as the sharpest and swiftest arrows from a bow. It is a principle in mechanics that the force with which all bodies strike, is in proportion to the size of these bodies, or the quantity of matter they contain, multiplied by the velocity with which they move. Therefore if the particles of light were not almost infinitely small, they would, of necessity prove destructive in the highest degree. If a particle of light were equal in size to the twelve hundred thousandth part of a small grain of sand, — supposing light to be material — we should be no more able to withstand its force than we should that of sand shot point blank from the mouth of a cannon. Every object would be battered and perforated by such celestial artillery, till our world were laid in ruins, and every living being destroyed. And herein are the wisdom and benevolence of the Creator displayed * The manner in which the motion of light was discovered is ex- plained in the author's work, entitled fc Celestial Scenery,' pp. 369 — 371, and the circumstances which led to the discovery of the aberra- tion of light are stated and illustrated in his volume on the 6 Sidereal Heavens,' pp. 71—74, and pp. 284-292. 14 THE PRACTICAL ASTRONOMER. in making the particles of light so extremely small as to render them in some degree proportionate to the greatness of the force with which they are impelled ; otherwise, all nature would have been thrown into ruin and confusion, and the great globes of the universe shattered to atoms. We have many proofs, besides the above, that the particles of light are next to infinitely small. "We find that they penetrate with facility the hardest substances, such as crystal, glass, various kinds of precious stones, and even the diamond itself, though among the hardest of stones; for such bodies could not be transparent, unless light found an easy passage through their pores. When a candle is lighted in an elevated situation, in the space of a second or two, it will fill a cubical space (if there be no interruption) of two miles around it, in every direction, with luminous par- ticles, before the least sensible part of its substance is lost by the candle : — that is, it will in a short instant, fill a sphere four miles in diameter, twelve and a half miles in circumference, and containing thirty-three and a half cubical miles with particles of light; for an eye placed in any part of this cubical space w r ould perceive the light emitted by the candle. It has been calculated that the num- ber of particles of light contained in such a space cannot be less than four hundred septillions — a num- ber which is six billions of times greater than the number of grains of sand which could be contained in the whole earth considered as a solid globe, and supposing each cubic inch of it to contain ten hundred thousand grains. Such is the inconceiv- able tenuity of that substance which emanates from all luminous bodies, and which gives beauty and splendour to the universe ! This may also be evinced by the following experiment. Make a GENERAL PROPERTIES OF LIGHT. 15 small pin-hole in a piece of black paper, and hold the paper upright facing a row of candles placed near each other, and at a little distance behind the black paper, place a piece of white paste- board. On this pasteboard the rays which flow from all the candles through the small hole in the black paper, will form as many specks of light as there are candles, each speck being as clear and distinct as if there were only one speck from a single candle. This experiment shows that the streams of light from the different candles pass through the small hole without confusion, and consequently, that the particles of light are ex- ceedingly small. For the same reason we can easily see through a small hole not more than of an inch in diameter, the sky, the trees, houses, and nearly all the objects in an extensive landscape, occupying nearly an entire hemisphere, the light of all which may pass through this small aperture. 3. Light is sent forth in all directions from every visible point of luminous bodies. If we hold a sheet of paper before a candle, or the sun, or any other source of light, we shall find that the paper is illuminated in whatever position we hold it, provided the light is not obstructed by its edge or by any other body. Hence, wherever a spectator is placed with regard to a luminous body, every point of that part of its surface which is toward him will be visible, when no intervening object intercepts the passage of the light. Hence, like- wise, it follows, that the sun illuminates, not only an immense plane extending along the paths of the planets, from the one side of the orbit of Uranus to the other, but the whole of that sphere, or solid space, of which the distance of Uranus is the radius. The diameter of this sphere is three 16 THE PRACTICAL ASTRONOMER. thousand six hundred millions of miles, and it, consequently, contains about 24,000,000,000,000, 000,000,000,000,000, or twenty-four thousand quartillions of cubical miles, — every point of which immense space is filled with the solar beams. Not only so, but the whole cubical space which intervenes between the sun and the nearest fixed stars is more or less illuminated by his rays. For, at the dis- tance of Sirius, or any other of the nearest stars, the sun would be visible, though only as a small twinkling orb ; and consequently, his rays must be diffused, however faint, throughout the most distant spaces whence he is visible. The diameter of this immense sphere of light cannot be less than forty billions of miles, and its solid contents 33 9 500, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000,000, or, thirty-three thousand, five hundred sextillions of cubical miles. All this immense, and incomprehensible space is filled with the radiations of the solar orb; for were an eye placed in any one point of it, where no extraneous body interposed, the sun would be visible either as a large luminous orb, or as a small twinkling star. But he can be visible only by the rays he emits, and which enter the organs of vision. How inconceivably immense, then, must be the quantity of rays which are thrown off in all direc- tions from that luminary which is the source of our day ! Every star must likewise be considered as emitting innumerable streams of radiance over a space equally extensive, so that no point in the universe can be conceived w T here absolute darkness prevails, unless in the interior regions of planetary bodies. 4. The effect of light upon the eye is not instan- taneous, but continues for a short space of time. This may be proved and illustrated by the follow- GENERAL PROPERTIES OF LIGHT. 17 ing examples : — If a stick — or a ball connected with a string — be whirled round in a circle, and a certain degree of velocity given it, the object will appear to fill the whole circle it describes. If a lighted firebrand be whirled round in the same rapid manner, a complete circle of light will be exhibited. This experiment obviously shows that the impression made on the eye by the light from the ball or the firebrand — when in any given point of the circle — is sufficiently lasting to re- main till it has described the whole circle, and again renews its effect, as often as the circular motion is continued. The same is proved by the following considerations : — We are continually shutting our eyes, or winking ; and, during the time our eyes are shut, on such occasions, we should lose the view of surrounding objects, if the impression of light did not continue a certain time while the eye-lid covers the pupil ; but experience proves that during such vibrations of the eye-lids, the light from surrounding objects is not sensibly intercepted. If we look for some time steadily at the light of a candle, and particularly, if we look directly at the sun, without any interposing medium, or if we look for any considerable time at this luminary, through a telescope with a coloured glass interposed — in all these cases, if we shut our eyes immediately after viewing such objects, we shall still perceive a faint image of the object, by the impression which its light has made upon our eyes. ' With respect to the duration of the impression of light, it has been observed that the teeth of a cog-wheel in a clock were still visible in succession, when the velocity of rotation brought 246 teeth through a given fixed point in a second. In this case it is clear that if the impression made on the 18 THE PRACTICAL ASTRONOMER. eye by the light reflected from any tooth, had lasted without sensible diminution for the 246th part of a second, the teeth would have formed one unbroken line, because a new tooth would have continually arrived in the place of the inte- rior one before its image could have disappeared. If a live coal be whirled round, it is observed that the luminous circle is complete, when the rotation is performed in the p| of a second. In this instance we see that the impression was much more durable than the former. Lastly, if an observer sitting in a room direct his sight through a window, to any particular object out of doors, for about half a minute, and then shut his eyes and cover them with his hands, he will still con- tinue to see the window, together with the out- line of the terrestrial objects bordering on the sky. This appearance will remain for near a minute, though occasionally vanishing and chang- ing colour in a manner that brevity forbids our minutely describing. From these facts we are authorized to conclude, that all impressions of light on the eye, last a considerable time, that the brightest objects make the most lasting im- pressions ; and that, if the object be very bright, or the eye weak, the impression may remain for a time so strong, as to mix with and confuse the subsequent impressions made by other objects. In the last case the eye is said to be dazzled by the light.'* The following experiment has likewise been sug- gested as a proof of the impression which light makes upon the eye. If a card, on both sides of which a figure is drawn, for example, a bird and a cage, be made to revolve rapidly on the straight line which divides it symmetrically, the eye will perceive both * Nicolson's Introduction to Natural Philosophy, vol. 1. GENERAL PROPERTIES OF LIGHT. 19 figures at the same time, provided they return successively to the same place. M. D'Arcy found by various experiments, that, in general, the im- pression which light produces on the eye, lasts about the eighth of a second. M. Plateau, of Brussels, found that the impression of different colours lasted the following periods ; the numbers here stated being the decimal parts of a second. Flame, 0.242. or nearly one fourth of a second ; Burning coal, 0.229; White, 0.182, or, a little more than one sixth of a second; Blue s 0.186; Yellow, 0.173 ; Red, 0.184. 5. Light, though extremely minute, is supposed to have a certain degree of force or momentum. In order to prove this, the late ingenious Mr. Mitchell contrived the following experiment. He constructed a small vane in the form of a common weathercock, of a very thin plate of copper, about an inch square, and attached to one of the finest harpsicord wires, about ten inches long, and nicely balanced at the other end of the wire, by a grain of very small shot. The instrument had also fixed to it in the middle, at right angles to the length of the wire, and in an horizontal di- rection, a small bit of a very slender sewing needle, about half an inch long, which was made magnetical. In this state the whole instrument might weigh about ten grains. The vane was supported in the manner of the needle in the mariner's compass, so that it could turn with the greatest ease ; and to prevent its being affected by the vibrations of the air, it was enclosed in a glass case or box. The rays of the sun were then thrown upon the broad part of the vane or copper plate, from a concave mirror of about two feet diameter, which, passing through the front glass of the box, were collected into the focus of the 20 THE PRACTICAL ASTRONOMER. mirror upon the copper plate. In consequence of this the plate began to move with a slow motion of about an inch in a second of time, till it had moved through a space of about two inches and a half, when it struck against the back of the box. The mirror being removed, the instrument returned to its former situation, and the rays of the sun being again thrown upon it, it again began to move, and struck against the back of the box as before. This was repeated three or four times with the same success. On the above experiment, the following calcu- lation has been founded : If we impute the motion produced in this experiment to the im- pulse of the rays of light, and suppose that the instrument weighed ten grains, and acquired a velocity of one inch in a second, we shall find that the quantity of matter contained in the rays fall- ing upon the instrument in that time amounted to no more than one twelve hundreth -millionth part of a grain, the velocity of light exceeding the velocity of one inch in a second in the pro- portion of about 12,000,000,000 to 1. The light in this experiment was collected from a surface of about three square feet, which reflecting only about half what falls upon it, the quantity of matter contained in the rays of the sun incident upon a foot and a half of surface in one second of time, ought to be no more than the twelve hundred-millionth part of a grain. But the den- sity of the rays of light at the surface of the sun is greater than that at the earth in the proportion of 45,000 to 1 ; there ought therefore to issue from one square foot of the sun's surface in one second of time, in order to supply the waste by light 45^oih P ar t of a grain of matter, that is, a little more than two grains a day, or about GENERAL PROPERTIES OF LIGHT. 21 4,752,000 grains, or 670 pounds avoirdupoise, nearly, in 6,000 years, a quantity which would have shortened the sun's diameter no more than about ten feet, if it were formed of the density of water only. If the above experiment be considered as hav- ing been accurately performed, and if the calcula- tions founded upon it be correct, it appears that there can be no grounds for apprehension that the sun can ever be sensibly diminshed by the immense and incessant radiations proceeding from his body on the supposition that light is a mate- rial emanation. For the diameter of the sun is no less than 880,000 miles ; and, before this dia- meter could be shortened, by the emission of light, one English mile, it would require three millions, one hundred and sixty-eight thousand years, at the rate now stated ; and, before it could be shortened ten miles, it would require a period of above thirty-one millions of years. And although the sun were thus actually diminished, it would produce no sensible effect or derange- ment throughout the planetary system. We have no reason to believe that the system, in its present state and arrangements, was intended to endure for ever, and before that luminary could be so far reduced, during the revolutions of eternity, as to produce any irregularities in the system, new arrangements and modifications might be intro- duced by the hand of the All Wise and Omnipo- tent Creator. Besides, it is not improbable that a system of means is established by which the sun and all the luminaries in the universe receive back again a portion of the light which they are con- tinually emitting, either from the planets from whose surfaces it is reflected, or from the millions of stars whose rays are continually traversing the 22 THE PRACTICAL ASTRONOMER. immense spaces of creation, or from some other sources to us unknown. 6. The intensity of light is diminished in pro- portion to the square of the distance from the lumi- nous body. Thus, a person at two feet distance from a candle, has only the fourth part of the light he would have at one foot, at three feet dis- tance the ninth part, at four feet the sixteenth part, at five feet the twenty fifth part, and so on for other distances. Hence the light received by the planets of the Solar system decreases in pro- portion to the squares of the distances of these bodies from the sun. This may be illustrated by the following figure, Figure 1. Suppose the light which flows from a point A, and passes through a square hole B, is re- ceived upon a plane C, parallel to the plane of the hole — or, let the figure C be considered as the shadow of the plane B. When the distance of C is double of B, the length and breadth of the shadow C will be each double of the length and breadth of the plane B, and treble when AD is treble of AB, and so on, which may be easily GENERAL PROPERTIES OF LIGHT. 23 examined by the light of a candle placed at A. Therefore the surface of the shadow C, at the dis- tance AC — double of AB, is divisible into four squares, and at a treble distance, into nine squares, severally equal to the square B, as repre- sented in the figure. The light, then, which falls upon the plane B being suffered to pass to double that distance, will be uniformly spread over four times the space, and consequently will be four times thinner in every part of that space. And at a treble distance it will be nine times thinner, and at a quadruple distance sixteen times thinner than it was at first. Consequently the quantities of this rarifled light received upon a surface of any given size and shape when removed succes- sively to these several distances, will be but one- fourth, one-ninth, one-sixtenth, of the whole quan- tity received by it at the first distance AB. In conformity with this law, the relative quan- tities of light on the surfaces of the planets may be easily determined, when their distances from the sun are known. Thus, the distance of Uranus from the sun is 1,800,000,000 miles,which is about nineteen times greater than the distance of the earth from the same luminary. The square of 19 is 361 ; consequently the earth enjoys 361 times the intensity of light when compared with that of Uranus ; in other words, this distant planet enjoys only the ^ part of the quantity of light which falls upon the earth. This quantity, how- ever, is equivalent to the light we should enjoy from the combined effulgence of 348 full moons ; and if the pupils of the eyes of the inhabitants of this planet be much larger than ours, and the retina of the eye be endued with a much greater degree of nervous sensibility, they may per- ceive objects with as great a degree of splen- 24 THE PRACTICAL ASTRONOMER. dour as we perceive on the objects which sur- round us in this world. Following out the same principle, we find that the quantity of light en- joyed by the planet Mercury is nearly seven times greater than that of the Earth, and that of Venus nearly double of what we enjoy — that Mars has less than the one half — Jupiter the one twenty- seventh part — and Saturn only the one ninetieth part of the light which falls upon the Earth. That the light of these distant planets, however, is not so weak as we might at first imagine appears from the brilliancy they exhibit, when viewed in our nocturnal sky, either with the telescope or with the unassisted eye — and likewise from the circumstance that a very small portion of the Sun — such as the one fortieth or one fiftieth part diffuses a quantity of light sufficient for most of the purposes of life, as is found in the case of total eclipses of the Sun, when his western limb begins to be visible, only like a fine luminous thread, for his light is then sufficient to render distinctly visible all the parts of the surrounding landscape. 7. It is by light reflected from opake bodies that most of the objects around us are rendered visible. When a lighted candle is brought into a dark room, not only the candle but all other bodies in the room become visible. Rays of the sun passing into a dark room render luminous a sheet of paper on which they fall, and this sheet in its turn enlightens, to a certain extent, the whole apartment, and renders objects in it visible, so long as it receives the rays of the sun. In like manner, the moon and the planets are opake bodies, but the light of the sun falling upon them, and being reflected from their surfaces, renders them visible. Were no light to fall on them from the sun, or were they not endued with a power of GENERAL PROPERTIES OF LIGHT. 25 reflecting it, they would be altogether invisible to our sight. When the moon comes between us and the sun, as in a total eclipse of that luminary, as no solar light is reflected from the surface next the earth, she is invisible — only the curve or out- line of her figure being distinguished by her sha- dow. In this case, however, there is a certain portion of reflected light on the lunar hemisphere next the earth, though not distinguishable during a solar eclipse. The earth is enlightened by the sun, and a portion of the rays which fall upon it is reflected upon the dark hemisphere of the moon which is then towards the earth. This reflected light from the earth is distinctly perceptible, when the moon appears as a slender crescent, two or three days after new moon — when the earth re- flects its light back on the moon, in the same manner as the full moon reflects her light on the earth. Hence, even at this period of the moon, her whole face becomes visible to us, but its light is not uniform or of equal intensity. The thin crescent on which the full blaze of the solar light falls, is very brilliant and distinctly seen, while the other part, on which falls only a comparatively feeble light from the earth, appears very faint, and is little more than visible to the naked eye, but With a telescope of moderate power, — if the atmos- phere be very clear — it appears beautifully distinct, so that the relative positions of many of the lunar spots may be distinguished. The intensity of reflected light is very small, when compared with that which proceeds directly from luminous bodies. M. Bouguer, a French philosopher, who made a variety of experiments to ascertain the proportion of light emitted by the heavenly bodies, concluded ftjom these experi- ments, that the light transmitted from the sun to c 26 THE PRACTICAL ASTRONOMER. the earth is at least 300,000 times as great as that which descends to us from the full moon — and that, of 300,000 rays which the moon receives, from 170,000 to 200,000 are absorbed. Hence we find that, however brilliant the moon may- appear at night — in the day time she appears as obscure as a small portion of dusky cloud to which she happens to be adjacent, and reflects no more light than a portion of whitish cloud of the same size. And as the full moon fills only the ninety thousandth part of the sky, it would require at least ninety thousand moons to produce as much light as we enjoy in the day-time under a cloudy sky. As the moon and the planets are rendered visi- ble to us only by light reflected from their surfaces, so it is in the same way that the images of most of the objects around us are conveyed to our organs of vision. We behold all the objects which compose an extensive landscape, — the hills and vales, the woods and lawns, the lakes and rivers, and the habitations of man — in consequence of the capacity with which they are endued of sending forth reflected rays to the eye, from every point of their surfaces and in all directions. In connec- tion with the reflection of light, the following curious observation may be stated. Baron Funk, visiting some silver mines in Sweden, observed, that, 6 in a clear day, it w r as as dark as pitch underground in the eye of a pit, at sixty or seventy fathoms deep ; whereas, in a cloudy or rainy day, he could see to read even at 106 fathoms deep. Enquiring of the miners, he was informed that this is always the case ; and reflect- ing upon it, he imagined it arose from this cir- cumstance, that when the atmosphere is full of clouds, light is reflected from them into the pit GENERAL PROPERTIES OF LIGHT. 27 in all directions, and that thereby a considerable proportion of the rays are reflected perpendicu- larly upon the earth : whereas when the atmos- phere is clear, there are no opaque bodies to reflect the light in this manner, at least in a suf- ficient quantity ; and rays from the sun himself can never fall perpendicularly in that country/ — The reason here assigned is, in all probability, the true cause of the phenomenon now described. 8. It is supposed by some philosophers that light is subject to the same laws of attraction that govern all other material substances — and that it is imbibed and forms a constituent part of certain bodies. This has been inferred from the pheno- mena of the Bolognian stone, and what are generally called the solar phosphori. The Bolog- nian stone was first discovered about the year 1630, by Leascariolo, a shoe-maker of Bologna. Having collected together some stones of a shining appearance at the bottom of Monte Paterno, and being in quest of some alchemical secret, he put them into a crucible to calcine them — that is, to reduce them to the state of cinders. Having taken them out of the crucible, and exposed them to the light of the sun, he afterwards happened to carry them into a dark place, when to his surprise, he observed that they possessed a self-illuminating power, and continued to emit faint rays of light for some hours afterwards. In consequence of this discovery, the Bolognian spar came into con- siderable demand among natural philosophers and the curious in general ; and the best way of pre- paring it seems to have been hit upon by the family of Zagoni, who supplied all Europe with Bolognian phosphorus, till the discovery of more powerful phosphoric substances put an end to their monopoly. — In the year 1677, Baldwin, a native c 2 28 THE PRACTICAL ASTRONOMER. ofMisnia, observed that chalk dissolved in aqua- fortis exactly resembled the Bolognian stone in its property of imbibing light, and emitting it after it was brought into the dark ; and hence it has obtained the name of Baldwin's phosphorus. In 1730 M. du Fay directed his attention to this subject, and observed that all earthy sub- stances susceptible of calcination, either by mere fire, or when assisted by the previous action of nitrous acid, possessed the property of becoming more or less luminous, when calcined and exposed for a short time in the light — that the most per- fect of these phosphori were limestones, and other kinds of carbonated lime, gypsum, and particu- larly the topaz, and that some diamonds were also observed to be luminous by simple exposure to the sun's rays. Sometime afterwards, Beccaria discovered that a great variety of other bodies were convertible into phosphori by exposure to the mere light of the sun, such as, organic animal remains, most compound salts, nitre and borax — all the farinaceous and oily seeds of vegetable substances, all the gums and several of the resins— the white woods and vegetable fibre, either in the form of paper or linen ; also starch and loaf-sugar proved to be good phosphori, after being made thoroughly dry, and exposed to the direct rays of the sun. Certain animal substances by a similar treatment were also converted into phosphori; particularly bone, sinew, glue, hair, horn, hoof, feathers, and fish-shells. The same property was communicated to rock crystal and some other of the gems, by rubbing them against each other so as to roughen their surfaces, and then placing them for some minutes in the focus of a lens, by which the rays of light were concentrated upon GENERAL PROPERTIES OF LIGHT. 29 them, at the same time that they were also mode- rately heated. In the year 1768 Mr. Canton contributed some important facts in relation to solar phosphori, and communicated a method of preparing a very powerful one, which, after the inventor, is usually called Cantons phosphorus. He affirms that his phosphorus, enclosed in a glass flask, and herme- tically sealed, retains its property of becoming luminous for at least four years, without any appa- rent decrease of activity. It has also been found that, if a common box smoothing-iron, heated in the usual manner, be placed for half a minute on a sheet of dry, white paper, and the paper be then exposed to the light, and afterwards examined in a dark closet, it will be found that the whole paper will be luminous, that part, however, on which the iron had stood being much more shining than the rest. From the above facts it would seem that certain bodies have the power of imbibing light and again emitting it, in certain circumstances, and that this power may remain for a considerable length of time. It is observed that the light which such bodies emit bears an analogy to that which they have imbibed. In general, the illuminated phos- phorus is reddish ; but when a weak light only has been admitted to it, or when it has been received through pieces of white paper, the emitted light is pale or whitish. — Mr. Morgan, in the seventy- fifth volume of the Philosophical Transactions, * treats the subject of light at considerable length ; and as a foundation for his reasoning, he assumes the following data : — 1 . That light is a body, and like all others, subject to the laws of attraction. 2. That light is a heterogeneous body ; and that .the same attractive power operates with different 80 THE PRACTICAL ASTRONOMER. degrees of force on its different parts. To the principle of attraction, likewise, Sir Isaac Newton has referred the most extraordinary phenomena of light, Refraction and Inflection. He has also endeavoured to show that light is not only subject to the law of attraction but of repulsion also, since it is repelled or reflected from certain bodies. If such principles be admitted, then, it is highly pro- bable that the phosphorescent bodies to which we have adverted have a power of attracting or imbib- ing the substance of light, and of retaining or giving it out under certain circumstances, and that the matter of light is incorporated at least with the surface of such bodies. But on this subject, as on many others, there is a difference of opinion among philosophers.* * Light of a phosphoric nature, is frequently emitted from various putrescent animal substances which, in the ages of superstition, served to astonish and affright the timorous. We learn from Fabri- cius, an Italian, that three young men, residing at Padua, having bought a lamb, and eaten part of it on Easter Day, 1592, several pieces of the remainder which they kept till the following day, shone like so many candles when they were casually viewed in the dark. The astonishment of the whole city was excited by this phenomenon, and a part of the flesh was sent to Fabricius, who was Professor of anatomy, to be examined by him. He observed, that those parts which were soft to the touch and transparent in candle-light, were the most resplendent : and also that some pieces of kid's flesh which had hap- pened to have lain in contact with them were luminous, as well as the fingers and other parts of the bodies of those persons who touched them. Bartholin gives an account of a similar phenomenon, which happened at Montpelier in 1641. A poor woman had bought a piece of flesh in the market, intending to make use of it the following day, but happening not to be able to sleep well that night, and her bed and pantry being in the same room, she observed so much light come from the flesh as to illuminate all the place where it hung. We may judge of the terror and astonishment of the woman herself, when we find that a part of this luminous flesh was carried as a very extraordinary curiosity to Henr}% Duke of Conde, the Governor of the place, who viewed it several hours with the greatest astonishment. The light was as if gems had been scattered over the surface, and continued till the flesh began to putrify, when it vanished, which it was be- lieved to do in the form of a cross. Hence the propriety of instruct- ing the mass of the community in the knowledge of the facts con- nected with the material system, and the physical causes of the various phenomena of nature. GENERAL PROPERTIES OF LIGHT. 31 9. Light is found to produce a remarkable effect on Plants and Flowers, and other vegetable produc- tions. Of all the phenomena which living vegeta- bles exhibit there are few that appear more extra- ordinary than the energy and constancy with which their stems incline toward the light. Most of the discous flowers follow the sun in his course. They attend him to his evening retreat, and meet his rising lustre in the morning with the same unerring law. They unfold their flowers on the approach of this luminary ; they follow his course by turning on their stems, and close them as soon as he disappears. If a plant, also, is shut up in a dark room, and a small hole afterwards opened by which the light of the sun may enter, the plant will turn towards that hole, and even alter its own shape in order to get near it ; so that though it was straight before, it will in time become crooked, that it may get near the light. Vegetables placed in rooms where they receive light only in one direction, always extend themselves in that direc- tion. If they receive light in two directions, they direct their course towards that which is strongest. It is not the heat but the light of the sun which the plant thus covets ; for, though a fire be kept in the room, capable of giving a much stronger heat than the sun, the plant will turn away from the fire in order to enjoy the solar light. Trees growing in thick forests, where they only receive light from above, direct their shoots almost invaria- bly upwards, and therefore become much taller and less spreading than such as stand single. The green colour of plants is likewise found to depend on the sun's light being allowed to shine on them ; for without the influence of the solar light, they are always of a white colour. It is found by experiment that, if a plant which has 32 THE PRACTICAL ASTRONOMER. been reared in darkness be exposed to the light of day, in two or three days it will acquire a green colour perceptibly similar to that of plants which have grown in open day-light. If we expose to the light one part of the plant, whether leaf or branch, this part alone will become green. If we cover any part of a leaf with an opake substance, this place will remain white, while the rest be- comes green. The whiteness of the inner leaves of cabbages is a partial effect of the same cause, and many other examples of the same kind might easily be produced. M. Decandolle, who seems to have paid particular attention to this subject, has the following remarks : 6 It is certain, that between the white state of plants vegetating in darkness, and complete verdure, every possible intermediate degree exists, determined by the in- tensity of the light. Of this any one may easily satisfy himself by attending to the colour of a plant exposed to the full day-light ; it exhibits in succession all the degrees of verdure. I had already seen the same phenomenon, in a particular manner, by exposing plants reared in darkness to the light of lamps. In these experiments, I not only saw the colour come on gradually, according to the continuance of the exposure to light ; but I satis- fied myself, that a certain intensity of permanent light never gives to a plant more than a certain degree of colour. The same fact readily shows itself in nature, when we examine the plants that grow under shelter or in forests, or when we exa- mine in succession the state of the leaves that form the heads of cabbages.' * It is likewise found that the perspiration of vegetables is increased or diminished, in a certain measure by the degree of light which falls upon * Memoires de la Soc. d'Aroncil, vol. ii. GENERAL PROPERTIES OF LIGHT. 33 them. The experiments of Mr. P. Miller and others, prove that plants uniformly perspire most in the forenoon, though the temperature of the air in which they are placed should be unvaried. M. Guettard likewise informs us that a plant ex- posed to the rays of the sun, has its perspiration increased to a much greater degree than if it had been exposed to the same heat under the shade. Vegetables are likewise found to be indebted to light for their smell, taste, combustibility, matu- rity, and the resinous principle, which equally depend upon this fluid. The aromatic substances, resins, and volatile oil are the productions of southern climates, where the light is more pure, constant, and intense. In fine, another remarkable property of light on the vegetable kingdom is that, when vegetables are exposed to open day- light, or to the sun's rays, they emit oxygen gas or vital air. It has been proved that, in the pro- duction of this effect, the sun does not act as a body that heats. The emission of the gas is determined by the light : pure air is therefore separated by the action of light, and the operation is stronger as the light is more vivid. By this continual emission of vital air, the Almighty inces- santly purifies the atmosphere, and repairs the loss of pure air occasioned by respiration, combustion, fermentation, putrefaction, and numerous other processes which have a tendency to contaminate this fluid so essential to the vigor and comfort of animal life ; so that, in this way, by the agency of light, a due equilibrium is always maintained be- tween the constituent parts of the atmosphere. In connection with this subject the following curious phenomenon may be stated, as related by M. Haggern, a Lecturer on Natural History in Sweden. One evening he perceived a faint flash c 5 34 THE PRACTICAL ASTRONOMER* of light repeatedly dart from a marigold. Sur- prised at such an uncommon appearance, he re- solved to examine it with attention ; and, to be assured it was no deception of the eye, he placed a man near him, with orders to make a signal at the moment when he observed the light. They both saw it constantly at the same moment. The light was most brilliant on marigolds of an orange or flame colour, but scarcely visible on pale ones. The flash was frequently seen on the same flower two or three times in quick succession ; but more commonly at intervals of several minutes ; and when several flowers in the same place emitted their light together, it could be observed at a con- siderable distance. The phenomenon was remarked in the months of July and August at sun-set, and for half an hour when the atmosphere was clear ; but after a rainy day, or when the air was loaded with vapours, nothing of it was seen. The fol- lowing flowers emitted flashes more or less vivid, in this order: — 1. The Marigold, 2. Monk's hood, 3. The Orange Lily, 4. The Indian Pink. As to the cause of this phenomenon, different opinions may be entertained. From the rapidity of the flash and other circumstances, it may be conjectured that electricity is concerned in producing this appearance. M. Haggern, after having observed the flash from the orange lily, the antheras of which are at considerable distance from the petals, found that the light proceeded from the petals only ; whence he concludes, that this electrical light is caused by the pollen which, in flying off, is scat- tered on the petals. But, perhaps, the true cause of it still remains to be ascertained. 10. Light has been supposed to produce a cer- tain degree of influence on the propagation of sound? — M. Parolette, in a long paper in the GENERAL PROPERTIES OF LIGHT. 35 ' Journal de Physique/ vol. 68, which is copied into 6 Nicholson's Philosophical Journal, vol. 25, pp, 28 — 39, — has offered a variety of remarks, and detailed a number of experiments on this subject. The author states the following circumstances as having suggested the connection between light and sound. ' In 1803, I lived in Paris, and being accustomed to rise before day to finish a work on which I had long been employed, I found myself frequently disturbed by the sound of carriages, as my windows looked into one of the most fre- quented streets in that city. This circumstance which disturbed me in my studies every morning, led me to remark, that the appearance of day-break peculiarly affected the propagation of the sound: from dull and deep, which it was before day, it seemed to me to acquire a more sonorous sharp- ness in the period that succeeded the dissipation of darkness. The rolling of the wheels seemed to announce the friction of some substances grown more elastic ; and my ear on attending to it per- ceived this difference diminish, in proportion as the sound of wheels was confounded with those excited by the tumult of objects quitting their nocturnal silence. Struck with this observation, I attempted to discover whether any particular causes had deceived my ears. I rose several times be-fore day for this purpose alone, and was every time confirmed in my suspicion, that light must have a peculiar influence on the propagation of sound. This variation, however, in the manner in which the air gave sounds might be the effect of the agitation of the atmosphere produced by the rarefaction the presence of the sun occasioned ; but the situation of my windows, and the usual direction of the morning breeze, militated against this argument.' 36 THE PRACTICAL ASTRONOMER. The author then proceeds to give a description of a very delicate instrument, and various appa- ratus for measuring the propagation and intensity of sound, and the various experiments both in the dark, and in day-light, and likewise under different changes of the atmosphere, which were made with his apparatus — all of which tended to prove that light had a sensible influence in the propagation of sound. But the detail of these experiments and their several results would be too tedious to be here transcribed. — The night has generally been considered as more favourable than the day for the transmission of sound. ' That this is the case (says Parolette) with respect to our ears cannot be doubted; but this argues nothing against my opinion. We hear further by night on account of the silence, and this always contributes to it, while the noise of a wind favourable to the propa- gation of a sound, may prevent the sound from being heard.' In reference to the cause which produces the effect now stated, he proposes the following queries. * Is the atmospheric air more dense on the appearance of light than in darkness? Is this greater density of the air or of the elastic fluid that is subservient to the propagation of sound, the effect of aeriform substances kept in this state through the medium of light ? ' He is disposed, on the whole, to conclude, that the effect in question is owing to the action of light upon the oxygen of the atmosphere, since oxygen gas is found by experiment to be best adapted to the transmission of sound. Our author concludes his communication with the following remarks : — ' Light has a velocity 900,000 times as rapid as that of sound. Whe- ther it emanate from the sun and reach to our earth, or act by means of vibrations agitating the GENERAL PROPERTIES OF LIGHT. 37 particles of a fluid of a peculiar nature — the par- ticles of this fluid must be extremely light, elastic and active. Nor does it appear to me unreason- able, to ascribe to the mechanical action of these particles set in motion by the sun, the effects its presence occasions in the vibrations that proceed from sonorous bodies. The more deeply we in- vestigate the theory of light, the more we must perceive, that the powers by which the universe is moved reside in the imperceptible particles of bodies ; and that the grand results of nature are but an assemblage of an order of actions that take place in its infinitely small parts; consequently, we cannot institute a series of experiments more interesting than those which tend to develope the properties of light. Our organs of sense are so immediately connected with the fluid that enlightens us, that the notion of having acquired an idea of the mode of action of this fluid presents itself to our minds, as the hope of a striking advance in the knowledge of what composes the organic mechanism of our life, and of that of beings which closely follow the rank assigned to the human species.' Such is a brief description of some of the lead- ing properties of light. Of all the objects that present themselves to the philosophic and con- templative mind, light is one of the noblest and most interesting. The action it exerts on all the combinations of matter, its .extreme divisibility, the rapidity of its propagation, the sublime wonders it reveals, and the office it performs in what con- stitutes the life of organic beings, lead us to con- sider it as a substance acting the first part in the economy of nature. The magic power which this emanation from the heavens exerts on our organs 38 THE PRACTICAL ASTRONOMER. Of vision, in exhibiting to our view the sublime spectacle of the universe, cannot be sufficiently admired. Nor is its power confined to the organs of sight ; all our senses are, in a greater or less degree, subjected to the action of light, and all the objects in this lower creation — whether in the animal, the vegetable, or the mineral kingdoms — are, to a certain extent, susceptible of its in- fluence. Our globe appears to be little more than an accumulation of terrestrial materials introduced into the boundless ocean of the solar light, as a theatre on which it may display its exhaustless power and energy, and give animation, beauty and sublimity to every surrounding scene — and to regulate all the powers of nature* and render thern subservient to the purposes for which they were ordained. This elementary substance appears to be universal in its movements, and in its influence. It descends to us from the solar orb. It wings its way through the voids of space, along a course of ninety -five millions of miles, till it arrives at the outskirts of our globe ; it passes freely through the surrounding atmosphere, it strikes upon the clouds and is reflected by them ; it irradiates the mountains, the vales, the forests, the rivers, the seas, and all the productions of the vegetable king- dom, and adorns them with a countless assemblage of colours. It scatters and disperses its rays from one end of creation to another, diffusing itself throughout every sphere of the universe. It flies without intermission.from star to star, and from suns to planets, throughout the boundless sphere of immensity, forming a connecting chain and a medium of communication among all the worlds and beings within the wide empire of Omnipo- tence. When the sun is said "to rule over the day/' it GENERAL PROPERTIES OF LIGHT. 39 is intimated that he acts as the vicegerent of the Almighty, who has invested him with a mechani- cal power of giving light, life and motion to all the beings susceptible of receiving impressions from his radiance. As the servant of his creator he distributes blessings without number among all the tribes of sentient and intelligent existence. When his rays illumine the eastern sky in the morn- ing, all nature is enlivened with his presence. When he sinks beneath the western horizon, the flowers droop, the birds retire to their nests, and a mantle of darkness is spread over the landscape of the world. When he approaches the equinox in spring, the animal and vegetable tribes revive, and nature puts on a new and a smiling aspect. When he declines towards the winter solstice, dreariness and desolation ensue, and a temporary death takes place among the tribes of the vegeta- ble world. — This splendid luminary, whose light embellishes the whole of this lower creation, forms the most lively representation of Him who is the source and the centre of all beauty and perfection. " God is a sun," the sun of the moral and spirit- ual universe, from whom all the emanations of knowledge, love and felicity descend. c< He covereth himself with light as with a garment." and Fig. 21. shows the direction of diverging rays, or those which proceed from a near object. These rays proceeding from an object further from the mirror than the true focal point, as from D to A and to B, are reflected converging and meet at a point F, further from the mirror than the focal point of parallel rays. If the distance of the ra- figure 21. 92 THE PRACTICAL ASTRONOMER. diant, or object D, be equal to the radius CE, then will the focal distance be likewise equal to the radius : That is, if an object be placed in the center of a concave speculum, the image will be reflected upon the object, or they will seem to meet and embrace each other in the centre. If the distance of the radiant be equal to half the radius, its image will be reflected to an infinite distance, for the rays will then be parallel. If, therefore, a luminous body be placed at half the radius from a concave speculum, it will enlighten places directly before it at great distances. Hence their use when placed behind a candle in a common lantern ; Hence their utility in throwing light upon objects in the Magic Lantern and Phantasmagoria, and hence the vast importance of very large mir- rors of this description, as now used in most of our Light Houses, for throwing a brilliant light to great distances at sea to guide the mariner when directing his course under the cloud of night. When converging rays fall upon a concave mir- ror, they are reflected more converging and unite at a point between the focus of parallel rays and the mirror ; that is, nearer the mirror than one half the radius ; and their precise degree of con- vergency will be greater than that wherein they converged before reflection. Of the images formed by Concave Mirrors. If rays proceeding from a distant object fall upon a concave speculum, they will paint an image or representation of the object on its focus before the mirror. This image will be inverted, because the rays cross at the points where the image is formed. We have already seen that a convex glass forms an image of an object behind it ; the rays of light ON THE REFLECTION OF LIGHT. 93 from objects pass through the glass, and the pic- ture is formed on the side farthest from the object. But in concave mirrors the images of distant ob- jects — and of all objects that are farther from its surface than its principal focus — are formed before the mirror, or on the same side as the object. In almost every other respect, however, the effect of a concave mirror is the same as that of a convex lens, in regard to the formation of images, and the course pursued by the rays of light, except that the effect is produced in the one case by refraction, and in the other by reflection. The following figure represents the manner in which images are formed by concave mirrors. GF represents the reflecting surface of the mirror ; OAB, the object ; and IaM, the image formed by the mirror. The rays proceeding from O, will be carried to the mirror, in the direction OG, and according to the law that the angle of incidence is equal to the angle of reflection, will be reflected to I, in the direction GI. In like manner the rays figure 22. O 94 THE PRACTICAL ASTRONOMER. from B, will be reflected from F to M, the rays from A, will be reflected to a, and so of all the intermediate rays, so that an inverted image of the object OB, will be formed at IM. If the rays proceeded from objects at a very great dis- tance the image would be formed in the real focus of the mirror, or at one-fourth the diameter of the sphere from its surface ; but near objects, which send forth diverging rays, will have their images formed a little farther from the surface of the mirror. If we suppose a real object placed at IM, then OB will represent its magnified image, which will be larger than the object, in proportion to its dis- tance from the mirror. This may be experimen- tally illustrated by a concave mirror and a candle. Suppose a concave mirror whose focal distance is five inches, and that a candle is placed before it, at a little beyond its focus, (as at IM) — suppose at five and a half inches, — and that a wall or white screen receives the image, at the distance of five feet six inches from the mirror, an image of the candle will be formed on the wall which will be twelve times longer and broader than the candle itself. In this way concave mirrors may be made to magnify the images of objects to an indefinite extent. This experiment is an exact counterpart of what is effected in similar circumstances by a convex lens, as described p. 74 ; the mirror per- forming the same thing by reflection, as the lens did by refraction. From what has been stated in relation to con- cave mirrors it will be easily understood how they make such powerful burning-glasses. Suppose the focal distance of a concave mirror to be twelve inches, and its diameter or breadth twelve inches. When the sun's rays fall on such a mirror, they ON THE REFLECTION OF LIGHT. 95 form an image of the sun at the focal point whose diameter is found to be about one-tenth of an inch. All the rays which fall upon the mirror are con- verged into this small point; and consequently their intensity is in proportion as the square of the surface of the mirror is to the square of the image. The squares of these diameters are as 14,400 to 1; and consequently the density of the sun's rays, in the focus, is to their density on the surface of the mirror as 14,400 to 1. That is, the heat of the solar rays in the focus of such a mirror will be fourteen thousand four hundred times greater than before — a heat which is capable of producing very powerful effects in melting and setting fire to sub- stances of almost every description. Were we desirous of forming an image by a concave speculum which shall be exactly equal to the object, the object must be placed exactly in the centre ; and, by an experiment of this kind, the centre of the concavity of a mirror may be found. In the cases now stated, the images of objects are all formed in the front of the mirror, or be- tween it and the object. But there is a case in which the image is formed behind the mirror. This happens when the object is placed between the mirror and the focus of parallel rays, and then the image is larger than the object. In fig. 23, GF is a concave mirror, whose focus of parallel rays is at E. If an object OB be placed a little within this focus, as at A, a large image IM will be seen behind the mirror, somewhat curved and erect, which will be seen by an eye looking directly into the front of the mirror. Here the image ap- pears at a greater distance behind the mirror than the object is before it, and the object appears mag- nified in proportion to its distance from the focus 96 THE PRACTICAL ASTRONOMER. figure 23. and the mirror. If the mirror be one inch focal distance , and the object be placed eight-tenths of an inch from its surface, the image would be five times as large as the object in length and breadth, and consequently twenty-five times larger in sur- face. In this way small objects may be magnified by reflection, as such objects are magnified by re- fraction, in the case of deep convex lenses. When such mirrors are large, for example six inches di- ameter, and eight or ten inches focal distance, they exhibit the human face as of an enormous bulk. This is illustrated by the following figure. Let c n, Fig. 24, represent the surface of a concave mirror, and A a human face looking into it, the face will appear magnified as represented by the image behind the mirror d q. Suppose a ray a c proceeding from the forehead, and another m n from the chin ; these rays are reflected to the per- son's eye at o, which consequently sees the image in the lines of reflection od,oq, and in the angle ON THE REFLECTION OF LIGHT. 97 figure 24, B o q , and consequently magnified much beyond the natural size, and at a small distance behind the mirror. If we suppose the side T u to represent a convex mirror, and the figure d q a head of an ordinary size, then the figure A will represent the dimin- ished appearance which a person's face exhibits, when viewed in such a mirror. It will not only appear reduced, but somewhat distorted ; because from the form of the mirror, one part of the object is nearer to it than another, and consequently will be reflected under a different angle. The effect we have now mentioned as produced by concave mirrors, will only take place when the eye is nearer the mirror than its principal focus. If the spectator retire beyond this focus — suppose to the distance of five or six feet, he will not see the image behind the mirror; but he will see his image in a diminished form, hanging upside down, and suspended in the air, in a line between his F 98 THE PRACTICAL ASTRONOMER. eye and the mirror. In this case, his image is formed before the mirror as represented at IM fig. 22. In this situation, if you hold out your hand towards the mirror, the hand of the image will come out towards your hand, and, when at the centre of concavity, it will be of an equal size with it, and you may shake hands with this aerial image. If you move your hand farther, you will find the hand of the image pass by your hand, and come between it and your body. If you move your hand towards either side, the hand of the image will move towards the other side; the image moving always in a contrary direction to the ob- ject. All this while the by-standers, if any, see nothing of the image, because none of the reflected rays that form it can enter their eyes, — The fol- lowing figure represents a phenomenon produced in the same manner, a b is a concave mirror of a large size ; c represents a hand presented before the mirror, at a point farther distant than its focus. figure 25. A B In this case, an inverted image of the hand is formed which is seen hanging in the air at M. The ON THE REFLECTION OF LIGHT. 99 rays c and d go diverging from the two opposite points of the object, and by the action of the mirror, they are again made to converge to points at o and s where they cross, form an image, and again proceed divergent to the eye.'* In consequence of the properties of concave mirrors, now described, many curious experiments and optical deceptions have been exhibited. The appearance of images in the air, suspended between the mirror and the object, have sometimes been displayed with such dexterity and an air of mys- tery, as to have struck with astonishment those who were ignorant of the cause. In this way birds, flying angels, spectres and other objects have been exhibited, and when the hand attempts to lay hold on them, it finds them to be nothing, and they seem to vanish into air. An apple or a beau- tiful flower is presented, and when a spectator attempts to touch it, it instantly vanishes, and a death's head immediately appears, and seems to snap at his fingers. A person with a drawn sword appears before him, in an attitude as if about to run him through, or one terrific phantom starts up after another, or sometimes the resemblances of deceased .persons are made to appear, as if, by the art of conjuration, they had been forced to return from the world of spirits. In all such exhibitions, a very large concave mirror is requisite, a brilliant light must be thrown upon the objects, and every * Small glass mirrors for performing some of the experiments, and illustrating some of the principles above alluded to, — may be made of the flattest kind of common watch glasses, by foliating or covering with tin leaf and quicksilver the convex surfaces of such glasses. Their focal distances will generally be from one to two inches. Such mir- rors afford a very large and beautiful view of the eye, when held within their focal distance of that organ. Such mirrors will also serve the purpose of reflecting light on the objects viewed by microscopes. Larger mirrors, of from four to eight inches diameter, may be had of the optician at different prices varying from five to ten or fifteen shillings. 100 THE PRACTICAL ASTRONOMER. arrangement is made, by means of partitions, &c, to prevent either the light, the mirror, or the ob- ject from being seen by the spectators. The fol- lowing representation (fig. 26.) shows one of the methods by which this is effected : a is a large concave mirror, either of metal or of glass, placed on the back part of a dark box, d is the performer, concealed from the spectators by the cross parti- tion c ; e is a strong light, which is likewise con- cealed by the partition i, which is thrown upon the actor D r or upon any thing he may hold in his hand. If he hold a book, as represented in the figure, the light reflected from it will pass between the partitions c and i to the mirror, and will be figure 26. reflected from thence to z, where the image of the book will appear so distinct and tangible, that a ON THE REFLECTION OF LIGHT. 101 spectator locking through the opening at x, will imagine that it is in his power to take hold of it. In like manner, the person situated at d, may ex- hibit his own head or body — a portrait, a painting, a spectre, a landscape, or any object or device which he can strongly illuminate. There is another experiment, made with a con- cave mirror, which has somewhat puzzled philo- sophers to account for the phenomena. Take a glass bottle AC, (fig. 27) and fill it with water to the point B ; leave the upper part BC empty, and cork it in the common manner. Place this bottle opposite a concave mirror, and beyond its focus, that it may appear reversed, and, before the mirror place yourself still further distant from the bottle, and it will appear in the situation a b c. Now, it figure 27. 102 THE PRACTICAL ASTRONOMER. is remarkable in this apparent bottle, that the water, which, according to the laws of catoptrics, should appear at A B, appears on the contrary at b c, and consequently, the part a b appears empty. If the bottle be inverted and placed before the mirror, its image will appear in its natural erect position, and the water which is in reality at BC (fig. 28) is seen at a b. If while the bottle is inverted, it be uncorked, and the water run gently out, it w 7 ill appear, that, while the part BC is emptying, that of a b in the image is filling, and, what is remarkable, as soon as the bottle is empty, the illusion ceases, the image also appear- ing entirely empty. — The remarkable circumstances in this experiment are, first, not only to see the object where it is not, but also where its image is not ; and secondly, that of two objects which are really in the same place, as the surface of the bottle and the water it contains, the one is seen at one place, and the other at another ; and to see the bottle in the place of its image, and the water where neither it nor its image are. The following experiments are stated by Mr. Ferguson in his " Lectures on select Subjects/'&c. " If a fire be made in a large room, and a smooth mahogany table be placed at a good distance near the wall, before a large concave mirror, so placed that the light of the fire may be reflected from the mirror to its focus upon the table ; if a person stand by the table, he will see nothing upon it but a longish beam of light : but if he stand at a dis- tance toward the fire, not directly between the fire and mirror, he will see an image of the fire upon the table, large and erect. And if another person who knows nothing of the matter beforehand should chance to come into the room, and should look from the fire toward the table, he would be ON THE REFLECTION OF LIGHT. 103 startled at the appearance ; for the table would seem to be on fire, and by being near the wainscot, to endanger the whole house. In this experiment there should be no light in the room but what pro- ceeds from the fire ; and the mirror ought to be at least fifteen inches in diameter. If the fire be darkened by a screen, and a large candle be placed at the back of the screen, a person standing by the candle will see the appearance of a very fine large star, or rather planet, upon the table, as bright as Venus or Jupiter. And if a small wax taper — whose flame is much less than the flame of the candle— be placed near the candle, a satellite to the planet will appear on the table ; and if the taper be moved round the candle, the satellite will go round the planet." Many other illustrations of the effects of concave specula might have been given, but I shall con- clude this department by briefly stating some of the general properties of speculums. 1. There is a great resemblance between the properties of convex lenses and concave mirrors. They both form an inverted focal image of any re- mote object, by the convergence of the pencil of rays. In those instruments whose performances are the effects of reflection, as reflecting telescopes, the concave mirror is substituted in the place of the convex lens. The whole effect of these instru- ments, in bringing to view remote objects in hea- ven and on earth, entirely depend on the property of a concave mirror in forming images of objects in its focus. 2. The image of an object placed beyond the centre, is less than the object ; if the object be placed between the principal focus and the centre, the image is greater than the object. In both cases the image is inverted. 3. When the object is placed between the focus and the 104 THE PRACTICAL ASTRONOMER. mirror, the image situated behind the mirror is greater than the object, and it has the same direc- tion : in proportion as the object approaches the focus, the image becomes larger and more distant. These and similar results are proved by placing a lighted candle at different distances from a concave mirror. 4. An eye cannot see an image in the air except it be placed in the diverging rays ; but if the image be received on a piece of white paper, it may be seen in any position of the eye, as the rays are then reflected in every direction. 5. If a picture drawn according to the rules of perspec- tive, be placed before a large concave speculum, a little nearer than its principal focus, the image of the picture will appear extremely natural, and very nearly like the real objects whence it was taken. Not only are the objects considerably magnified, so as to approach to their natural size, but they have also different apparent distances, as in nature, so that the view of the inside of a church appears very like what it is in reality, and representations of landscapes appear very nearly, as they do from the spot whence they were taken. In this respect a large concave speculum may be made to serve nearly the same purpose, as the Optical Diagonal Machine, in viewing perspective prints. 6. The concave speculum is that alone which is used as the great mirror which forms the first image in re- flecting telescopes ; and it is likewise the only kind of speculum used as the small mirror, in that con- struction of the instrument called the Gregorian Reflector. Quantity of light reflected by polished surfaces. As this is a circumstance connected with the construction of reflecting telescopes, it may not be ON THE REFLECTION OF LIGHT. 105 improper, in this place, to state some of the results of the accurate experiments of M. Bonguer on this subject. This philosopher ascertained that of the light reflected from mercury, or quicksilver, more than one-fourth is lost, though it is probable that no substances reflect more light than this. The rays were received at an angle of eleven and a half degrees of incidence, measured from the sur- face of the reflecting body, and not from the per- pendicular. The reflection from water was found to be almost as great as that of quicksilver ; so that in very small angles it reflects nearly three- fourths of the direct light. This is the reason why so strong a reflection appears on water, when one walks, in still weather, on the brink of a lake op- posite to the sun. The direct light of the sun diminishes gradually as it approaches the horizon, while the reflected light at the same time grows stronger ; so that there is a certain elevation of the sun in which the united force of the direct and reflected light will be the greatest possible, and this is when he is twelve or thirteen degrees in altitude. On the other hand, light reflected from water at great angles of incidence is extremely small. When the light was perpendicular, it re- flected no more than the thirty-seventh part which mercury does in the same circumstances, and only the fifty-fifth part of what fell upon it in this case. Using a smooth piece of glass, one line in thick- ness, he found that, when it was placed at an angle of fifteen degrees with the incident rays, it reflected 628 parts of 1000 which fell upon it; at the same time, a metallic mirror which he tried in the same circumstances, reflected only 561 of them. At a less angle of incidence much more light was reflected ; so that at an angle of three degrees, the glass reflected 700 parts, and the metal something F 5 106 THE PRACTICAL ASTRONOMER. less, as in the former case. The most striking ob- servations made by this experimenter relate to the very great difference in the quantity of light re- flected at different angles of incidence. He found that for 1000 incident rays, the reflected rays, at different angles of incidence, were as follows. Angles of Rays reflected Rays reflected incidence by water by glass 5° 501 549 10 333 412 15 211 299 30 65 . 112 50 22 34 70 18 25 90 18 25 With regard to such mirrors as the specula of reflecting telescopes, it will be found, in general, that they reflect little more than the one half of the rays which fall upon them. Uncommon appearances in nature produced by the combined influences of Reflection and Refrac- tion. The reflection and refraction of the rays of light frequently produce phenomena which astonish the beholders, and which have been regarded by the ignorant and the superstitious, as the effects of supernatural agency. Of these phenomena I shall state a few examples. One of the most striking appearances of this kind is what has been termed the Fata Morgana, or optical appearances of figures in the sea and the air, as seen in the Faro of Messina. The follow- ing account is translated from a work of Minasi, who witnessed the phenomenon, and wrote a dis- ON THE REFLECTION OF LIGHT. 107 sertation on the subject. " When the rising sun shines from that point whence its incident ray forms an angle of about forty-five degrees to the sea of Riggio, and the bright surface of the water in the bay is not disturbed either by the wind or the cur- rent, the spectator being placed on an eminence of the city, with his back to the sun and his face to the sea ; — on a sudden there appear on the water, as in a catoptric theatre, various multiplied objects, that is to say, numberless series of pilasters, arches, castles well delineated, regular columns, lofty towers, superb palaces, with balconies and windows, extended alleys of trees, delightful plains with herds and flocks, armies of men on foot and horse- back, and many other strange images, in their na- tural colours and proper actions, passing rapidly in succession along the surface of the sea, during the w r hole of the short period of time, while the above mentioned causes remain. — But, if in ad- dition to the circumstances now described, the at- mosphere be highly impregnated with vapour and dense exhalations, not previously dispersed by the winds or the sun, it then happens that, in this vapour, as in a curtain extended along the channel, at the height of about thirty palms, and nearly down to the sea, the observer will behold the scene of the same objects, not only reflected from the surface of the sea, but likewise in the air, though not so distant or well defined, as the former objects from the sea. — Lastly, if the air be slightly hazy or opake, and at the same time dewy and adapted to form the iris, the then above-mentioned ob- jects will appear only at the surface of the sea, as in the first case, but all vividly coloured or fringed with red, green, blue and other prismatic colours." * * Nicholson's Journal of Natural Philosophy, &c. 4to. series, p. 225. 108 THE PRACTICAL ASTRONOMER. It is somewhat difficult to account for all the appearances here described ; but, in all probability, they are produced by a calm sea, and one or more strata of superincumbent air differing in refractive and consequently in reflective power. At any rate reflection and refraction are some of the essential causes which operate in the production of the phenomena. The Mirage, seen in the deserts of Africa, is a phenomenon, in all probability produced by a similar cause. M. Monge, who accompanied the French army to Egypt, relates that, when in the desert between Alexandria and Cairo, the mirage of the blue sky was inverted, and so mingled with the sand below, as to give to the desolate and arid wilderness an appearance of the most rich and beautiful country. They saw, in all directions, green islands, surrounded with extensive lakes of pure, transparent water. Nothing could be con- ceived more lovely and picturesque than the land- scape. In the tranquil surface of the lakes, the trees and houses with which the islands were co- vered, were strongly reflected with vivid and varied hues, and the party hastened forward to enjoy the cool refreshments of shade and stream which these populous villages proffered to them. When they arrived, the lake on whose bosom they floated, the trees among whose foliage they were embowered, and the people who stood on the shore inviting their approach, had all vanished, and nothing re- mained but an uniform and irksome desert of sand and sky, with a few naked huts and ragged Arabs. Had they not been undeceived by their nearer ap- proach, there was not a man in the French army who would not have sworn that the visionary trees and lakes had a real existence in the midst of the desert. ON THE REFLECTION OF LIGHT. 109 Dr. Clark observed precisely the same appear- ances at Rosetta. The city seemed surrounded with a beautiful sheet of water ; and so certain was his Greek interpreter — who was unacquainted with the country — of this fact, that he was quite indignant at an Arab who attempted to explain to him that it was a mere optical delusion. At length they reached Rosetta in about two hours, without meeting with any water ; and on looking back on the sand they had just crossed, it seemed to them as if they had waded through a vast blue lake. On the 1st of August, 1798, Dr. Vince observed at Ramsgate a ship which appeared as at a, (fig. 29.) the topmast being the only part of it that figure 29. was seen above the horizon. An inverted image of it was seen at b, immediately above the real ship a, and an erect image at c, both of them 110 THE PRACTICAL ASTRONOMER, being complete and well defined. The sea was dis- tinctly seen between them, as at v w. As the ship rose to the horizon the image c gradually disap- peared, and while this was going on, the image b descended, but the mainmast of b did not meet the mainmast of a. The two images bc were perfectly visible when the whole ship was actually below the horizon. Dr. Vince then directed his telescope to another ship whose hull was just in the horizon, and he observed a complete inverted image of it, the mainmast of which just touched the mainmast of the ship itself. He saw at the same time several other ships whose images ap- peared in nearly a similar manner, in one of which the two images were visible when the whole ship was beneath the horizon. These phenomena must have been produced by the same causes which operated in the case formerly mentioned, in relation to Captain Scoresby, when he saw the figure of his father's ship inverted in the distant horizon. Such cases are, perhaps not uncommon, especially in calm and sultry weather, but they are seldom observed, except when a person's at- tention is accidentally directed to the phenomenon, and, unless he use a telescope, it will not be so distinctly perceived. The following phenomenon, of a description nearly related to the above, has been supposed to be chiefly owing to reflection. On the 18th of November, 1804, Dr. Buchan, when watching the rising sun, about a mile to the east of Brighton, just as the solar disk emerged from the surface of the water, saw the face of the cliff on which he was standing, a windmill, his own figure and the figure of his friend, distinctly represented, precisely opposite, at some distance from the ocean. This appearance lasted about ten minutes, ON THE REFLECTION OF LIGHT. Ill till the sun had risen nearly his own diameter above the sea. The whole then seemed to be elevated into the air and successively disappeared. The surface of the sun was covered with a dense fog of many yards in height, which gradually receded from the rays of the sun as he ascended from the horizon. The following appearance most probably arose chiefly from the refraction of the atmosphere. It was beheld at Ramsgate, by Dr. Vince of Cam- bridge and another gentleman. It is well known that the four turrets of Dover castle are seen at Ramsgate, over a hill which intervenes between a full prospect of the whole. On the 2nd of August, 1806, not only were the four turrets visible, but the castle itself appeared as though situated on that side of the hill nearest Ramsgate, and so striking was the appearance, that for a long time the Doctor thought it an illusion ; but at last, by accurate observation, was convinced that it was an actual image of the castle. He, with another in- dividual, observed it attentively for twenty minutes, but were prevented by rain from making further observations. Between the observers and the land from which the hill rises, there were about six miles of sea, and from thence to the top of the hill there was about the same distance, their own height above the surface of the water was about seventy feet.- — The cause of this phenomenon was, undoubtedly, unequal refraction. The air being more dense near the ground and above the sea than at greater heights, reached the eye of the observer, not in straight but in curvelinear lines. If the rays from the castle had in their path struck an eye at a much greater distance than Ramsgate, the probability is, that the image of the castle would have been inverted in the air; but in the 112 THE PRACTICAL ASTRONOMER. present case, the rays from the turret and the base of the castle had not crossed each other. To similar causes as those now alluded to are to be attributed such phenomena as the following : The Spectre of the Brocken. This is a wonder- ful and, at first sight, a terrific phenomenon, which is sometimes seen from the summit of one of the Hartz mountains in Hanover, which is about 3,300 feet above the level of the sea, and overlooks all the country fifteen miles round. From this mountain the most gigantic and terrific spectres have been seen, which have terrified the credulous, and gratified the curious, in a very high degree. M. Hawe who witnessed this phenomenon, says, the sun rose about four o'clock, after he had as- cended to the summit, in a serene sky, free of clouds ; and about a quarter past five, when look- ing round to see if the sky continued clear, he suddenly beheld at a little distance, a human figure of a monstrous size turned towards him, and glaring at him. While gazing on this gigantic spectre, with a mixture of awe and apprehension, a sudden gust of wind nearly carried off his hat, and he clapt his hand to his head to detain it, when to his great delight, the colossal spectre did the same. He changed his body into a variety of attitudes, all which the spectre exactly imitated, and then suddenly vanished without any apparent cause, and, in a short time as suddenly appeared. Being joined by another spectator, after the first visions had disappeared, they kept steadily looking for the aerial spectres, when two gigantic monsters suddenly appeared. These spectres had been long considered as preternatural, by the inhabi- tants of the adjacent districts, and the whole country had been filled with awe and terror. Some of the lakes of Ireland are found to be susceptible ON THE REFLECTION OF LIGHT. 113 of producing illusions, particularly the lake of Killarney. This romantic sheet of water is bounded on one side, by a semicircle of rugged mountains, and on the other by a flat morass ; and the vapours generated in the marshy and broken by the mountains, continually represent the most fantastic objects. Frequently men riding along the shore are seen as if they were moving across the lake, which is supposed to have given rise to the legend of O'Donougho, a magician who is said to be visible on the lake every May morn- ing. There can be little doubt that most of those visionary appearances which have been frequently seen in the sky and in mountainous regions, are phantoms produced by the cause to which I am adverting, such as armies of footmen and horse- men, which some have asserted to have been seen in the air near the horizon. A well authenticated instance of this kind occurred in the Highlands of Scotland :— Mr. Wren of Wetton Hall, and D. Stricket his servant, in the year 1744, were sitting at the door of the house in a summer evening, when they were surprised to see opposite to them on the side of Sonterfell hill — a place so extremely steep, that scarce a horse could walk slowly along it — the figure of a man with a dog pursuing several horses, all running at a most rapid pace. Onwards they passed till at last they disap- peared at the lower end of the Fell. In expec- tation of finding the man dashed to pieces by so tremendous a fall, they went early next morning and made a search, but no trace of man or horse, or the prints of their feet on the turf could be found. Sometime afterwards, about seven in the evening, on the same spot, they beheld a troop of horsemen advancing in close ranks and at a brisk 114 THE PRACTICAL ASTRONOMER. pace. The inmates of every cottage for a mile round beheld the wondrous scene, though they had formerly ridiculed the story told by Mr. Wren and his servant, and were struck with surprise and fear. The figures were seen for upwards of two hours, till the approach of darkness rendered them invisible. The various evolutions and changes through which the troops passed were distinctly visible, and were marked by all the observers. It is not improbable that these aerial troopers were produced by the same cause which made the castle of Dover to appear on the side of the hill next to Ramsgate, and it is supposed that they were the images of a body of rebels, on the other side of the hill, exercising themselves previous to the rebellion in 1745.* I shall mention only another instance of this description which lately occurred in France, and for a time caused a powerful sensation among all ranks. On Sunday the 17th of December, 1826, the clergy in the parish of Migne, in the vicinity of Poictiers, were engaged in the exercises of the Jubilee which preceded the festival of Christmas, and a number of persons to the amount of 3000 souls assisted in the service. They had planted as part of the ceremony, a large cross, twenty-five feet high, and painted red, in the open air beside the church. While one of the preachers, about five in the evening, was addressing the multitude, he reminded them of the miraculous cross which appeared in the sky to Constantine and his army, and the effect it produced — when suddenly a similar celestial cross appeared in the heavens just * There can be little doubt that some of the facts ascribed, in the western highlands of Scotland, to second sight, have been owing to the unusual refraction of the atmosphere ; as one of the peculiarities at- tributed to those who possessed this faculty was, that they were enabled to descry boats and ships, before they appeared in the horizon. ON THE REFLECTION OF LIGHT. 115 before the porch of the church about 200 feet above the horizon, and 140 feet in length, and its breadth from three to four feet, of a bright silver colour tinged with red. The curate and congre- gation fixed their wondering gaze upon this ex- traordinary phenomenon, and the effect produced on the minds of the assembly was strong and solemn : they spontaneously threw themselves on their knees ; and many, who had been remiss in their religious duties, numbly confessed their sins, and made vows of penance and reformation. A commission was appointed to investigate the truth of this extraordinary appearance, and a memorial stating the above and other facts was subscribed by more than forty persons of rank and intelli- gence, so that no doubt was entertained as to the reality of the phenomenon. By many it was considered as strictly miraculous, as having hap- pened at the time and in the circumstances men- tioned. But it is evident, from what we have already stated, that it may be accounted for on physical principles. The large cross of wood painted red was doubtless the real object which produced the magnified image. The state of the atmosphere, according to the descriptions given in the memorial, must have been favourable for the production of such images. The spectrum of the wooden cross must have been cast on the concave surface of some atmospheric mirror, and so re- flected back to the eyes of the spectators, from an opposite place — retaining exactly the same shape and proportions, but dilated in size ; and what is worthy of attention, it was tinged with red, the very colour of the object of which it was the re- flected image. Such phenomena as we have now described, and the causes of them which science is able to unfold, 116 THE PRACTICAL ASTRONOMER. are worthy of consideration, in order to divest the mind of superstitious terrors, and enable it clearly to perceive the laws by which the Almighty directs the movements of the material system. When any appearance in nature, exactly the reverse of every thing we could have previously conceived — presents itself to view, and when we know of no material cause by which it could be produced, the mind must feel a certain degree of awe and terror, and will naturally resort to supernatural agency as acting either in opposition to the established laws of the universe, or beyond the range to which they are confined. Besides the fears and appre- hensions to which such erroneous conceptions give rise, they tend to convey false and distorted im- pressions of the attributes of the Deity and of his moral government. Science, therefore, per- forms an invaluable service to man, by removing the cause of superstitious alarms, by investigating the laws and principles which operate in the physical system, and by assigning reasons for those occa- sional phenomena, which at first sight appeared beyond the range of the operation of natural causes. The late ingenious Dr. Wollaston illustrated the causes of some of the phenomena we have described, in the following manner. He looked along the side of a red hot poker at a word or object ten or twelve feet distant ; and at a dis- tance less than three eights of an inch from the line of the poker, an inverted image was seen, and within and without that image, an erect image, in consequence of the change produced, by the heat of the poker, in the density of the air. He also suggested the following experiment as another illustration of the same principle, namely, viewing an object through a stratum of spirit of wine lying ON THE REFLECTION OF LIGHT. 117 above water, or a stratum of water laid above one of syrup. He poured into a square phial a small quantity of clear syrup, and above this he poured an equal quantity of water which gradually com- bined with the syrup, as seen at A. fig. 30. The figure 30. word * Syrup,' on a card held behind the bottle, appeared erect when seen through the pure spirit, but inverted, when seen through the mixture of water and syrup. He afterwards put nearly the same quantity of rectified spirits of wine above the water, as seen at B, and he saw the appearance as represented, namely, the true place of the word ' Spirit,' and the inverted and erect images below. These substances, by their gradual incorporation, produce refracting power, diminishing from the spirit of wine to the water, or from the syrup to the water ; so that by looking through the mixed stra- tum, an inverted image of any object is seen be- hind the bottle. These experiments show that the mirage and several other atmospherical phenomena may be produced by variations in the refractive power of different strata of the atmosphere. 118 THE PRACTICAL ASTRONOMER. It is not unlikely that phenomena of a new and different description from any we have hitherto observed, may be produced from the same causes to which we have adverted. A certain optical writer remarks c If the variation of the refrac- tive power of the air takes place in a horizontal line perpendicular to the line of vision, that is, from right to left, then we may have a lateral Mirage, that is, an image of a ship may be seen on the right or left hand of the real ship, or on both, if the variation of refractive power is the same on each side of the line of vision, and a fact of this kind was once observed on the Lake of Geneva. If there should happen at the same time, both a vertical and a lateral variation of refractive power in the air, and if the variation should be such as to expand or elongate the object in both directions, then the object would be magni- fied as if seen through a telescope, and might be seen and recognized at a distance at which it w r ould not otherwise have been visible. If the refracting power, on the contrary, varied, so as to construct the object in both directions, the image of it would be diminished as if seen through a concave lens. Remarks and Reflections, in reference to the phenomena described above. Such, then, are some of the striking and in- teresting effects produced by the refraction and the reflection of the rays of light. As the forma- tion of the images of objects by convex lenses, lays the foundation of the construction of refract- ing telescopes and microscopes, and of all the discoveries they have brought to light, so the property of concave specula, in forming similar ON THE REFLECTION OF LIGHT. 119 images, is that on which the construction of Reflecting telescopes entirely depends. To this circumstance Herschel was indebted for the powerful telescopes he was enabled to construct — which were all formed on the principle of reflec- tion — and for all the discoveries they enabled him to make in the planetary system, and in the side- real heavens, The same principles which operate in optical instruments, under the agency of man, we have reason to believe, frequently act on a more expansive scale in various parts of the system of nature. The magnificent Cross which aston- ished the preacher and the immense congregation assembled at Migne, was, in all probability, formed by a vast atmospherical speculum formed by the hand of nature, and representing its objects on a scale far superior to that of human art ; and pro- bably, to the same cause is to be attributed the singular phenomenon of the coast of France having been made to appear within two or three miles of the town of Hastings, as formerly described, (see p. 53.) Many other phenomena which we have never witnessed, and of which we can form no conception, may be produced by the same cause operating in an infinity of modes. The facts we have stated above, and the variety of modes by which light may be refracted and reflected by different substances in nature, lead us to form some conceptions of the magnificent and diversified scenes which light may produce in other systems and worlds, under the arrangements of the all-wise and Beneficent Creator. Light, in all its modifications and varieties of colour and reflection, may be considered as the beauty and glory of the universe, and the source of unnum- bered enjoyments to all its inhabitants. It is a symbol of the Divinity himself ; for " God is 120 THE PRACTICAL ASTRONOMER. Light, and in Him is no darkness at all." It is a representative of Him who is exhibited in the Sacred oracles, as " The Sun of Righteousness/' and "the Light of the world." It is an emblem of the glories and felicities of that future world, where knowledge shall be perfected, and happiness complete ; for its inhabitants are designated " the saints in light ; " and it is declared in Sacred his- tory, to have been the first born of created beings. In our lower world, its effects on the objects which surround us, and its influences upon all sensitive beings, are multifarious and highly admirable. "While passing from infinitude to infinitude, it reveals the depth and immensity of the heavens, the glory of the sun, the beauty of the stars, the arrangements of the planets, the rainbow encom- passing the sky with its glorious circle, the embroidery of flowers, the rich clothing of the meadows, the valleys standing thick with corn, " the cattle on a thousand hills/' the rivers rolling through the plains, and the wide expanse of the ocean. But in other worlds the scenes it creates may be far more resplendent and magnificent. This may depend upon the refractive and reflec- tive powers with which the Creator has endowed the atmospheres of other planets, and the pecu- liar constitution of the various objects with which they are connected. It is evident, from what we already know of the reflection of light, that very slight modifications of certain physical principles, and very slight additions to the arrangements of our terrestrial system, might produce scenes of beauty, magnificence and splendour of which, at present, we can form no conception. And, it is not unlikely that by such diversities of arrange- ment, in other worlds, an infinite variety of natural scenery is produced throughout the universe. ON THE REFLECTION OF LIGHT. 121 In the arrangements connected with the planet Saturn, and the immense rings with which it is encompassed, and in the various positions which its satellites daily assume with regard to one another, to the planet itself, and to these rings — there is, in all probability, a combination of re- fractions, reflections, light, and shadows, which produce scenes wonderfully diversified, and sur- passing in grandeur what we can now distinctly conceive. In the remote regions of the heavens, there are certain bodies composed of immense masses of luminous matter, not yet formed into any regular system, and which are known by the name of Nebulce. What should hinder us from supposing that certain exterior portions of those masses form speculums of enormous size, as some parts of our atmosphere are sometimes found to do ? Such specula may be conceived to be hundreds and even thousands of miles in diameter, and that they may form images of the most distant objects in the heavens, on a scale of immense magnitude and extent, and which may be reflected, in all their grandeur, to the eyes of intelligences at a vast distance. And, if the organs of vision of such beings, be far superior to ours in acuteneas and penetrating power, they may thus be enabled to take a survey of an immense sphere of vision, and to descry magnificent objects at distances the most remote from the sphere they occupy. What- ever grounds there may be for such suppositions, it must be admitted, that all the knowledge we have hitherto acquired respecting the operation of light, and the splendid effects it is capable of pro- ducing, is small indeed, and limited to a narrow circle, compared with the immensity of its range, the infinite modifications it may undergo, and the w T ondrous scenes it may create in regions of crea- G 122 THE PRACTICAL ASTRONOMER. tion to which human eyes have never yet pene- trated, — and which may present to view objects of brilliancy and magnificence such as, " Eye hath not yet seen, nor ear heard, nor hath it entered into the heart of man to conceive/' ON THE COLOURS OF LIGHT. 123 CHAPTER V. SECT. I.— ON THE COLOURS OF LIGHT. We have hitherto considered light chiefly as a simple homogeneous substance, as if all its rays were white, and as if they were all refracted in the same manner by the different lenses on which they fall. Investigations however, into the nature of this wonderful fluid, have demonstrated that this is not the case, and that it is possessed of certain additional properties, of the utmost impor- tance in the system of nature. Had every ray of light been a pure white, and incapable of being separated into any other colours, the scene of the universe would have exhibited a very different aspect from what we now behold. One uniform hue would have appeared over the whole face of nature, and one object could scarcely have been distinguished from another. The different shades of verdure which now diversify every landscape, the brilliant colouring of the flowery fields, and almost all the beauties and sublimities which adorn this lower creation would have been with- drawn. But it is now ascertained that every ray of white light is composed of an assemblage of colours, whence proceed that infinite variety of G 2 124 THE PRACTICAL ASTRONOMER, shade and colour with which the whole of our terrestrial habitation is arrayed. Those colours are found not to be in the objects themselves, but in the rays of light which fall upon them, without which they would either be invisible, or wear an uniform aspect. In reference to this point, Gold- smith has well observed : ' The blushing beauties of the rose, the modest blue of the violet, are not in the flowers themselves, but in the light that adorns them. Odour, softness, and beauty of figure are their own ; but it is light alone that dresses them up in those robes w 7 hich shame the monarch's glory/ Many strange opinions and hypotheses were entertained respecting colours, by the ancients, and even by many modern writers, prior to the time of Sir Isaac Newton. The Pythagoreans called colour the superficies of bodies ; Plato said that it was a flame issuing from them. According to Zeno it is the first configuration of matter, and according to Aristotle, it is that which moves bodies actually transparent. Among the moderns, Des Cartes imagined that the difference of colour proceeds from the prevalence of the direct or rotatory motions of the particles of light. Grimaldi, Dechales, and others, thought the differences of colour depended upon the quick or slow vibrations of a certain elastic medium filling the whole ■universe. Rohault imagined that the different colours were made by the rays of light entering the eye at different angles with respect to the optic axis ; and Dr. Hook conceived that colour is caused by the sensation of the oblique or uneven pulse of light; and this being capable of no more than two varieties, he concluded that there could be no more than two primary colours. Such were some of the crude opinions w T hich prevailed ON THE COLOURS OF LIGHT. 125 before the era of the illustrious Newton, by whose enlightened investigations the true theory of colours was at last discovered. In the year 1666 this philosopher began to investigate the subject ; and finding the coloured image of the sun, formed by a glass prism, to be of an oblong and not of a circular form, as according to the laws of refrac- tion it ought to be, he was surprised at the great disproportion between its length and breadth, the former being Jive times the length of the latter ; and he began to conjecture that light is not homo- geneal, but that it consists of rays some of which are much more refrangible than others. Prior to this period, philosophers supposed that all light, in passing out of one medium into another of different density was equally refracted in the same or like circumstances ; but Newton dis- covered that this is not the fact ; but that there are different species of light, and that each species is disposed both to suffer a different degree of re- frangibility in passing out of one medium into another, — and to excite in us the idea of a different colour from the rest ; and that bodies appear of that colour which arises from the peculiar rays they are disposed to reflect. It is now, therefore, universally acknowledged, that the light of the sun, which to us seems perfectly homogeneal and white, is composed of no fewer than seven different colours, namely Red, Orange, Yellow, Green, Blue, Indigo and Violet. A body which appears of a red colour has the property of reflecting the red rays more powerfully than any of the others; a body of a green colour reflects the green rays more copiously than rays of any other colour, and so of the orange, yellow, blue, purple and violet. A body which is of a black colour, instead of re- flecting — absorbs all, or the greater part of the 126 THE PRACTICAL ASTRONOMER. rays that fall upon it ; and, on the contrary, a body that appears white reflects the greater part of the rays indiscriminately without separating the one from the other. Before proceeding to describe the experiments by which the above results were obtained, it may be proper to give some idea of the form and effects of the Prism by which such experiments are made. This instrument is triangular and straight, and generally about three or four inches long. It is commonly made of white glass, as free as possible from veins and bubbles, and other similar defects, and is solid throughout. Its lateral faces, or sides, should be perfectly plane and of a fine polish. The angle formed by the two faces, one receiving the ray of light that is refracted in the instrument, and the other affording it an issue on its returning into the air, is called the refracting angle of the prism, as ACB, (fig. 31.) The manner in which Newton performed his experiments, and established the discovery to which we have alluded, is as follows. In the window-shutter EG, (fig. 31.) of a dark room, a hole F, was made, of about one third of an inch diameter, and behind it was placed a glass prism ACB, so that the beam of light, SF, pro- ceeding directly from the sun was made to pass through the prism. Before the interposition of the prism, the beam proceeded in a straight line towards T, where it formed a round white spot ; but being now bent out of its course by the prism, it formed an oblong image OP, upon the white pasteboard, or screen LM, containing the seven colours marked in the figure — the red being the least, and the violet the most refracted from the original direction of the solar beam, ST. This oblong image is called the prismatic spectrum* If the refracting angle of the prism ACB, be 64 ON THE COLOURS OF LIGHT. 127 figure 31. E degrees, and the distance of the pasteboard from the prism about 18 feet, the length of the image OP will be about 10 inches, and the breadth 2 inches. The sides of the spectrum are right lines distinctly bounded, and the ends are semi- circular. From this circumstance it is evident that it is still the image of the sun, but elongated by the refractive power of the prism. It is evi- dent from the figure, that since some part of the beam, RO, is refracted much further out of its natural course WT, than some other part of the beam, as WP, the rays towards RO have a much greater disposition to be refracted than those toward WP ; and that this disposition arises from the naturally different qualities of those rays, is evident from this consideration, that the refracting angle or power of the prism is the same in regard to the superior part of the beam as to the inferior. By making a hole in the screen LM opposite any one of the colours of the spectrum, so as to allow that colour alone to pass — and by letting the colour thus separated fall upon a second prism — Newton found that the light of each of the colours was alike refrangible, because the second prism could not separate them into an oblong image, or into any other colour. Hence he 128 THE PRACTICAL ASTRONOMER. called all the seven colours simple or homogeneous, in opposition to white light, which he called com- pound or heterogeneous. With the prism which this philosopher used he found the lengths of the colours and spaces of the spectrum to be as follows : Red, 45 ; Orange, 27 ; Yellow, 40 ; Green, 60 ; Blue, 60 ; Indigo, 48 ; Violet, 80 : or 360 in all. But these spaces vary a little with prisms formed of different substances, and as they are not separated by distinct limits, it is difficult to obtain any thing like an accurate measure of their relative extents. Newton examined the ratio between the sines of incidence and refrac- tion of these decompounded rays (see p. 30,) and found that each of the seven primary colour- making rays, had certain limits within which they were confined. Thus let the sine of incidence in glass be divided into 50 equal parts, the sine of refraction into air of the least refrangible, and the most refrangible rays will contain respectively 77 and 78 such parts. The sines of refraction of all the degrees of red will have the intermediate degrees of magnitude, from 77 to 77 one-eighth ; Orange from 77 one-eighth to 77 one-fifth ; Yellow from 77 one-fifth to 77 one-third ; Green from 77 one-third to 77 one-half ; Blue from 77 one-half to 77 two-thirds ; Indigo from 77 two-thirds to 77 seven-ninths; and Violet from 77 seven-ninths to 78. From what has been now stated, it is evident that, in proportion as any part of an optic glass bears a resemblance to the form of a prism, the component rays that pass through it must be necessarily separated, and will consequently paint or tinge the object with colours. The edges of every convex lens approach to this form, and it is on this account that the extremities of objects when viewed through them are found to be tinged ON THE COLOURS OF LIGHT. 129 with the prismatic colours. In such a glass, there- fore, those different coloured rays will have dif- ferent foci, and will form their respective images at different distances from the lens. Thus, sup- pose LN (fig. 32.) to represent a double convex- figure 32. B lens, and OB an object at some distance from it. If the object OB was of a pure red colour, the rays proceeding from it would form a red image at Rr ; if the object was of a violet colour, an image of that colour would be formed at Vv, nearer the lens ; and if the object was white or any other combination of the colour-making rays, those rays would have their respective foci at different distances from the lens, and form a succession of images, in the order of the prismatic colours, between the space Rr and Vv. ■figure 33. This may be illustrated by experiment in the following manner. Take a card or slip of white G 5 130 THE PRACTICAL ASTRONOMER. pasteboard, as ABEF, (fig. 33.) and paint one half ABCD red, the other half CF, violet or indigo ; and tying black threads across it, set it near the flame of a candle G, then take a lens HI, and holding a sheet of white paper behind it, move it backwards and forwards upon the edge of a graduated ruler, till you see the black threads most distinctly in the image, and you will find the focus of the violet fe, much nearer than that of the red AC, which plainly shows that bodies of different colours can never be depicted by convex- lenses, without some degree of confusion. The quantity of dispersion of the coloured rays in convex lenses depends upon the focal length of the glass; the space which the coloured images occupy being about the twenty-eighth part. Thus if the lens be twenty-eight inches focal distance, the space between Rr and Vv (fig 32) will be about one inch ; if it be twenty-eight feet focus, the same space will be about one foot, and so on in proportion. Now, when such a succession of images formed by the different coloured rays, is viewed through an eye-glass, it will seem to form but one image, and consequently very indistinct, and tinged with various colours, and as the red figure Rr is largest, or seen under the greatest angle — the extreme parts of this confused image will be red, and a succession of the prismatic colours will be formed within this red fringe, as is generally found in common refracting-telescopes, constructed with a single object-glass. It is owing to this circumstance that the common refracting telescope cannot be much improved without having recourse to lenses of a very long focal distance ; and hence, about 150 years ago, such telescopes were constructed of 80, and 100, and 120 feet in length. But still the image was not formed so ON THE COLOURS OF LIGHT. 131 distinctly as was desired, and the aperture of the object-glass was obliged to be limited. This is a defect which was long regarded as without a remedy ; and even Newton himself despaired of discovering any means by which the defects of refracting telescopes might be removed and their improvement effected. This, however, was accom- plished by Dollond to an extent far surpassing what could have been expected, of which a par- ticular account will be given in the sequel. It was originally remarked by Newton, and the fact has since been confirmed by the experiments of Sir W. Herschel, that the different-coloured rays have not the same illuminating power. The violet rays appear to have the least illuminating effect ; the indigo more, and the effect increases in the order of the colours, — the green being very great; between the green and the yellow the greatest of all ; the yellow the same as the green ; but the red less than the yellow. Herschel also endeavoured to determine whether the power of the differently-coloured rays to heat bodies, varied with their power to illuminate them. He intro- duced a beam of light into a dark room, which was decomposed by a prism, and then exposed a very sensible thermometer to all the rays in suc- cession, and observed the heights to which it rose in a given time. He found that their heating power increased from the violet to the red. The mercury in the thermometer rose higher when its bulb was placed in the Indigo than when it was placed in the violet, still higher in blue, and high- est of all at red. Upon placing the bulb of the thermometer below the red, quite out of the spectrum, he was surprised to find that the mercury rose highest of all ; and concluded that rays pro- ceed from the sun, which have the power of heat- 132 THE PRACTICAL ASTRONOMER. ing, but not of illuminating bodies. These rays have been called invisible solar raj 7 s. They were about half an inch from the commencement of the red rays; at a greater distance from this point the heat began to diminish, but was very perceptible even at the distance of 1^ inch. He determined that the heating power of the red to that of the green rays, was 2f to 1, and 3^ to 1, in red to violet. He afterwards made experiments to collect those invisible calorific rays, and caused them to act independently of the light, from which he concluded that they are sufficient to account for all the effects produced by the solar rays in exciting heat ; that they are capable of passing through glass, and of being refracted and reflected, after they have been finally detached from the solar beam. M. Ritter of Jena, Wollaston, Beckman and others, have found that the rays of the spectrum are possessed of certain chemical properties — that beyond the least brilliant extremity, namely, a little beyond the violet ray, there are invisible rays which act chemically, while they have neither the power of heating nor illuminating bodies. Muriate of silver exposed to the action of the red rays becomes blackish ; a greater effect is produced by the yellow: a still greater by the violet, and the greatest of all by the invisible . rays beyond the violet. When phosphorus is exposed to the action of the invisible rays beyond the red, it emits white fumes ; but the invisible rays beyond the violet extinguish them. The influence of these rays is daily seen in the change produced upon vegeta- ble colours, which fade, when frequently exposed to the direct influence of the sum. What object they are destined to accomplish in the general economy of nature, is not yet distinctly known ; ON THE COLOURS OF LIGHT. 133 we cannot however doubt that they are essentially requisite to various processes going forward in the material system. And we know that, not only the comfort of all the tribes of the living world, but the very existence of the animal and vegetable creation depends upon the unremitting agency of the Calorific rays. It has likewise been lately discovered that certain rays of the spectrum, particularly the violet, possesses the property of communicating the magnetic power. Dr. Morichini, of Rome, appears to have been the first who found that the violet rays of the spectrum had this property. The result of his experiments, however, was involved in doubt, till it was established by a series of ex- periments instituted by Mrs. Somerville, whose name is so well known in the scientific world. This lady having covered half of a sewing-needle, about an inch long, with paper, she exposed the other half for two hours, to the violet rays. The needle had then acquired North polarity. The indigo rays produced nearly the same effect ; and the blue and green rays produced it in a still less degree. In the yellow, orange, red and invisible rays, no magnetic influence was exhibited, even though the experiment was continued for three successive days. The same effects were produced by enclosing the needle in blue or green glass, or wrapping it in blue and green ribbands one half of the needle being always covered with paper. One of the most curious discoveries of modern times, in reference to the solar spectrum, is that of Fraunhofer of Munich — one of the most dis- tinguished artists and opticians on the Continent.* * Fraunhofer was in the highest sense of the word, an Optician, an original discoverer in the most abstruse and delicate departments of this science — a competent mathematician, an admirable mechanist, and a man of a truly philosophical turn of mind. By his extraordi- ] 34 THE PRACTICAL ASTRONOMER. He discovered that the spectrum is covered with dark and coloured lines, parallel to one another, and perpendicular to the length of the spectrum 5 and he counted no less than 590 of these lines. In order to observe these lines, it is necessary to use prisms of the most perfect construction, of very pure glass, free of veins — to exclude all extrane- ous light, and even to stop those rays which form the coloured spaces, which we are not examining. It is necessary also to use a magnifying instru- ment, and the light must enter and emerge from the prism at equal angles. One of the important practical results of this discovery is, that those lines are fixed points in the spectrum, or rather, that they have always the same position in the coloured spaces in which they are found. Fra- unhofer likewise discovered in the spectrum pro- duced by the light of Venus, the same streaks, as in the solar spectrum ; in the spectrum of the light of Sirius, he perceived three large streaks which, according to appearance, had no resem- blance to those of the light of the sun ; one of nary talents, he was soon raised from the lowest station in a manu- facturing establishment to the direction of the optical department of the business, in which he original^ laboured as an ordinary workman. He then applied the whole power of his mind to the perfection of the achromatic telescope, the defects of which in reference to the optical properties of the materials used — he attempted to remedy ; and by a series of admirable experiments, succeeded in giving to optical deter- minations, the precision of astronomical observations, surpassing, in this respect all who had gone before him, except perhaps, the illustri- ous Newton. It was in the course of these researches, that he was led to the important discovery of the dark lines which occur in the solar spectrum. His achromatic telescopes are scattered over Europe, and are the largest and best that have hitherto been constructed. He died at Munich, at a premature age, in 1826; his death, it is said being accelerated by the unwholesome nature of the processes employed in his glass-house ; leaving behind him a reputation rarely attained by one so yonng. His Memoir " On the refractive and dispersive power of different species of glass, in reference to the improvement of Achromatic telescopes, and an account of the lines on the spectrum," will be found in the " Edinburgh Philosophical Journal," Vol. ix. pp. 288—299, and Vol. x. pp. 26—40, for 1823-4. ON THE COLOURS OF LIGHT. 135 them was in the green, two in the blue. The stars appear to differ from one another in their streaks. The electric light differs very much from the light of the sun and that of a lamp, in regard to the streaks of the spectrum— ' This experiment may also be made, though in an imperfect manner, by viewing a narrow slit be- tween two nearly closed window-shutters, through a very excellent glass prism held close to the eye, with the refacting angle parallel to the line of light. When the spectrum is formed by the sun's rays, either direct or indirect, as from the sky, clouds, rainbow, moon, or planets, the black bands are always found to be in the same parts of the spectrum, and under all circumstances to main- tain the same relative position, breadth and in- tensities.' From what has been stated in reference to the solar spectrum it will evidently appear, that white light is nothing else than a compound of all the prismatic colours ; and this may be still farther illustrated by shewing, that the seven primary colours, when again put together, recompose white light. This may be rudely proved for the purpose of illustration, by mixing together seven different powders, having the colours and propor- tion of the spectrum ; but the best mode, on the whole, is the following. Let two circles be drawn on a smooth round board, covered with white paper, as in fig. 34 : Let the outermost be divided into 360 equal parts ; then draw seven right lines as A,B,C, &c, from the center to the outermost circle, making the lines A and B include 80 degrees of that circle. The lines B and C, 40 degrees ; C and D, 60 ; D and E, 60 ; E and F, 48 ; F and G, 27 ; G and A, 45. Then between these two circles paint the space AG red, inclin- 136 THE PRACTICAL ASTRONOMER. figure 34. D ing to orange near G ; GF orange, inclining to yellow near F ; FE yellow, inclining to green near E ; ED green, inclining to blue near D; DC blue, inclining to indigo near C ; CB indigo, inclining to violet near B ; and BA violet, inclining to a soft red near A. This done, paint all that part of the board black which lies within the inner circle ; and putting an axis through the centre of the board, let it be turned swiftly round that axis, so that the rays proceeding from the above colour, may be all blended and mixed together in coming to the eye. Then the whole coloured part will appear like a white ring a little grayish — hot per- fectly white, because no art can prepare or lay on perfect colours, in all their delicate shades, as found in the real spectrum. That all the colours of light, when blended to- gether in their proper proportions, produce a pure white is rendered certain by the following expe- riment. Take a large convex glass, and place it ON THE COLOURS OF NATURAL OBJECTS. 187 in the room of the paper or screen on which the solar spectrum was depicted (LM fig. 31), the glass will unite all the rays which come from the prism, if a paper is placed to receive them, and you will see a circular spot of a pure lively white. The rays will cross each other in the focus of the glass, and, if the paper be removed a little further from that point, you will see the prismatic colours again displayed, but in an inverted order, owing to the crossing of the rays. SECT. 2, ON THE COLOURS OF NATURAL OBJECTS. From what has been stated above we may learn the true cause of those diversified hues exhibited by natural and artificial objects, and the variegated colouring which appears on the face of nature. It is owing to the surfaces of bodies being dis- posed to reflect one colour rather than another. When this disposition is such that the body re- flects every kind of ray, in the mixed state in which it receives them, that body appears white to us — which, properly speaking, is no colour, but rather the assemblage of all colours. If the body has a fitness to reflect one sort of rays more abundantly than others, by absorbing all the others, it will appear of the colour belonging to that species of rays. Thus, the grass is green, because it absorbs all the rays except the green. It is these green rays only which the grass, the trees, the shrubs, and all the other verdant parts of the landscape reflect to our sight, and which make them appear green. In the same manner the different flowers reflect their respective colours ; the rose, the red rays ; the violet, the blue ; the jonquil, the yellow ; the marigold, the 138 THE PRACTICAL ASTRONOMER. orange, and every object, whether natural or arti- ficial, appears of that colour which its peculiar texture is fitted to reflect. A great number of bodies are fitted to reflect at once several kinds of rays, and of consequence they appear under mixed colours. It may even happen, that of two bodies which should be green, for example, one may reflect the pure green of light, and the other the mixture of yellow and blue. This quality, which varies to infinity, occasions the different kinds of rays to unite in every possible manner, and every possible proportion ; and hence the inexhaustible variety of shades and hues which nature has diffused over the landscape of the w r orld. When a body absorbs nearly all the light which reaches it, that body appears black. It transmits to the eye so few reflected rays that it is scarcely per- ceptible in itself, and its presence and form make no impression upon us, unless as it interrupts the brightness of the surrounding space. Black is, therefore, the absence of all the coloured rays. It is evident, then, that all the various assem- blages of colours which we see in the objects around us, are not in the bodies themselves, but in the light which falls upon them. There is no colour inherent in the grass, the trees, the fruits, and the flowers, nor even in the most splendid and variegated dress that adorns a lady. All such objects are as destitute of colour, in themselves, as bodies which are placed in the centre of the earth, or as the chaotic materials out of which our globe was formed, before light was created. For where there is no light, there is no colour. Every object is black, or without colour, in the dark, and it only appears coloured as soon as light renders it visible. This is further evident from the following experiment. If we place a coloured ON THE COLOURS OF NATURAL OBJECTS. 139 body in one of the colours of the spectrum which is formed by the prism, it appears of the colour of the rays in which it is placed. Take, for ex- ample, a red rose, and expose it first to the red rays, and it will appear of a more brilliant ruddy hue. Hold it in the blue rays, and it appears no longer red, but of a dingy blue colour, and in like manner its colour will appear diffe- rent, when placed in all the other differently coloured rays. This is the reason why the colours of objects are essentially altered by the nature of the light in which they are seen. The colours of ribbons and various pieces of silk or woollen stuff are not the same when viewed by candle-light as in the day time. In the light of a candle or a lamp, blue often appears green, and yellow objects assume a whitish aspect. The reason is that the light of a candle is not so pure a white as that of the sun, but has a yellowish tinge, and there- fore, when refracted by the prism, the yellowish rays are found to predominate, and the superabun- dance of yellow rays gives to blue objects a green- ish hue. The doctrine we are now illustrating is one which a great many persons, especially among the fair sex, find it difficult to admit. They cannot conceive it possible that there is no colour really inherent in their splendid attire, and no tints of beauty in their countenances. ' What/ said a certain lady, 6 are there no colours in my shawl, and in the ribbons that adorn my head-dress — and, are we all as black as negroes in the dark ; I should almost shudder to think of it.' Such persons, however, need be in no alarm at the idea ; but may console themselves with the reflection, that, when they are stripped of all their coloured ornaments in the dark, they are certain that they 140 THE PRACTICAL ASTRONOMER. will never be seen by any one in that state; and therefore, there is no reason to regret the tempo- rary loss of those beauties which light creates — when they themselves and all surrounding objects are invisible* But, to give a still more palpable proof of this position, the following popular ex- periments may be stated. Take a pint of common spirit, and pour it into a soup dish, and then set it on fire ; as it begins to blaze, throw a handful of salt into the burning spirit, and keep stirring it with a spoon. Several handfuls may thus be successively thrown in, and then the spectators, standing around the flame, will see each other frightfully changed, their colours being altered into a ghastly blackness, in consequence of the nature of the light which falls upon them — which produces colours very different from those of the solar light. The following expe- riment, as described by Sir D. Brewster, illustrates the same principle. ' Having obtained the means of illuminating any apartment with yelloiv light, let the exhibition be made in a room with furniture of various bright colours, and with oil or water colour- ed paintings on the wall. The party which is to witness the experiment should be dressed in a diversity of the gayest colours ; and the brightest coloured flowers, and highly coloured drawings should be placed on the tables. The room being at first lighted with ordinary lights, the bright and gay colours of every thing that it contains will be finely displayed. If the white lights are now suddenly extinguished, and the yellow lamps lighted, the most appalling metamorphosis will be exhibited. The astonished individuals will no longer be able to recognise each other. All the furniture of the room, and all the objects it contains, will exhibit only one colour. The flowers ON THE COLOURS OF NATURAL OBJECTS. 141 will lose their hues ; the paintings and drawings will appear as if they were executed in China ink, and the gayest dresses, the brightest scarlets, the purest lilacs, the richest blues and the most vivid greens, will all be converted into one monotonous yellow. The complexions of the parties, too, will suffer a corresponding change. One pallid death- like yellow, * Like the unnatural hue Which autumn paints upon the perished leaf, will envelope the young and the old, and the sallow face will alone escape from the metamor- phosis. Each individual derives merriment from the cadaverous appearance of his neighbour, with- out being sensible that he is one of the ghastly assemblage/ From such experiments as these we might con- clude, that were the solar rays of a very different description from what they are now found to be, the colours which embellish the face of nature, and the whole scene of our sublunary creation would assume a new aspect, and appear very dif- ferent from what we now behold around us in every landscape. We find that the stars display great diversity of colour ; which is doubtless owing to the different kinds of light which are emitted from those bodies ; and hence we may con- clude, that the colouring thrown upon the various objects of the universe is different in every different system, and that thus, along with other arrangements, an infinite variety of colouring and of scenery is distributed throughout the immen- sity of creation. The atmosphere ) in consequence of its different refractive and reflective powers, is the source of a variety of colours which frequently embellish and 142 THE PRACTICAL ASTRONOMER. diversify the aspect of our sky. The air reflects the blue rays most plentifully, and must therefore transmit the red, orange, and yellow, more copiously than the other rays. When the sun and other heavenly bodies are at a high elevation, their light is transmitted without any perceptible change, but when they are near the horizon, their light must pass through a long and dense track of air, and must therefore be considerably modified before it reach the eye of the observer. The mo- mentum of the red rays being greater than that of the violet, will force their way through the resisting medium, while the violet rays will be either reflected or absorbed. If the light of the setting sun, by thus passing through a long track of air, be divested of the green, blue, indigo, and violet rays, the remaining rays which are trans- mitted through the atmosphere, will illuminate the western clouds, first with an orange colour ; and then, as the sun gradually sinks into the horizon, the track through which the rays must pass be- coming longer, the yellow and orange are reflected, and the clouds grow more deeply red, till at length the disappearance of the sun leaves them of a leaden hue by the reflection of the blue light through the air. Similar changes of colour are sometimes seen on the eastern and western fronts of white buildings. St. Paul's Church, in London, is frequently seen at sun-set, tinged with a very considerable degree of redness ; and the same cause occasions the moon to assume a ruddy colour, by the light transmitted through the atmosphere. From such atmospherical refractions and reflec- tions are produced those rich and beautiful hues with which our sky is gilded by the setting sun, and the glowing red which tinges the morning and evening clouds, till their ruddy glare is tern- ON THE COLOURS OF NATURAL OBJECTS. 143 pered by the purple of twilight, and the reflected azure of the sky. When a direct spectrum is thrown on colours darker than itself, it mixes with them : as the yellow spectrum of the setting sun, thrown on the green grass, becomes a greener yellow. But when a direct spectrum is thrown on colours brighter than itself, it becomes instantly changed into the reverse spectrum, which mixes with those brighter colours. Thus the yellow spectrum of the setting sun thrown on the luminous sky, be- comes blue, and changes with the colour or bright- ness of the clouds on which it appears. The red part of light being capable of struggling through thick and resisting mediums which inter- cept all other colours — is likewise the cause why the sun appears red when seen through a fog, — why distant light, though transmitted through blue or green glass, appears red — why lamps at a dis- tance, seen through the smoke of a long street, are red, while those that are near, are white. To the same cause it is owing that a diver at the bottom of the sea is surrounded with the red light which has pierced through the superincum- bent fluid, and that the blue rays are reflected from the surface of the ocean. Hence, Dr. Halley informs us that, when he was in a diving bell, at the bottom of the sea, his hand always appeared red in the water. The blue rays, as already noticed, being unable to resist the obstructions they meet with in their course through the atmosphere, are either reflected or absorbed in their passage. It is to this cause, that most philosophers ascribe the blue colour of the sky, the faintness and obscurity of distant objects, and the bright azure which tinges the mountains of a distant landscape. 144 THE PRACTICAL ASTRONOMER. SECT. 3. — PHENOMENA OF THE RAINBOW. Since the rays of light are found to be decom- posed by refracting surfaces, and reflected in an infinite variety of modes and shades of colour, we need not be surprised at the changes produced in any scene or object by the intervention of another, and by the numerous modifications of which the primary colours of nature are susceptible. The vivid colours which gild the rising and the setting sun, must necessarily differ from those which adorn its noon-day splendour. Variety of atmospheric scenery will thus necessarily be produced, greater than the most lively fancy can well imagine. The clouds will sometimes assume the most fantastic forms, and at other times will be irradiated with beams of light, or, covered with the darkest hues, will assume a lowering aspect, prognos- tive of the thunder's roar and the lightning's flash — all in accordance with the different rays that are reflected to our eyes, or the quantity absorbed by the vapours which float in the at- mosphere. Light, which embellishes with so much magni- ficence a pure and serene sky, by means of innu- merable bright starry orbs which are spread over it, sometimes, in a dark and cloudy sky, exhibits an ornament which, by its pomp, splendour and variety of colours, attracts the attention of every eye that has an opportunity of beholding it. At certain times, when there is a shower either around us, or at a distance from us in an opposite quarter to that of the sun, a species of arch or bow is seen in the sky, adorned with all the seven primary colours of light. This phenomenon, ON THE COLOURS OF NATURAL OBJECTS. 145 which is one of the most beautiful meteors in nature, has obtained the name of the Rainbow. The rainbow was, for ages, considered as an inexplicable mystery, and by some nations it was adored as a deity. Even after the dawn of true philosophy, it was a considerable time before any discovery of importance was made, as to the true causes which operate in the production of this phenomenon. About the year 1571, M. Fletcher of Breslau, made a certain approximation to the discovery of the true cause, by endeavouring to account for the colours of the rainbow by means of a double refraction and one reflection. A nearer approximation was made by Antonio de Dominis, bishop of Spalatro, about 1601. He maintained that the double refraction of Fletcher, with an intervening reflection, was sufficient to produce the colours of the bow, and also to bring the rays that formed them to the eye of the spec- tator, without any subsequent reflection. To verify this hypothesis, he procured a small globe of solid glass, and viewing it when it was exposed to the rays of the sun — with his back to that luminary — in the same manner as he had supposed the drops of rain were situated with respect to them, he observed the same colours which he had seen in the rainbow, and in the same order. But he could give no good reason why the bow should be coloured, and much less any satisfactory account of the order in which the colours appear. It was not till Sir I. Newton discovered the dif- ferent refrangibility of the rays of light, that a complete and satisfactory explanation could be given of all the circumstances connected with this phenomenon. As the full elucidation of this subject involves a variety of optical and mathematical investiga- a 146 THE PRACTICAL ASTRONOMER. tions, I shall do little more than explain the general principle on which the prominent pheno- mena of the rainbow may be accounted for, and some of the facts and results which theory and observation have deduced. We have just now alluded to an experiment with a glass globe : —If, then, we take either a solid glass globe, or a hollow globe filled with water, and suspend it so high in the solar rays above the eye, that the spectator, with his back to the sun, can see the globe red; — if it be lowered slowly, he will see it orange, then yellow, then green, then blue, then indigo, and then violet ; so that the drop at different heights, shall present to the eye the seven primitive colours in succession. In this case, the globe, from its form, will act in some measure like a prism, and the ray will be separated into its component parts. The following figure will more particularly illus- trate this point. Suppose A (fig. 35.) to repre- sent a drop of rain — which may be considered as a globe of glass in miniature, and will produce the same effect on the rays of light — and let Sd represent a ray from the sun falling upon the upper part of the drop at d. At the point of entering the drop, it will suffer a refraction, and instead of going forward to g, it will be bent to N. From N a part of the light will be reflected to q — some part of it will, of course, pass through the drop. By the obliquity with which it falls on the side of the drop at q, that part becomes a kind of prisms, and separates the ray into its primitive colours. It is found by computation that, after a ray has suffered two refractions and one reflection, as here represented, the least refrangible part of it, namely the red ray, will make an angle with the incident solar ray of 42° 2', as Sfq ; and the ON THE COLOURS OF NATURAL OBJECTS. 147 figure 35. violet, or greatest refrangible ray will make with the solar ray, an angle of 40° 17', as Scq; and thus all the particles of water within the difference of those two angles, namely 1° 45' — (supposing the ray to proceed merely from the centre of the sun) — willexhibit severally the coloursof the prism, and constitute the interior bow of the cloud. This holds good at whatever height the sun may chance to be in a shower of rain. If he be at a high altitude, the rainbow will be low ; if he be at a low elevation, the rainbow must be high ; and if a shower happen in a vale, when the spec- tator is on a mountain, he will sometimes see the H 2 148 THE PRACTICAL ASTRONOMER. bow in the form of a complete circle below him. We have at present described the phenomena only of a single drop ; but it is to be considered that in a shower of rain there are drops at all heights and at all distances ; and therefore the eye situated at g will see all the different colours. All those drops that are in a certain position with respect to the spectator will reflect the red rays, all those in the next station the orange, those in the next the green, and so on with regard to all the other colours. It appears, then, that the first or primary bow is formed by two refractions and one reflection ; but there is frequently a second bow, on the out- side of the other, which is considerably fainter. This is produced by drops of rain above the drop we have supposed at A. If B (fig. 35.) represent one of these drops, the ray to be sent to the eye enters the drop near the bottom, and suffers two refractions and two reflections, by which means the colours become reversed, that is, the violet is lowest in the exterior bow, and the red is lowest in the interior one, and the other colours are reversed accordingly. The ray T is refracted at r : a part of it is reflected from s to t, and at t it suffers another reflection from t to u. At the points s and T part of the ray passes through the drop on account of its transparency, towards w and x, and therefore we say that part only of the ray is reflected. By these losses and reflections the exterior bow becomes faint and ill-defined in comparison of the interior or primary bow. In this case the upper part of the secondary bow will not be seen when the sun is above 54° 10' above the horizon ; and the lower part of the bow will not be seen when the sun is 60° 58' above the horizon. ON THE COLOURS OF NATURAL OBJECTS. 149 For the further illustrations of this subject, we may introduce the following section of a bow, (fig. 36.) and, in order to prevent confusion in attempt- figure 36. ing to represent all the different colours — let us suppose only three drops of rain, and three dif- ferent colours, as shown in the figure. The spec- tator O being in the centre of the two bows, here represented, — the planes of which must be con- sidered as perpendicular to his view — the drops A,B, and C produce part of the interior bow by two refractions and one reflection as stated above, 150 THE PRACTICAL ASTRONOMER. and the drops D,E,F will produce the exterior bow by two refractions and two reflections, the sun's rays being represented by 3,3. It is evident that the angle COP is less than the angle BOP, and that the angle AOP is the greatest of the three. The largest angle, then, is formed by the red rays, the middle one consists of the green, and the smallest the purple or violet. All the drops of rain, therefore, that happen to be in a certain position with respect to the spectator's eye, will reflect the red rays, and form a band or semicircle of red, and so of the other colours from drops in other positions. If the spectator alters his station, he will see a bow, but not the same as before ; and if there be many spectators, they will each see a different bow, though it appears to be the same. The rainbow assumes a semicircular appearance, because it is only at certain angles that the refracted rays are visible to our eyes, as is evident from the experiment of the glass globe formerly alluded to, which will refract the rays only in a certain position. We have already stated that the red rays make an angle of 42° 2', and the violet an angle of 40° 17'. Now, if a line be drawn horizontally from the spectator's eye, it is evident that angles formed with this line, of a certain dimension, in every direction, will produce a circle, as will appear by attaching a cord of a given length to a certain point, round which it may turn as round its axis ; and, in every point will describe an angle with the horizontal line of a certain and determinate extent. Sometimes it happens that three or more bows are visible, though with different degrees of dis- tinctness. I have more than once observed this phenomenon, particularly in Edinburgh, in the month of August, 1825, when three rainbows ON THE COLOURS OF NATURAL OBJECTS. 151 were distinctly seen in the same quarter of the sky ; and, if 1 recollect right, a fragment of a fourth made its appearance. This happens when the rays suffer a third or fourth reflection; but, on account of the light lost by so many reflections, such bows are, for the most part, altogether imperceptible. If there were no ground to intercept the rain and the view of the observer, the rainbow would form a complete circle, the centre of which is diametrically opposite to the sun. Such circles are sometimes seen in the spray of the sea or of a cascade, or from the tops of lofty mountains, when the showers happen in the vales below. Rain- bows of various descriptions are frequently ob- served rising amidst the spray and exhalations of waterfalls, and among the waves of the sea whose tops are blown by the wind into small drops. There is one regularly seen, when the sun is shining, and the spectator in a proper position, at the fall of Staubbach, in the bosom of the Alps ; one near Schaffhausen ; one at the cascade of Lauffen ; and one at the cataract of Niagara in North America. A still more beautiful one is said to be seen at Terni, where the whole current of the river Velino, rushing from a steep preci- pice of nearly 200 feet high, presents to the spectator below, a variegated circle, over-arching the fall, and two other bows suddenly reflected on the right and left. Don Ulloa, in the account of his journeys in South America, relates that circu- lar rainbows are frequently seen on the mountains above Quito in Peru. It is said that a rainbow was once seen near London, caused by the exhala- tions of that city, after the sun had been below the horizon more than twenty minutes,* A naval * Philosophical Transactions. Vol. 50. p. 294. 152 THE PRACTICAL ASTRONOMER, friend, says Mr. Bucke, informed me, that, as he was one day watching the sun's effect upon the exhalations near Juan Fernandez, he saw upwards of five-and-twenty ires marince animate the sea at the same time. In these marine bows the con- cave sides were turned upwards, the drops of water rising from below, and not falling from above, as in the instances of the aerial arches. Rainbows are also occasionally seen on the grass, in the morning dew, and likewise when the hoar- frost is descending. Dr. Langwith once saw a bow lying on the ground, the colours of which were almost as lively as those of a common rain- bow. It was not round but oblong, and was extended several hundred yards. The colours took up less space, and were much more lively in those parts of the bow which were near him than in those which were at a distance. When M. Labillardiere was on Mount Teneriffe, he saw the contours of his body traced on the clouds beneath hirn in all the colours of the solar bow. He had previously witnessed this phenomenon on the Kesrouan in Asia Minor. The rainbows of Greenland are said to be frequently of a pale white, fringed with a brownish yellow, arising from the rays of the sun being reflected from a frozen cloud. The following is a summary view of the princi- pal facts which have been ascertained respecting the rainbow : — 1. The rainbow can only be seen when it rains, and in that point of the heavens which is opposite to the sun. 2. Both the primary and secondary bows are variegated with all the prismatic colours — the red being the highest colour in the primary, or brightest bow, and the violet the highest in the exterior. 3. The primary rainbow can never be a greater arc than a semi- PHENOMENA OF THE RAINBOW. 153 circle ; and when the sun is set, no bow, in ordinary circumstances, can be seen. 4. The breadth of the inner or primary bow — supposing the sun but a point — is 1° 45' ; and the breadth of the exterior bow 3° 12', which is nearly twice as great as that of the other ; and the distance between the bows is 8° 55'. But since the body of the sun subtends an angle of about half a degree, by so much will each bow be increased, and their distance diminished ; and therefore the breadth of the interior bow will be 2° 15', and that of the exterior, 3° 42', and their distance 8° 25'. The greatest semidiameter of the interior bow, on the same grounds, will be 42° 17\ and the least of the exterior bow 50° 43'. 5. When the sun is in the horizon, either in the morning or evening, the bows will appear complete semicircles. On the other hand, when the sun's altitude is equal to 42° 2' or to 54° 10', the summits of the bows will be depressed below the horizon. Hence, during the days of summer, within a certain interval each day, no visible rainbows can be formed, on account of the sun's high altitude above the horizon. 6. The altitude of the bows above the horizon, or surface of the earth, varies, according to the elevation of the sun. The altitude, at any time, may be taken by a common quadrant, or other angular instrument ; but, if the sun's alti- tude at any particular time be known, the height of the summit of any of the bows may be found, by subtracting the sun's altitude from 42 0 2' for the inner bow, and from 54° 10', for the outer. Thus, if the sun's altitude were 26°, the height of the primary bow would 16° 2', and of the secondary, 28° 3'. It follows, that the height and the size of the bows diminish as the altitude of the sun increases. 7. If the sun's altitude is more H 5 15i THE PRACTICAL ASTRONOMER. than 4£ degrees, and less than 54°, the exterior bow may be seen though the interior bow is invisi- ble. 8. Sometimes only a portion of an arch will be visible while all the other parts of the bow are invisible. This happens when the rain does not occupy a space of sufficient extent to complete the bow ; and the appearance of this position, and even of the bow itself, will be various, according to the nature of the situation, and the space occupied by the rain. The appearance of the rainbow may be pro- duced by artificial means, at any time when the sun is shining and not too highly elevated above the horizon. This is effected by means of arti- ficial fountains or Jet d'eaus, which are intended to throw up streams of water to a great height. These streams, when they spread very wide, and blend together in their upper parts, form, when falling, a shower of artificial rain. If, then, when the fountain is playing, we move between it and the sun, at a proper distance from the fountain, till our shadow point directly towards it, and look at the shower, — we shall observe the colours of the rainbow, strong and vivid ; and, what is particu- larly worthy of notice, the bow appears, notwith- standing the nearness of the shower, to be as large, and as far off, as the rainbow which we see in a natural shower of rain. The same experi- ment may be made by candle-light, and with any instrument that will form an artificial shower. Lunar Rainbows. — A lunar bow is sometimes formed at night by the rays of the moon striking on a rain-cloud, especially when she is about the full. But such a phenomenon is very rare. Aris- totle is said to have considered himself the first who had seen a lunar rainbow. For more than a hundred years prior to the middle of the last cen- PHENOMENA OF THE RAINBOW. 155 tury, we find only two or three instances recorded in which such phenomena are described with accu- racy. In the philosophical transactions for 1783, however, we have an account of three having been seen in one year, and all in the same place, but they are by no means common phenomena. I have had an opportunity within the last twenty years of witnessing two phenomena of this des- cription — one of which was seen at Perth, on a sabbath evening, in the autumn of 1825, and the other at Edinburgh, on Wednesday, the 9th of September 1840, about eight o'clock in the even- ing — of both which I gave a detailed description in some of the public journals. The Moon, in both cases, was within a day or two of the full ; the arches were seen in the northern quarter of the heavens, and extended nearly from east to west, the moon being not far from the southern meridian. The bows appeared distinct and well defined, but no distinct traces of the prismatic colours could be perceived on any of them. That which appeared in 1825 was the most distinctly formed, and continued visible for more than an hour. The other was much fainter, and lasted little more than half an hour, dark clouds having obscured the face of the moon. These bows bore a certain resemblance to some of the luminous arches which sometimes accompany the Aurora Borealis, and this latter phenomenon has not un- frequently been mistaken for a Lunar rainbow ; but they may be always distinguished by attend- ing to the phases and position of the moon. If the moon be not visible above the horizon, if she be in her first or last quarter, or if any observed phenomenon be not in a direction opposite to the moon, we may conclude with certainty that, what- ever appearance is presented, there is no lunar rainbow. 156 THE PRACTICAL ASTRONOMER. The rainbow is an object which has engaged universal attention, and its beautiful colours and form have excited universal admiration. The poets have embellished their writings with many beautiful allusions to this splendid meteor; and the playful school-boy, while viewing the 1 bright enchantment/ has frequently run ' to catch the falling glory.' When its arch rests on the oppo- site sides of a narrow valley, or on the summits of two adjacent mountains, its appearance is both beautiful and grand. In all probability, its figure first suggested the idea of arches, which are now found of so much utility in forming aqueducts and bridges, and for adorning the architecture of palaces and temples. It is scarcely possible seri- ously to contemplate this splendid phenomenon, without feeling admiration and gratitude towards that wise and beneficent Being, whose hands have bent it into so graceful and majestic a form, and decked it with all the pride of colours. " Look upon the rainbow," says the son of Sirach,* and praise Him that made it : very beautiful it is in the brightness thereof. It compasseth the heaven about with a glorious circle, and the hands of the Most High have bended it." To this grand ethe- rial bow, the inspired writers frequently allude as one of the emblems of the majesty and splen- dour of the Almighty. In the prophecies of Ezekiel, the throne of Deity is represented as adorned with a brightness " like the appearance of the bow that is in the cloud in the day of rain — the appearance of the likeness of the glory of Jehovah." And, in the visions recorded in the Book of the Revelations, where the Most High is represented as sitting upon a throne ; " there was a rainbow round about the throne, in sight like * Ecclesiasticus xliii. 11, 12. PHENOMENA OF THE RAINBOW. 157 unto an emerald/' as an emblem of his propitious character and of his faithfulness and mercy. After the deluge, this bow was appointed as a sign and memorial of the covenant which God made with Noah and his sons, that a flood of waters should never again be permitted to deluge the earth and its inhabitants ; — and as a pledge of inviolable fide- lity and Divine benignity. When, therefore, we at any time behold " the bow in the cloud," we have not only a beautiful and sublime phenomenon presented to the eye of sense, but also a memorial exhibited to the mental eye, assuring us, that, " While the earth remaineth, seed-time and har- vest, and cold and heat, and summer and winter, and day and night, shall not cease"* On the broad sky is seen 44 A dewy cloud, and in the cloud a bow- Conspicuous, with seven listed colours gay Betokening peace with God and covenant new. — He gives a promise never to destroy The earth again by flood, nor let the sea Surpass his bounds, nor rain to drown the world." Milton. Par. Lost, Book XI, * It is a question which has been frequently started — Whether there was any rainbow before the flood ? Some have conceived that the rainbow was something of a miraculous production, and that it was never seen before the flood. The equivocal sense of the word 4 set ' in our translation, has occasioned a mistaken impression of this kind. The Hebrew word thus translated, signifies more properly 6 1 do give,' or 'I appoints The whole passage in reference to this circumstance, literally translated, runs thus ; — " I appoint my bow which is in the cloud, that it may be for a sign or token of a covenant between me and the earth ; and it shall come to pass when I bring a cloud over the earth, and the bow shall be seen in the cloud, that I will remem- ber my covenant that is between me and you," &c. As the rainbow is produced by the immutable laws of refraction and reflection, as applied to the rays of the sun striking on drops of falling rain, the phenomenon must have been occasionally exhibited from the beginning of the world : unless we suppose that there was no rain before the flood, and that the constitution of things in the physical system was very different from what it is now. The passage affirms no more than that the rainbow was then appointed to be a symbol of the covenant be- tween God and man, and although it may have been frequently seen 158 THE PRACTICAL ASTRONOMER, SECT. 4. REFLECTIONS ON THE BEAUTY AND UTILITY OF COLOURS. Colour is one of the properties of light which constitutes, chiefly, the beauty and sublimity of of the universe. It is colour, in all its diversified shades, which presents to our view that almost in- finite variety of aspect which appears on the scene of nature, which gives delight to the eye and the imagination, and which adds a fresh pleasure to every new landscape we behold. Every flower which decks our fields and gardens is compounded of different hues ; every plain is covered with shrubs and trees of different degrees of verdure ; and almost every mountain is clothed with herbs and grass of different shade from those which appear on the hills and landscape with which it is surrounded. In the country, during summer, nature is every day, and almost every hour, vary- ing her appearance, by the multitude and variety of her hues and decorations, so that the eye wan- ders with pleasure over objects continually diversi- fied, and extending as far as the sight can reach. In the flowers with which every landscape is adorned, what a lovely assemblage of colours, and what a wonderful art in the disposition of their shades ! Here, a light pencil seems to have laid on the delicate tints ; there, they are blended according to the nicest rules of art. Although green is the general colour which prevails over the scene of sublunary nature, yet it is diversified by a thou- sand different shades, so that every species of tree, before, it would serve the purpose of a sign equally well, as if it had been miraculously formed for this purpose, and even better, as its frequent appearance, according to natural laws, is a perpetual memo- rial to man of the divine faithfulness and mercy. PHENOMENA OF THE RAINBOW. 159 shrub and herb, is clothed with its own peculiar verdure. The dark green of the forests is thus easily distinguished from the lighter shades of corn- fields and the verdure of the lawns. The system of animated nature likewise, displays a diversified assemblage of beautiful colours. The plumage of birds, the brilliant feathers of the peacock, the ruby and emerald hues which adorn the little humming-bird, and the various embellishments of many species of the insect tribe, present to the eye, in every region of the globe, a scene of di- versified beauty and embellishment. Nor is the mineral kingdom destitute of such embellishments. For some of the darkest and most unshapely stones and pebbles, when polished by the hand of art, display a mixture of the most delicate and variegated colours. All which beauties and varie- ties in the scene around us are entirely owing to that property, in every ray of light, by which it is capable of being separated into the primitive colours. To the same cause, likewise, are to be ascribed those beautiful and diversified appearances, which frequently adorn the face of the sky, — the yellow, orange and ruby hues which embellish the firma- ment at the rising of the sun, and when he is about to descend below the western horizon ; and those aerial landscapes, so frequently beheld in tropical climes, where rivers, castles and mountains, are depicted rolling over each other along the circle of the horizon. The clouds, especially in some countries, reflect almost every colour in nature. Sometimes they wear the modest blush of the rose ; sometimes they appear like stripes of deep vermillion, and sometimes as large brilliant masses tinged with various hues ; now they are white as ivory, and now as yellow as native gold. In some 160 THE PRACTICAL ASTRONOMER. tropidal countries, according to St. Pierre, the clouds roll themselves up into enormous masses as white as snow, and are piled upon each other, like the Cordeliers of Peru, and are moulded into the shape of mountains, of caverns and of rocks. When the sun setsbehind this magnificent aerial net-work, amultitudeof luminousrays are transmitted through each particular interstice, which produce such an effect, that the two sides of the lozenge illumi- nated by them, have the appearance of being begirt with a fillet of gold ; and the other two which are in the shade, seem tinged with a superb ruddy orange. Four or five divergent streams of light, emanating from the setting sun up to the zenith, clothe with fringes of gold the un- determinate summits of this celestial barrier, and proceed to strike with the reflexes of their fires the pyramids of the collateral aerial mountains, which then appear to consist of silver and vermi- lion. — In short, colour diversifies every sublunary scene, whether on the earth or in the atmosphere, it imparts a beauty to the phenomena of falling stars, of luminous arches, and the coruscations of the Aurora Borealis, and gives a splendour and sublimity to the spacious vault of heaven. Let us now consider for a moment, what would be the aspect of nature, if, instead of the beautiful variety of embellishments which now appear on every landscape, and on the concave of the sky, — one uniform colour had been thrown over the scenery of the universe. Let us conceive the whole of terrestrial nature to be covered with snow, so that not an object on earth should appear with any other hue, and that the vast expanse of the firmament presented precisely the same uni- form aspect. What would be the consequence ? The light of the sun would be strongly reflected UTILITY AND BEAUTY OF COLOURS. 161 from all the objects within the bounds of our ho- rizon, and would produce a lustre which would dazzle every eye. The day would acquire a greater brightness than it now exhibits, and our eyes might, after some time, be enabled freely to ex- patiate over the surrounding landscape ; but every thing, though enlightened, would appear confused, and particular objects would scarcely be distin- guishable, A tree, a house or a church, near at hand, might possibly be distinguished, on account of its elevation above the general surface of the ground, and the bed of a river by reason of its being depressed below it. But we should be obliged rather to guess, and to form a conjecture as to the particular object we wished to distinguish, than to arrive at any certain conclusion respecting it ; and if it lay at a considerable distance, it would be impossible, with any degree of probability, to discriminate any one object from another. Not- withstanding the universal brightness of the scene, the uniformity of colour thrown on every object, would most certainly prevent us from distinguish- ing a church from a palace, a cottage from a knoll or a heap of rubbish, a splendid mansion from rugged rocks, the trees from the hills on which they grow, or a barren desert from rich and fer- tile plains. In such a case, human beings would be confounded, and even friends and neighbours be at a loss to recognize one another. The vault of heaven, too, would wear a uniform aspect. Neither planets nor comets would be visi- ble to any eye, nor those millions of stars which now shine forth with so much brilliancy, and di- versify the nocturnal sky. For, it is by the con- trast produced by the deep azure of the heavens and the white radiance of the stars, that those bodies are rendered visible. Were they depicted 162 THE PRACTICAL ASTRONOMER. on a pure white ground, they would not be distin- guished from that ground, and would consequently be invisible, unless any of them occasionally as- sumed a different colour. Of course, all that beautiful variety of aspect which now appears on the face of sublunary nature — the rich verdure of the fields, the stately port of the forest, the rivers meandering through the valleys, the splendid hues that diversify and adorn our gardens and meadows, the gay colouring of the morning and evening clouds, and all that variety which distinguishes the different seasons, would entirely disappear. As every landscape would exhibit nearly the same aspect, there would be no inducement to the poet and the philosopher to visit distant countries to investigate the scenes of nature, and journeyings from one region to another would scarcely be pro- ductive of enjoyment. Were any other single colour to prevail, nearly the same results would ensue. Were a deep ruddy hue to be uniformly spread over the scene of creation, it would not only be offensive to the eye, but would likewise prevent all distinction of objects. Were a dark blue or a deep violet to prevail, it would produce a similar effect, and at the same time, present the scene of nature as covered with a dismal gloom. Even if creation were arrayed in a robe of green, which is a more pleasant colour to the eye — were it not diversified with the different shades it now exhibits, every object would be equally undistin- guishable. Such would have been the aspect of creation, and the inconveniences to which we should have been subjected, had the Creator afforded us light without that intermixture of colours which now appears over all nature, and which serves to discriminate one object from another. Even our UTILITY AND BEAUTY OF COLOURS. 163 very apartments would have been tame and insipid, incapable of the least degree of ornament, and the articles with which they are furnished, almost undistinguishable, so that in discriminating one object from another, we should have been as much indebted to the sense of touch as to the sense of vision. Our friends and fellow men would have presented no objects of interest in our daily asso- ciations. The sparkling eye, the benignant smile, the modest blush, the blended hues of white and vermillion in the human face, and the beauty of the female countenance, would all have vanished, and we should have appeared to one another as so many moving marble statues cast nearly in the same mould. But, what would have been worst of all, the numerous delays, uncertainties and per« plexities to which we should have been subjected, had we been under the necessity, every moment, of distinguishing objects by trains of reasoning, and by circumstances of time, place, and relative position ? An artist, when commencing his work in the morning, with a hundred tools of nearly the same size and shape around him, would have spent a considerable portion of his time before he could have selected those proper for his purpose, or the objects to which they were to be applied ; and in every department of society, and in all our excur- sions from one place to another, similar difficulties and perplexities would have occurred. The one half of our time must thus have been employed in uncertain guesses, and perplexing reasonings, respecting the real nature and individuality of objects, rather than in a regular train of thinking and of employment ; and after all our perplexities and conjectures, we must have remained in the utmost uncertainty, as to the thousands of scenes and objects, which are now obvious to us, through 164 THE PRACTICAL ASTRONOMER. the instrumentality of colours, as soon as we open our eyes. In short, without colour, we could have had no books nor writings : we could neither have corres- ponded with our friends by letters, nor have known any thing with certainty, of the events which happened in former ages. No written revelation of the will of God, and of his character, such as we now enjoy, could have been handed down to us from remote periods and generations. The discoveries of science, and the improvements of art, would have remained unrecorded. Universal ignorance would have prevailed throughout the world, and the human mind have remained in a state of demoralization and debasement. All these, and many other inconveniences and evils would have inevitably followed, had not God painted the rays of light with a diversity of colours, And hence we may learn, that the most important scenes and events in the universe, may depend upon the existence of a single principle in nature, and even upon the most minute circumstances, which we may be apt to overlook, in the arrange- ments of the material world. In the existing state of things in the visible creation, we cannot but admire the Wisdom and Beneficence of the Deity, in thus enabling us to distinguish objects by so easy and expeditious a mode as that of colour, which in a moment, discri- nates every object and its several relations. We rise in the morning to our respective employments, and our food, our drink, our tools, our books, and whatever is requisite for our comfort, are at once discriminated. Without the least hesitation or uncertainty, and without any perplexing process of reasoning, we can lay our hands on whatever articles we require. Colour clothes every object UTILITY AND BEAUTY OF COLOURS. 165 with its peculiar livery, and infallibly directs the hand in its movements, and the eye in its surveys and contemplations. But, this is not the only end which the Divine Being had in view, in impressing on the rays of light a diversity of colours. It is evident, that he likewise intended to minister to our pleasures, as well as to our wants. To every man of taste, and almost to every human being, the combination of colours in flowers, the delicate tints with which they are painted, the diversified shades of green with which the hills and dales, the mountains and the vales are arrayed ; and that beautiful variety which appears in a bright sum- mer day, on all the objects of this lower creation — are sources of the purest enjoyment and delight. It is colour, too, as well as magnitude, that adds to the sublimity of objects. Were the canopy of heaven of one uniform hue, it would fail in pro- ducing those lofty conceptions, and those delight- ful and transporting emotions, which a contempla- tion of its august scenery is calculated to inspire. Colours are likewise of considerable utility in the intercourse of general society. They serve both for ornaments, and for distinguishing the different ranks and conditions of the community : they add to the beauty and gracefulness of our furniture and clothing. At a glance, they enable us at once to distinguish the noble from the ignoble, the prince from his subjects, the master from his ser- vant, and the widow clothed with sable weeds from the bride adorned with her nuptial ornaments. Some colours, then, are of so much value and importance, that they may be reckoned as holding a rank among the noblest natural gifts of the Crea- tor. As they are of such essential service to the inhabitants of our globe, there can be no doubt that they serve similar or analogous purposes 166 THE PRACTICAL ASTRONOMER. throughout all the worlds in the universe. The colours displayed in the solar beams are common to all the globes which compose the planetary- system, and must necessarily be reflected, in all these diversified hues, from objects on their sur- faces. The light which radiates from the fixed stars displays a similar diversity of colours. Some of the double stars are found to emit light of different hues ; — the larger star exhibiting light of a ruddy or orange hue, and the smaller one a radiance which approaches to blue or green. There is therefore reason to conclude, that the objects connected with the planets which revolve round such stars — being occasionally enlightened by suns of different hues — will display a more variegated and splendid scenery of colouring than is ever beheld in the world on which we dwell ; and that one of the distinguishing characteristics of dif- ferent worlds, in regard to their embellishments, may consist in the splendour and variety of colours with which the objects on those surfaces are adorned. In the metaphorical description of the glories of the New Jerusalem, recorded in the Book of Revelation, one of the chief characteris- tics of that city is said to consist in the splendour and diversity of hues with which it is adorned. It is represented as " coming down from heaven, prepared as a bride adorned for her husband" and as reflecting all the beautiful and variegated colours which the finest gems on earth can exhibit ; evidently indicating, that splendour and variety of colouring are some of the grandest features of celestial scenery. On the whole, the subject of colours, when seriously considered, is calculated to excite us to the adoration of the goodness and intelligence of that Almighty Being whose wisdom planned all UTILITY AND BEAUTY OF COLOURS. 167 the arrangements of the universe, and to inspire us with gratitude for the numerous conveniences and pleasures we derive from those properties and laws he has impressed on the material system. He might have afforded us light, and even splendid illumination, without the pleasures and advantages which diversified colours now produce, and man and other animated beings might have existed in such a state. But, what a very different scene would the world have presented from what it now exhibits ! Of how many thousands of pleasures should we have been deprived ! and to what numerous inconveniences and perplexities should we have been subjected ! The sublimity and glories of the firmament, and the endless beauties and varieties which now embellish our terrestrial system, would have been for ever unknown, and man could have had little or no incitement to study and investigate the works of his Creator. In this, as well as in many other arrangements in nature, w T e have a sensible proof of the presence and agency of that Almighty Intelligence " in whom we live, and move, and have our being." None but an infinitely Wise and Beneficent Being, in- timately present in all places, could thus so regu- larly create in us by means of colour, those exqui- site sensations which afford so much delight, and which unite us, as it were, with every thing around us. In the diversity of hues spread over the face of creation, we have as real a display of the Divine presence as Moses enjoyed at the burning bush. The only difference is, that the one was out of the common order of Divine procedure, and the other in accordance with those permanent laws which regulate the economy of the universe. In every colour, then, which we contemplate, we have a sensible memorial of the presence of that 168 THE PRACTICAL ASTRONOMER. Being u whose Spirit garnished the heavens and laid the foundations of the earth," and whose " merciful visitation" sustains us every moment in existence. But the revelation of God to our senses, through the various objects of the ma- terial world, has become so familiar, that we are apt to forget the Author of all our enjoyments, even at the moment when we are investigating his works and participating of his benefits. " O that men would praise Jehovah for his goodness, and for his wonderful works towards the children of men." PART II. ON TELESCOPES. CHAPTER I. HISTORY OF THE INVENTION OF TELESCOPES. The telescope is an optical instrument for viewing objects at a distance. Its name is com- pounded of two Greek words, — -njXe, which signifies, at a distance, or far off, and ax°n €lv , to view, or to contemplate. By means of telescopes, remote objects are represented as if they were near, small apparent magnitudes are enlarged, confused objects are rendered distinct, and the invisible and obscure parts of very distant scenes are ren- dered perceptible and clear to the organ of vision. The telescope is justly considered as a grand and noble instrument. It is not a little surprising that it should be in the power of man to invent and construct an instrument by which objects, too remote for the unassisted eye to distinguish, should be brought within the range of distinct i 170 THE PRACTICAL ASTRONOMER. vision, as if they were only a few yards from our eye, and that thousands of august objects in the heavens, which had been concealed from mortals for numerous ages, should be brought within the limits of our contemplation, and be as distinctly perceived, as if we had been transported many millions of miles from the space we occupy, through the celestial regions. The celebrated Huygens remarks, in reference to this instrument, that, in his opinion, c the wit and industry of man has not produced any thing so noble and so worthy of his faculties as this sort of knowledge ; (namely of the telescope) insomuch that if any particular person had been so diligent and sagacious as to invent this instrument from the principles of nature and geometry, — for my part, I should have thought his abilities were more than human ; but the case is so far from this, that the most learned men have not yet been able sufficiently to explain the reason of the effects of this casual invention/ The persons who constructed the first telescopes, and the exact period when they were first invented, are involved in some degree of obscurity. It does not certainly appear that such instruments were known to the ancients, although we ought not to be perfectly decisive on this point. The cabinets of the curious contain some very ancient gems, of admirable workmanship, the figures on which are so small, that they appear beautiful through a magnifying glass, but altogether con- fused and indistinct to the naked eye : and, there- fore, it may be asked, if they cannot be viewed, how could they be wrought, without the assistance of glasses ? And as some of the ancients have declared that the moon has a form like that of the earth, and has plains, hills, and valleys in it, — how could they know this — unless by mere conjecture, THE INVENTION OF TELESCOPES. 171 without the use of a telescope ? And how could they have known that the Milky Way is formed by the combined rays of an infinite number of stars? For Ovid states, in reference to this zone, * its ground-work is of stars.' But whatever knowledge the ancients may have possessed of the telescope or other optical glasses, it is quite evi- dent that they never had telescopes of such size and power as those which we now possess ; and that no discoveries in the heavens, such as are now brought to light, were made by any of the ancient astronomers ; otherwise some allusions to them must have been found in their writings. Among the moderns, the illustrious Friar Bacon appears to have acquired some rude ideas respect- ing the construction of telescopes. 6 Lenses and specula' says he, 6 may be so figured that one object may be multiplied into many, that those which are situated at a great distance may be made to appear very near, that those which are small may be made to appear very large, and those which are obscure very plain ; and we can make stars to appear wherever we will/ From these expressions, it appears highly probable, that this philosopher was acquainted with the general prin- ciple both of telescopes and microscopes, and that he may have constructed telescopes of small mag- nifying power, for his own observation and amuse- ment, although they never came into general use. He was a man of extensive learning, and made so rapid a progress in the sciences, when attending the university of Paris, that he was esteemed the glory of that seat of learning. He prosecuted his favourite study of experimental philosophy with unremitting ardour ; and in this pursuit, in the course of twenty years, he expended no less than £2000 in experiments, instruments, and in I 2 172 THE PRACTICAL ASTRONOMER. procuring scarce books. In consequence of such extraordinary talents, and such astonishing pro- gress in the sciences, in that ignorant age, he was represented, by the envy of his illiterate frater- nity, as having dealings with the devil ; and, under this pretence, he was restrained from reading lec- tures, and at length, in 1278, when sixty-four years of age, he was imprisoned in his cell, where he remained in confinement for ten years. He shone like a single bright star in a dark hemis- phere — the glory of our country — and died at Oxford, in the y ear in the eightieth year of his age. c Friar Bacon/ says the Rev. Mr. Jones, e may be considered as the first of English philo- sophers ; his profound skill in mechanics, optics, astronomy, and chemistry, would make an honour- able figure in the present age. But he is entitled to further praise, as he made all his studies sub- servient to theology, and directed all his writings, as much as could be, to the glory of God. He had the highest regard for the sacred scriptures, and was persuaded they contain the principles of all true science.' The next person who is supposed to have ac- quired a knowledge of telescopes, was Joannes Baptista Porta, of Naples, who flourished in the sixteenth century. He discovered the Camera Obscura — the knowledge of which might naturally have led to the invention of the telescope ; but it does not appear that he ever constructed such an instrument. Des Cartes considers James Metius, a Dutchman, as the first constructor of a telescope, and says, that ' as he was amusing himself with making mirrors and burning-glasses, he casually thought of looking through two of his lenses at a time, and found that distant objects appeared very large and distinct.' Others say that this great THE INVENTION OF TELESCOPES. 173 discovery was first made by John Lippersheim, a maker of spectacles at Middleburg, or rather by his children, who were diverting themselves with looking through two glasses at a time, and placing them at different distances from each other. But Borellus, who wrote a book 6 on the invention of the telescope,' gives this honour to Zacharias Jansen, another spectacle-maker in the same town, who, he says, made the first telescope in 1590. Jansen was a diligent inquirer into nature, and, being engaged in such pursuits, he was trying what use could be made of lenses for those pur- poses, when he fortunately hit upon the construc- tion. Having found the arrangement of glasses which produced the effect desired, he enclosed them in a tube, and ran with his instrument to prince Maurice, who, immediately conceiving that it might be of use to him in his wars, desired the author to keep it a secret. Such are the rude conceptions and selfish views of princely warriors, who would apply every invention in their power for the destruction of mankind. But the telescope was soon destined to more noble and honourable achievments. Jansen, it is said, directed his in- strument towards celestial objects, and distinctly saw the spots on the surface of the moon, and dis- covered many new stars, particularly seven pretty considerable ones in the Great Bear. His son Joannes is said to have noted the lucid circle near the lower limb of the moon, now named Tycho, from whence several bright rays seem to dart in different directions. In viewing Jupiter, he per- ceived two, sometimes three, and at the most four small stars, a little above or below him, and thought that they performed revolutions around him. This was, probably, the first observation of the satellites of Jupiter, though the person who 174 THE PRACTICAL ASTRONOMER. made it was not aware of the importance of his discovery.* It is not improbable that different persons about Middleburgh hit upon the invention, in different modes, about the same time. Lippersheim seems to have made his first rude telescope by adjusting two glasses on a board, and supporting them on brass circles.f Other workmen, parti- cularly Metius and Jansen, in emulation of each other, seem to have made use of that discovery, and by the new form they gave it, made all the honour of it their own. One of them, considering the effects of light as injurious to distinctness, placed the glasses in a tube blackened within. The other, still more cautious, placed the same glasses within tubes capable of sliding one in another, both to vary the prospects, by lengthen- ing the instrument, according to the pleasure of the observer, and to render it portable and com- modious. Thus, it is probable that different persons had a share in the invention, and jointly contributed to its improvement. At any rate, it is undoubtedly to the Dutch that we owe the original invention. The first telescope made by Jansen, did not exceed fifteen or sixteen inches in length, and therefore its magnifying power could not have been very great. The famous Galileo has frequently been sup- posed to have been the inventor of the telescope, but he acknowledges that he had not the honour of being the original inventor, having first learned * Though Borellus mentions this circumstance, yet there is some reason to doubt the accuracy of this statement, as young Jansen ap- pears to have been at that period, not more than six years old ; so that it is more probable that Galileo was the first discoverer of Jupiter's satellites. f The reader may see an engraving of this instrument in the author's work entitled 4 the Improvement of Society.'' — p. 209. THE INVENTION OF TELESCOPES. 175 from a German, that such an instrument had already been made ; although, from his own ac- count, it appears that he had actually re-invented this instrument. The following is the account, in his own words, of the circumstances which led him to construct a telescope. ( Nearly ten months ago (namely in April or May 1609) it was reported that a certain Dutchman had made a perspective through which many distant objects appeared dis- tinct as if they were near : several effects of this wonderful instrument were reported, which some believed and others denied : but, having it con- firmed to me a few days after by a letter from the noble John Badoverie, at Paris, I applied myself to consider the reason of it, and by what means I might contrive a similar instrument, which I after- wards attained to by the doctrine of refractions. And, first, I prepared a leaden tube, to whose ex- tremities I fitted two spectacle-glasses, both of them plain on one side, and on the other side, one of them was spherically convex, and the other concave. Then applying my eye to the concave, I saw objects appear pretty large and pretty near me. They appeared three times nearer and nine times larger in surface than to the naked eye : and soon after 1 made another, which represented objects about sixty times larger, and eight times nearer ; and, at last, having spared no labour nor expense, I made an instrument so excellent, as to show things almost a thousand times larger, and above thirty times nearer, than to the naked eye/ In another part of his writings, Galileo informs us that 6 he was at Venice when he heard of Prince Maurice's instrument, but nothing of its construc- tion ; that the first night, after he returned to Padua, he solved the problem, and made his in- strument the next day ; and soon after, presented 176 THE PRACTICAL ASTRONOMER. it to the Doge at Venice, who, to do him honour for his grand invention, gave him the ducal letters which settled him for life in his lectureship at Padua ; and the Republic, on the twenty-fifth of August in the same year (1610) more than tripled his salary as professor.' The following is the account which this philo- sopher gives of the process of reasoning, which led him to the construction of a telescope : — e I argued in the following manner. The contrivance consists either of one glass or more— one is not sufficient, since it must be either convex, concave, or plane ; the last does not produce any sensible alteration in objects, the concave diminishes them ; it is true that the convex magnifies, but it renders them confused and indistinct; consequently one glass is insuflicent to produce the desired effect. Proceeding to consider two glasses, and bearing in mind that the plane glass causes no change, I determined that the instrument could not consist of the combination of a plane glass with either of the other two. I therefore applied myself to make experiments on combinations of the two other kinds ; and thus obtained that of which I was in search.' If the true inventor is the person who makes the discovery by reasoning and reflection, by tracing facts and principles to their consequences, and by applying his invention to important purposes, then, Galileo may be con- sidered as the real inventor of the telescope. No sooner had he constructed this instrument — before he had seen any similar one— than he directed his tube to the celestial regions, and his unwearied diligence and ardour were soon rewarded by a series of new and splendid discoveries. He descried the four satellites of Jupiter, and marked the periods of their revolutions ; he discovered the THE INVENTION OF TELESCOPES. 177 phases of Venus, and thus was enabled to adduce a new proof of the Copernican system, and to remove an objection that had been brought against it. He traced on the lunar orb, a resemblance to the structure of the earth, and plainly perceived the outlines of mountains and vales, casting their shadows over different parts of its surface. He observed, that when Mars was in quadrature, his figure varied slightly from a perfect circle ; and that Saturn consisted of a triple body, having a small globe on each side — which deception was owing to the imperfect power of his telescope, which was insufficient to show him that the phe- nomenon was in reality a ring. In viewing the sun, he discovered large dark spots on the surface of that luminary, by which he ascertained that that mighty orb performed a revolution round its axis. He brought to view multitudes of stars imperceptible to the naked eye, and ascertained that those nebulous appearances in the heavens which constitute the Milky Way, consist of a vast collection of minute stars, too closely compacted together to produce an impression on our unas- sisted vision. The results of Galileo's observations were given to the world in a small work, entitled ' Nuncius Sidereus/ or, ' News from the starry regions,' which produced an extraordinary sensation among the learned. These discoveries soon spread throughout Europe, and were incessantly talked of, and were the cause of much speculation and debate among the circles of philosophers. Many doubted ; many positively refused to believe so novel and unlooked-for announcements, because they ran counter to the philosophy of Aristotle, and all the preconceived notions which then pre- vailed in the learned world. It is curious, and I 5 178 THE PRACTICAL ASTRONOMER. may be instructive, to consider to what a length of absurdity, ignorance and prejudice carried many of those who made pretensions to learning and science. Some tried to reason against the facts alleged to be discovered, others contented them- selves, and endeavoured to satisfy others, with the simple assertion that such things were not, and could not possibly be ; and the manner in which they supported themselves in their incredulity was truly ridiculous. 1 O my dear Kepler/ says Galileo in a letter to that astronomer, 6 how I wish we could have one hearty laugh together. Here at Padua is the principal professor of philosophy, whom I have repeatedly and urgently requested to look at the moon and planets through my glass, which he pertinaciously refuses to do, lest his opi- nions should be overturned. Why are you not here ? what shouts of laughter we should have at this glorious folly ! and to hear the professor of philosophy at Pisa labouring with the Grand Duke with logical arguments, as if with magical incantations, to charm the new planets out of the sky.' Another opponent of Galileo, one Christ- mann, says in a book he published, 6 We are not to think that Jupiter has four satellites given him by nature, in order, by revolving round him, to immortalize the Medici who first had notice of the observation. These are the dreams of idle men, who love ludicrous ideas better than our laborious and industrious correction of the hea- vens. Nature abhors so horrible a chaos ; and to the truly wise, such variety is detestable.' One Martin Horky, a would-be philosopher, declared to Kepler, 'I will never concede his four new planets to that Italian from Padua, though I should die for it / and he followed up this declaration, by publishing a book against Galileo, in which he THE INVENTION OF TELESCOPES. 179 examines four principal questions respecting the alleged planets; 1. Whether they exist? 2. What they are? 3. What they are like? 4. Why they are ? The first question is soon dis- posed of by declaring positively that he has ex- amined the heavens with Galileo's own glass, and that no such thing as a satellite about Jupiter exists. To the second, he declares solemnly that he does not more surely know, that he has a soul in his body than that reflected rays are the sole cause of Galileo's erroneous observations. In regard to the third question, he says, that these planets are like the smallest fly compared to an elephant ; and finally, concludes on the fourth, that the only use of them is to gratify Galileo's * thirst of gold/ and to afford himself a subject of discussion. Kepler, in a letter to Galileo, when alluding to Horky, says, ' He begged so hard to be forgiven, that I have taken him again into favour upon this preliminary condition — that I am to ^how him Jupiter's satellites, and he is to see them, and own that they are there.' The following is a specimen of the reasoning of certain pretended philosophers of that age against the discoveries of Galileo. Sizzi, a Flo- rentine astronomer, reasons in this strain: ' There are seven windows given to animals in the domi- cile of the head, through which the air is admitted to the rest of the tabernacle of the body to en- lighten, to warm and to nourish it; two nostrils, two eyes, two ears, and a mouth ; so in the hea- vens, or the great world, there are two favourable stars, two unpropitious, two luminaries, and Mer- cury alone undecided and indifferent. From which and many other similar phenomena in nature, such as the seven metals, &c, we gather that the number of planets is necessarily seven. More- 180 THE PRACTICAL ASTRONOMER. over, the satellites are invisible to the naked eye, and therefore can exert no influence on the earth, and therefore would be useless, and therefore do not exist. Besides, as well the Jews as other ancient nations have adopted the division of the week into seven days, and have named them from the seven planets. Now, if we increase the num- ber of the planets, this whole system falls to the ground/ The opinions which then prevailed in regard to Galileo's observations on the moon, were such as the following : — Some thought that the dark shades on the moon's surface arose from the interposition of opaque bodies floating between her and the sun, which prevent his light from reaching those parts ; others imagined that, on account of her vicinity to the earth, she was partly tainted with the imperfections of our terrestrial and elementary nature, and was not of that entirely pure and refined substance of which the more remote heavens consist : and a third party looked on her as a vast mirror, and maintained that the dark parts of her surface were the re- flected images of our earthly forests and mountains. Such learned nonsense is a disgrace to our species, and to the rational faculties with which man is endowed, and exhibits, in a most ludicrous manner, the imbecility and prejudice of those who made bold pretensions to erudition and philo- sophy. The statement of such facts, however, may be instructive, if they tend to guard us against those prejudices and pre-conceived opinions, which prevent the mind from the cordial reception of truth, and from the admission of improvements in society which run counter to long-established cus- toms. For the same principles and prejudices, though in a different form, still operate in society and retard the improvement of the social state, THE INVENTION OF TELESCOPES. 181 the march of science, and the progress of Chris- tianity. How ridiculous is it for a man, calling himself a philosopher, to be afraid to look through a glass to an existing object in the heavens, lest it should endanger his previous opinions ! And how foolish is it to resist any improvement or reforma- mation in society, because it does not exactly accord with existing opinions, and with ' the wisdom of our ancestors/ It is not a little surprising, that Galileo should have first hit on that construction of a telescope which goes by his name, and which was formed with a concave glass next the eye. This construc- tion of a telescope is more difficult to be under- stood, in theory, than one which is composed solely of convex glasses; and its field of view is com- paratively very small, so that it is almost useless when attempted to be made of a great length. In the present day, we cannot help wondering that Galileo and other astronomers, should have made such discoveries as they did with such an instru- ment, the use of which. must have required a great degree of patience and address. Galileo's best telescope, which he constructed 6 with great trouble and expense,' magnified the diameters of objects only thirty-three times; but its length is not stated — which would depend upon the focal distance of the concave eye-glass. If the eye- glass was two inches focus, the length of the in- strument would be five feet four inches ; if it was only one inch, the length would be two feet eight inches, which is the least we can allow to it— the object-glass being thirty-three inches focus, and the eye-glass placed an inch within this focus. With this telescope, Galileo discovered the satel- lites of Jupiter, the crescent of Venus, and the other celestial objects to which we have already 182 THE PRACTICAL ASTRONOMER. alluded. The telescopes made in Holland, are supposed to have been constructed solely of con- vex glasses, on the principle of the astronomical telescope ; and, if so, Galileo's telescope was in reality a new invention. Certain other claimants of the invention of the telescope, have appeared, besides those already mentioned. Francis Fontana, in his 6 celestial observations,' says, that he was assured by a Mr. Hardy, advocate of the parliament of Paris, a person of great learning and undoubted integrity, that on the death of his father, there was found among his things an old tube, by which distant objects were distinctly seen, and that it was of a date long prior to the telescope lately invented, and had been kept by him as a secret. Mr. Leo- nard Digges, a gentleman who lived near Bristol, in the seventeenth century, and was possessed of great and various knowledge, positively asserts in his ' Stratoticos' and in another work, that his father, a military gentleman, had an instrument which he used in the field, by which he could bring distant objects near, and could know a man at the distance of three miles. Mr. Thomas Digges, in the preface to his * Pantometria/ pub- lished in 1591, declares, " My father, by his con- tinual painful practices, assisted by demonstrations mathematical, was able, and sundry times hath by proportional glasses, duly situate in convenient angles, not only discovered things far of, read letters, numbered pieces of money, with the very coin and superscription thereof, cast by some of his friends of purpose, upon downs in open fields, but also, seven miles off, declared what hath been done that instant, in private places. He hath also, sundry times, by the sun-beams, fired powder and discharged ordnance half a mile and THE INVENTION OF TELESCOPES. 183 more distant, and many other matters far more strange and rare, of which there are yet living divers witnesses.' It is by no means unlikely, that persons accus- tomed to reflection, and imbued with a certain degree of curiosity, when handling spectacle- glasses, and amusing themselves with their magni- fying powers and other properties, might sometimes hit upon the construction of a telescope ; as it only requires two lenses of different focal dis- tances to be held at a certain distance from each other, in order to show distant objects magnified. Nay, even one lens, of a long focal distance, is sufficient to constitute a telescope of a moderate magnifying power, as I shall show in the sequel. But such instruments, when they happened to be constructed accidentally, appear to have been kept as secrets, and confined to the cabinets of the curious, so that they never came into general use ; and as their magnifying power would probably be comparatively small, the appearance of the heavenly bodies would not be much enlarged by such instruments — nor is it likely that they would be often directed to the heavens. On the whole, therefore, we may conclude that the period when instruments of this description came into general use, and were applied to useful purposes, was when Galileo constructed his first telescopes. 184 OF THE CAMERA OBSCURA. CHAPTER II. OF THE CAMERA OBSCURA. Before proceeding to a particular description of the different kinds of telescopes, I shall first give a brief description of the Camera Obscura, as the phenomena exhibited by this instrument tend to illustrate the principle of a refracting telescope. The term Camera Obscura literally signifies a darkened vault or roof ; and hence it came to de- note a chamber, or box, or any other place made dark for the purpose of optical experiments. The camera obscura, though a simple, is yet a very curious and noble contrivance ; as it naturally and clearly explains the manner in which vision is performed, and the principle of the telescope, and entertains the spectator with a most exquisite pic- ture of surrounding objects, painted in the most accurate proportions and colours by the hand of nature. The manner of exhibiting the pictures of objects in a dark room is as follows : — In one of the window -shutters of a room which com- mands a good prospect of objects not very distant, a circular hole should be cut of four or five inches diameter. In this hole an instrument should be placed, called a Scioptric ball, which has three parts, a frame, a ball, and a lens. The ball has OF THE CAMERA OBSCURA. 185 a circular hole cut through the middle, in which the lens is fixed, and its use is, to turn every way so as to take in a view of objects on every side. The chamber should be made perfectly dark ; and a white screen, or a large sheet of elephant paper, should be placed opposite to the lens, and in its focus, to receive the image. If then, the objects without be strongly enlightened by the sun, there will be a beautiful living picture of the scene de- lineated on the white screen, where every object is beheld in its proportions, and with its colours even more vivid than life ; green objects appear in the picture more intensely green, and yellow, blue, red or white flowers appear much more beautiful in the picture than in nature ; if the lens be a good one, and the room perfectly dark, the perspec- tive is seen in perfection. The lights and shadows are not only perfectly just, but also greatly heigh- tened; and, what is peculiar to this delineation, and which no other picture or painting can exhi- bit — the motions of all the objects are exactly ex- pressed in the picture ; the boughs of the trees wave, the leaves quiver, the smoke ascends in a waving form, the people walk, the children at their sports leap and run, the horse and cart move along, the ships sail, the clouds soar and shift their aspects, and all as natural as in the real objects ; the motions being somewhat quicker, as they are performed in a more contracted scene. These are the inimitable perfections of a pic- ture, drawn by the rays of light as the only pencil in nature's hand, and which are finished in a moment ; for no sensible interval elapses before the painting is completed, when the ground on which it is painted is prepared and adjusted. In comparison of such a picture, the finest produc- tions of the most celebrated artists, the propor- 186 THE PRACTICAL ASTRONOMER. tions of Raphael, the natural tints and colouring of Titian, and the shadowing of the Venetians, are but coarse and sorry daubings, when set in competition with what nature can exhibit by the rays of light passing through a single lens. The Camera obscura is at the same time the painter's assistant, and the painter's reproach. From the picture it forms he receives his best instructions, and is shown what he should endeavour to attain ; and hence, too, he learns the imperfections of his art, and what it is impossible for him to imitate. As a proof of this, the picture formed in the dark chamber will bear to be magnified to a great ex- tent, without defacing its beauty, or injuring the fineness of its parts ; but the finest painted land- scape, if viewed through a high magnifier will appear only as a coarse daubing. The following scheme will illustrate what has been now stated respecting the dark chamber. EF represents a darkened room, in the side of which, IK, is made the circular hole V, in which, on the inside, is fixed the scioptric ball. At some considerable distance from this hole is exhibited a landscape of houses, trees, and other objects, ABCD, which are opposite to the window. The rays which flow from the different objects which compose this landscape, to the lens at V, and which pass through it, are converged to their respective foci, on the opposite wall of the chamber HG or on a white moveable screen placed in the focus of the lens, where they all combine to paint a lively and beautiful picture of the range of objects directly opposite, and on each side, so far as the lens can take in. Though I have said, that a scioptric ball and socket are expedient to be used in the above ex- periment, yet where such an instrument is not at OF THE CAMERA OBSCURA, 187 figure 37. hand, the lens may be placed in a short tube made of pasteboard or any other material, and fixed in the hole made in the window shutter. The only imperfection attending this method is, that the lens can exhibit those objects only which lie directly opposite the window. Some may be disposed to consider it as in imper- fection in this picture, that all the objects appear in an inverted position ; as they must necessarily do, according to what we formerly stated respect- ing the properties of convex lenses, (p. 103). There are, however, different modes of viewing the picture as if it were erect. For, if we stand before the picture, and hold a common mirror against our breast at an acute angle with the pic- ture, and look down upon it, we shall see all the images of the objects as if restored to their erect position ; and by the reflection of the mirror, the 188 THE PRACTICAL ASTRONOMER, picture will receive such a lustre as will make it still more delightful. Or, if a large concave mirror were placed before the picture at such a distance, that its image may appear before the mirror, it will then appear erect and pendulous in the air in the front of the mirror. Or, if the image be received on a frame of paper, we may stand behind the frame, with our face towards the window, and look down upon the objects, when they will appear as if erect. The experiment of the Camera Obscura may serve to explain and illustrate the nature of a common refracting telescope. Let us suppose, that the lens in the window-shutter represents the object-glass of a refracting telescope. This glass forms an image in its focus, which is in every respect an exact picture or representation of the objects before it ; and consequently the same idea is formed in the mind, of the nature, form, magni- tude, and colour of the object — whether the eye at the centre of the glass views the object itself, or the image formed in its focus. For, as for- merly stated, the object and its image are both seen under the same angles by the eye placed at the centre of the lens. Without such an image as is formed in the camera obscura — depicted either in the tube of a telescope or in the eye itself — no telescope could possibly be formed. If we now suppose that, behind the image formed in the dark chamber, we apply a convex lens of a short focal distance to view that image, then the image will be seen distinctly, in the same manner as we view common objects, such as a leaf or a flower, with a magnifying glass; consequently, the object itself will be seen distinct and magnified. And, as the same image is nearer to one lens than the other, it will subtend a larger angle at the OF THE CAMERA OBSCURA. 189 nearest lens, and of course, will appear larger than through the other, and consequently the object will be seen magnified in proportion. For exam- ple, let us suppose the lens in the camera obscura, or the object lens of a telescope, to be five feet, or sixty inches focal distance, at this distance from the glass, an image of the distant objects opposite to it will be formed. If now, we place a small lens two inches focal distance beyond this point, or five feet two inches from the object-glass, the objects, when viewed through the small lens, will appear considerably magnified, and apparently much nearer than to the naked eye. The degree of magnifying power is in proportion to the focal distances of the two glasses ; that is, in the pre- sent case, in the proportion of two inches, the focus of the small lens, to sixty inches, the focus of the object lens. Divide sixty by two, the quotient is thirty, which gives the magnifying power of such a telescope, that is, it represents objects thirty times nearer, or under an angle thirty times larger than to the naked eye. If the eye- glass, instead of being two inches, were only one and a half inch focus, the magnifying power would be in the proportion of one and a half to sixty, or forty times. If the eye-glass were three inches focus, the magnifying power would be twenty times; and so on, with regard to other proportions. In all cases, where a telescope is composed of only two convex lens, the magnifying power is deter- mined, by dividing the focal distance of the object- glass, by the focal distance of the eye-glass y and the quotient expresses the number of times the object is magnified, in length and breadth. This and various other particulars, will be more fully illustrated in the sequel. In performing experiments with the camera 190 THE PRACTICAL ASTRONOMER. obscura in a darkened chamber, it is requisite that the following particulars be attended to : — 1. That the lens be well figured, and free from any veins or blemishes that might distort the picture. 2. That it be placed directly against the object whose image we wish to see distinctly delineated. 3. The lens should be of a proper size both as to its breadth and focal distance. It should not be less than three or four feet focal distance, otherwise the picture will be too small, and the parts of objects too minute to be distinctly perceived ; nor should it exceed fifteen or eighteen feet, as in this case the picture will be faint, and of course not so pleasing. The best medium as to focal distance, is from five to eight or ten feet. The aperture, too, or breadth of the glass, should not be too small, otherwise the image will be obscure, and the minute parts of it invisible for want of a suffi- cient quantity of light. A lens of six feet focal distance, for example, will require an aperture of at least two inches. Lenses of a shorter focal distance require less apertures, and those of a longer focal distance larger. But if the aperture be too large, the image will be confused, and in- distinct, by the admission of too much light. 4. We should never attempt to exhibit the images of objects, unless when the sun is shining and strongly illuminating the objects, except in the case of very near objects placed in a good light. As one of the greatest beauties, in the pheno- mena of the dark chamber, consists in the exqui- site appearance and contrast of light and shadows, nothing of this kind can be perceived but from objects directly illuminated by the sun. 5. A south window should never be used in the fore- noon, as the sun cannot then enlighten the north side of an object ; and besides, his rays would be OF THE CAMERA OBSCURA. 191 apt to shine upon the lens, which would make the picture appear with a confused lustre. An east window is best in the afternoon, and a western in the morning ; but a north window is in most cases to be preferred, especially in the forenoon, when the sun is shining with his greatest strength and splendour. In general, that window ought to be used which looks to the quarter opposite to that in which the sun is shining. The picture should be received upon a very white surface, as the finest and whitest paper, or a painted cloth, bordered with black ; as white bodies reflect most copiously the incident rays, while black surfaces absorb them. If the screen could be bent into the concave segment of a sphere, of which the focal distance of the double convex lens which is used, is the radius, the parts of the picture adjacent to the extremities would appear most distinct. Sir D. Brewster informs us that, having tried a number of white substances of different degrees of smoothness, and several metallic surfaces, on w r hich to receive the image, he happened to receive the picture on the silvered back of a looking-glass, and was surprised at the brilliancy and distinctness with which external objects were represented. To remove the spheri- cal protuberances of the tin foil, he ground the surface very carefully with a bed of hones which he had used for working the plane specula of Newtonian telescopes. By this operation, which may be performed without injuring the other side of the mirror, he obtained a surface finely adapted for the reception of images. The minute parts of the landscape were formed with so much pre- cision, and the brilliancy of colouring was so uncommonly fine, as to equal, if not exceed the images that are formed in the air by means of concave specula. 192 THE PRACTICAL ASTRONOMER. The following additional circumstances may be stated respecting the phenomena exhibited in the dark chamber. A more critical idea may be formed of any movement in the picture here pre- sented than from observing the motion of the object itself. For instance, a man walking in a picture appears to have an undulating motion, or to rise up and down every step he takes, and the hands seem to move almost exactly like a pendu- lum ; whereas scarcely any thing of this kind is observed in the man himself, as viewed by the naked eye. Again, if an object be placed just twice the focal distance from the lens without the room, the image will be formed at the same distance from the lens within the room, and consequently will be equal in magnitude to the object itself. The recognition of this principle may be of use to those concerned in drawing, and who may wish, at any time, to form a picture of the exact size of the object. If the object be placed further from the lens than twice its focal length, the image will be less than the object. If it be placed nearer, the image will be greater than the life. In regard to immoveable objects, such as houses, gardens, trees, &c., we may form the images of so many different sizes, by means of different lenses, the shorter focus making the lesser picture, and the longer focal distance the largest. The experiments with the camera obscura, may likewise serve to illustrate the nature of vision, and the functions of the human eye. The frame or socket of the scioptric ball may represent the orbit of the natural eye. The ball, which turns every way, resembles the globe of the eye, move- able in its orbit. The hole in the ball may repre- sent the pupil of the eye ; the convex lens cor- ON THE CAMERA OBSCURA. 193 responds to the crystalline humour, which is shaped like a lens, and contributes to form the images of objects on the inner part of the eye. The dark chamber itself, is somewhat similar to the internal part of the eye, which is lined all around, and under the retina, with a membrane, over which is spread a mucous of a very black colour. The white wall or frame of white paper to receive the picture of objects, is a fair representa- tion of the retina of the eye, on which all the images of external objects are depicted. Such are some of the general points of resemblance be- tween the apparatus connected with the dark chamber, and the organ of vision ; but the human eye is an organ of such exquisite construction, and composed of such a number and variety of delicate parts, that it cannot be adequately represented by any artificial contrivance. The darkened chamber is frequently exhibited in a manner somewhat different from what we have above described, as in the following scheme, (fig. 38) which is termed the revolving camera obscura. In this construction, KH represents a plane mirror or metallic reflector, placed at half a right angle to the convex lens HI, by which, rays proceeding from objects situated in the direction O are re- flected to the lens, which forms an image of the objects on a round white table at T, around which several spectators may stand, and view the pic- ture, as delineated on a horizontal plane. The reflector, along with its case, is capable of being turned round, by means of a simple apparatus connected with it, so as to take in, in succession, all the objects which compose the surrounding scene. But as the image here is received on a flat surface, the rays fm, en, will have to diverge farther than the central rays dc ; and hence the K 194 THE PRACTICAL ASTRONOMER. figure 38. representation of the object, near the sides, will be somewhat distorted; to remedy which, the image should be received on a concave surface, as ah or PS. This is the general plan of those Camera Obscuras, fitted up in large wooden tents, which are frequently exhibited in our large cities, and removed occasionally from one town to another. Were an instrument of this kind fitted up on a small scale, a hole might be made in one of the sides, as at E, where the eye could be applied to view the picture. The focal distances of the lenses used in large instruments of this kind, are generally from eight to twelve feet, in which case they produce a telescopic effect upon distant objects, ON THE CAMERA OBSCURA. 195 so as to make them appear nearer than when viewed with the naked eye. The camera obscura is frequently constructed in a portable form, so as to be carried about for the purpose of delineating landscapes. The fol- lowing is a brief description of the instrument in figure 39. B this form. AC is a convex lens placed near the end of a tube or drawer, which is moveable in the side of a square box, within which is a plane mirror DE, reclining backward in an angle of forty-five degrees from the perpendicular pn. The pencils of rays flowing from the object OB, and passing through the convex lens — instead of proceeding forward and forming the image HI, are reflected upward by the mirror, and meet in points as FG, at the same distance at which they would have met at H and I, if they had not been intercepted by the mirror. At FG, the image of the object OB is received either on a piece of oiled paper, or more frequently on a plane un- polished glass, placed in the horizontal situation FG, which receives the images of all objects, opposite to the lens, and on which, or on an oiled paper placed upon it, their outlines may be traced by a pencil. The moveable tube on which the K 2 196 THE PRACTICAL ASTRONOMER. lens is fixed, serves to adjust the focus for near and distant objects, till their images appear dis- tinctly painted on the horizontal glass at FG. Above is shown the most common form of the box of this kind of Camera Obscura. A is the position of the lens, BC, the position of the mirror, D, the plane unpolished glass on which the images are depicted, GH a moveable top or screen to prevent the light from injuring the picture, and EF, the moveable tube. The Daguerreotgpe. — An important, and some- what surprising discovery has lately been made, in relation to the picture formed by the Camera Obscura. It is found, that the images formed by this instrument are capable of being indelibly fixed on certain surfaces previously prepared for the purpose, so that the picture is rendered perma- nent. When a Camera is presented to any object or landscape strongly illuminated by the sun, and the prepared ground for receiving the image is ad- justed, and a certain time allowed to elapse till the rays of light produce their due effect, in a few minutes or even seconds, a picture of the objects opposite to the lens is indelibly impressed upon THE DAGUERREOTYPE. 197 the prepared plate, in all the accurate proportions and perspective, which distinguish the images formed in a dark chamber — which representations may be hung up in apartments, along with other paintings and engravings ; and will likely retain their beauty and lustre for many years. These are pictures of nature's own workmanship finished in an extremely short space of time, and with the most exquisite delicacy and accuracy. The effect is evidently owing to certain chemical properties in the rays of light ; and opens a new field for experiment and investigation to the philosopher. The only defect in the picture is, that it is not coloured ; but, in the progress of experiments on this subject, it is not unlikely that even this ob- ject may be accomplished, in which case, we should be able to obtain the most accurate landscapes and representations of all objects, which can possibly be formed. This art or discovery goes by the name of the Daguerreotype from M. Daguerre, a Frenchman, who is supposed to have been the first discoverer, and who received a large premium from the French government for disclosing the process, and making the discovery public. Several improvements and modifications, in reference to the preparation of the plates, have been made since the discovery was first announced, about the be- ginning of 1839 ; and the pictures formed on this principle, are frequently distinguished by the name of Photogenic drawings ; and are now exhibited at most of our public scientific institutions. This new science or art, has been distinguished by different names. It was first called Photogra- phy, from two Greek words, signifying writing by light : it was afterwards called the art of Photogenic Drawing, or drawing produced by light. M. Daguerre gave it the name of Heliography, or 198 THE PRACTICAL ASTRONOMER. writing by the sun, all which appellatives are de- rived from the Greek, and are expressive, in some degree, of the nature of the process. We shall, however, make use of the term Daguerreotype, derived from the name of the inventor. As it does not fall within our plan to give any minute descriptions of the Daguerreotype process, we shall just give a few general hints in reference to it, referring those who wish for particular de- tails, to the separate treatises which have been published respecting it. The first thing necessary to be attended to in this art is, the preparation of the plate on which the drawing is to be made. The plate consists of a thin leaf of copper, plated with silver; both metals together, not being thicker than a card. The object of the copper is simply to support the silver, which must be the purest that can be procured. But though the copper should be no thicker than to serve the purpose of support, it is necessary that it should be so thick as to prevent the plate from being warped, which would produce a distortion of the images traced upon it. This plate must be polished ; — and for this purpose, the following articles are required — a phial of olive oil— some very fine cotton — pumice- powder, ground till it is almost impalpable, and tied up in a piece of fine muslin, thin enough to let the powder pass through without touching the plate when the bag is shaken. A little nitric acid diluted with sixteen times, by measure, its own quantity of water — a frame of wire on which to place the plate, when being heated — a spirit lamp to make the plate hot — a small box with inclined sides within, and having a lid to shut it up close — and a square board large enough to hold the drawing, and having catches at the side to keep it steady. THE DAGUERREOTYPE. 199 To the above prerequisites, a good Camera Obscura is, of course, essentially necessary. This instrument should be large enough to admit the plate of the largest drawing intended to be taken. The lens which forms the image of the object, should, if possible, be achromatic, and of a con- siderable diameter. In an excellent instrument of this description, now before me, the lens is an achromatic, about 3 inches diameter, but capable of being contracted to a smaller aperture. Its focal distance is about 17 inches ; and the box, exclusive of the tube which contains the lens, is 15 inches long, I3i inches broad, and 11 inches deep. It forms a beautiful and well-defined pic- ture of every well-enlightened object to which it is directed. Before the plate is placed in the camera, there are certain operations to be performed. 1. The surface of the plate should be made perfectly smooth, or highly polished. For this purpose, it must be laid flat, with the silver side upwards, upon several folds of paper for a bedding ; and having been well polished in the usual way, the surface must be powdered equally and carefully with fine pumice enclosed in the muslin bag. Then taking a little cotton wool, dipped in olive oil, it must be rubbed over the plate with rounding strokes, and then crossing them by others which commence at right angles with the first. This process must be repeated frequently, changing the cotton, and renewing the punice powder every time. A small portion of cotton must now be moistened with the diluted nitric acid, and applied equally to the whole surface. The next thing to be done is to make the plate thoroughly and equally hot, by holding the plate with a pair of pincers, by the corner, over a charcoal fire, and 200 THE PRACTICAL ASTRONOMER. when the plate is sufficiently hot, a white coating will be observed on the silver, which indicates that that part of the operation is finished. An even cold surface is next wanted, such as a metallic plate cooled almost to the freezing point by muriate of soda, and to this the heated plate must be suddenly transferred. 2. The next operation is to give the plate a coating of Iodine. This is accomplished by fix- ing the plate upon a board, and then putting it into a box containing a little dish with iodine divided into small pieces, with its face downward, and supported with small brackets at the corners. In this position, the plate must remain till it as- sume a full gold colour, through the condensation of the iodine on its surface — which process should be conducted in a darkened apartment. The re- quisite time for the condensation of the iodine varies from five minutes to half an hour. When this process is satisfactorily accomplished, the plate should be immediately fixed in a frame with catches and bands, and placed in the Camera ; and the transference from one receptacle to another should be made as quickly as possible, and with only so much light as will enable the operator to see what he is doing. 3. The next operation is to obtain the drawing. Having placed the Camera in front of the scene to be represented, and the lens being adjusted to the proper focus, the ground-glass of the Camera is withdrawn, and the prepared plate is substi- tuted for it ; and the whole is left till the natural images are drawn by the natural light from the object. The time necessary to leave the plate for a complete delineation of the objects, depends upon the intensity of the light. Objects in the shade will require more time for their delineation than THE DAGUERREOTYPE. 201 those in the broad light. The full clear light of the south of Europe, Spain, Italy, and particu- larly, the more glowing brilliancy of tropical coun- tries, will effect the object much more speedily than the duller luminosity of a northern clime. Some hours of the day are likewise more favour- able than others. Daguerre states, that ' the most favourable, is from 7 a.m. to 3 o'clock p.m., and that a drawing could be effected in Paris in 3 or 4 minutes, in June and July, which would require 5 or 6, in May and August, and 7 or 8 in April and September/ In the progress of this art, at the present time, portraits and other objects are frequently delineated in the course of a few seconds. 4. Immediately after removing the plate from the Camera, it is next placed over the vapour of mercury, which is placed in a cup at the bottom of a box, and a spirit lamp applied to its bottom, till the temperature rise to 140 of Fahrenheit. This process is intended to bring out the image, which is not visible when withdrawn from the Camera ; but in the course of a few minutes a faint tracery will begin to appear, and in a very short time the figure will be clearly developed. 5. The next operation is to fix the impression. In order to this, the coating on which the design was impressed must be removed, to preserve it from being decomposed by the rays of light. For this purpose, the plate is placed in a trough con- taining common water, plunging, and withdrawing it immediately, and then plunging it into a solu- tion of salt and water, till the yellow coating has disappeared. Such is a very brief sketch of the photogenic processes of Daguerre. Other substances, how- ever, more easily prepared, have been recom- k 5 202 THE PRACTICAL ASTRONOMER. mended by Mr. Talbot, F.R.S., who appears, about the same time, to have invented a process somewhat similar to that of Daguerre. The fol- lowing are his directions for the preparation of Photogenic Paper. The paper is to be dipped into a solution of salt in water, in the proportion of half an ounce of salt to half a pint of water. Let the super- fluous moisture drain off, and then, laying the paper upon a clean cloth, dab it gently with a napkin, so as to prevent the salt collecting in one spot more than another. The paper is then to be pinned down by two of its corners on a draw- ing board, by means of common pins, and one side washed or wetted with the Photogenic fluid, using the brush prepared for that purpose, and taking care to distribute it equally. Next dry the paper as rapidly as you can at the fire, and it will be fit for use for most purposes. If, when the paper is exposed to the sun's rays, it should assume an irregular tint, a very thin extra wash of the fluid will render the colour uniform, and at the same time somewhat darker. Should it be required to make a more sensitive description of paper, after the first application of the fluid, the solution of salt should be applied, and the paper dried at the fire. Apply a second wash of the fluid, and dry it at the fire again : employ the salt a third time, dry it, — and one application more of the fluid will, when dried, have made the paper extremely sensitive. When slips of such papers, differently prepared, are exposed to the action of day light, those which are soonest affected by the light, by becoming dark, are the best prepared. When photogenic drawings are finished in a perfect way, the designs then taken on the plate or paper are exceedingly beautiful and correct, PHOTOGENIC DRAWINGS, 203 and will bear to be inspected with a considerable magnifying power, so that the most minute por- tions of the objects delineated may be distinctly perceived. We have seen portraits, finished in this way by a London artist, with an accuracy which the best miniature painter could never attempt— every feature being so distinct, as to bear being viewed with a deep magnifier. And in landscapes and buildings, such is the delicacy and accuracy of such representations, that the marks of the chisel and the crevices in the stones may frequently be seen by applying a magnifying lens to the picture ; so that we may justly ex- claim, in the words of the Poet : ' Who can paint like nature ! That light — which is the first- born of Deity, which pervades all space, and illuminates all worlds — in the twinkling of an eye, and with an accuracy which no art can imitate, depicts every object in its exact form and propor- tions, superior to every thing that human genius can produce. The Photogenic art, in its progress, will doubt- less' be productive of many highly interesting and beneficial effects. It affords us the power of representing, by an accurate and rapid process, all the grand and beautiful objects connected with our globe — the landscapes peculiar to every country — the lofty ranges of mountains which distinguish Alpine regions — the noble edifices which art has reared — -the monumental remains of antiquity — and every other object which it would be interesting for human beings to contemplate ; so that in the course of time, the general scenery of our world, in its prominent parts, might be exhibited to almost every eye. The commission of the French Chambers, when referring to this art, has the following remark, f To copy the 204 THE PRACTICAL ASTRONOMER. millions upon millions of hieroglyphics which cover even the exterior of the great monuments of Thebes and Memphis, of Carnac, &c, would require scores of years and legions of designers. By the assistance of the Daguerreotype, a single man could finish that immense work.' — This instrument lays down objects, which the visual organs of man would overlook, or might be unable to perceive, with the same minuteness and nicety, that it delineates the most prominent features of a landscape. The time-stained excrescences on a tree, the blades of grass, the leaf of a rose, the neglected weed, the moss on the summit of a lofty tower, and similar objects, are traced with the same accuracy as the larger objects in the sur- rounding scene. It is not improbable, likewise, that this art (still in its infancy) when it approximates to perfection, may enable us to take representations of the sub- lime objects in the heavens. The sun affords sufficient light for this purpose ; and there appears no insurmountable obstacle in taking, in this way, a highly magnified picture of that luminary, which shall be capable of being again magnified by a powerful microscope. It is by no means improbable, from experiments that have hitherto been made, that we may obtain an accurate delineation of the lunar world from the moon her- self. The plated disks prepared by Daguerre receive impressions from the action of the lunar rays to such an extent as permits the hope that photographic charts of the moon may soon be obtained ; and, if so, they will excel in accuracy all the delineations of this orb that have hitherto been obtained ; and if they should bear a microscopic power, objects may be perceived on the lunar surface which have hitherto been invisible. Nor PHOTOGENIC DRAWINGS. 205 is it impossible that the planets Venus, Mars, Jupiter and Saturn, may be delineated in this way, and objects discovered which cannot be descried by means of the telescope. It might perhaps be considered as beyond the bounds of probability to expect that even distant Nebulce, might thus be fixed, and a delineation of their objects produced which shall be capable of being magnified by microscopes. But we ought to con- sider that the art is yet only in its infancy — that plates of a more delicate nature than those hitherto used, may yet be prepared, and that other properties of light may yet be discovered, which shall facilitate such designs. For, we ought now to set no boundaries to the discoveries of science, and to the practical applications of scientific discovery which genius and art may accomplish. In short, this invention leads to the conclusion, that we have not yet discovered all the wonderful properties of that Luminous Agent which per- vades the universe, and which unveils to us its beauties and sublimities — and that thousands of admirable objects and agencies may yet be dis* closed to our view through the medium of light, as philosophical investigators advance in their researches and discoveries. In the present instance, as well as in many others, it evidently appears, that the Creator intends, in the course of his providence, by means of scientific researches, gradually to open to the view of the inhabitants of our world the w r onders, the beauties and the sub- limities of his vast creation, to manifest his infinite wisdom, and his superabundant goodness, and to raise our souls to the contemplation and the love of Him who is the original source of all that is glorious and beneficent in the scene of nature. 206 THE PRACTICAL ASTRONOMER. CHAPTER III. ON THE OPTICAL ANGLE, AND THE APPARENT MAGNITUDE OF OBJECTS. In order to understand the principle on which telescopes represent distant objects as magnified, it may be expedient to explain what is meant by the angle of vision, and the apparent magnitudes under which different objects appear, and the same object, when placed at different distances. The optical angle is the angle contained under two right lines drawn from the extreme points of figure 40. an object to the eye. Thus AEB or CED (fig. 40.) is the optical or visual angle, or the angle under which the object AB or CD, appears to the eye at E. These two objects, being at different distances, are seen under the same angle, although CD is evidently larger than AB. On the retina THE OPTICAL ANGLE, 207 of the eye, their images are exactly of the same size, and so is the still larger object FG. The apparent magnitude of objects denotes their magnitude as they appear to us, in contra- distinction from their real or true magnitude, and it is measured by the visual angle ; for whatever objects are seen under the same or equal angles appear equal, however different their real magni- tudes. If a half-crown or half-dollar be placed at about 120 yards from the eye, it is just percep- tible as a visible point, and its apparent magnitude, or the angle under which it is seen, is very small. At the distance of thirty or forty yards, its bulk appears sensibly increased, and we perceive it to be a round body ; at the distance of six or eight yards, we can see the king or queen's head engraved upon it ; and at the distance of eight or ten inches from the eye it will appear so large, that it will seem to cover a large building placed within the distance of a quarter of a mile, in other words, the apparent magnitude of the half-crown held at such a distance, will more than equal that of such a building, in the picture on the retina, owing to the increase of the optical angle. If we suppose A (fig. 41.) to represent the apparent size of the half-crown at nine yards distance, then we say it figure 41. I 208 THE PRACTICAL ASTRONOMER. is seen under the small angle FED. B will re- present its apparent magnitude at 4^ yards distant under the angle HEG, and the circle C, its appa- rent magnitude at 3 yards distant, under the large angle KEI. This may be otherwise illustrated by the fol- lowing figure. Let AB (fig. 42.) be an object figure 42. viewed directly by the eye QR. From each ex- tremity A and B draw the lines AN,BM, intersect- ing each other in the crystalline humour in I : then is AIB the optical angle which is the measure of the apparent magnitude or length of the object AB. From an inspection of this figure, it will evidently appear that the apparent magnitudes of objects will vary according to their distances. Thus AB, CD, EF, the real magnitudes of which are unequal, may be situated at such distances from the eye, as to have their apparent magni- tudes all equal, and occupying the same space on the retina MN, as here represented. In like manner, objects of equal magnitude, placed at unequal distances, will appear unequal. The THE OPTICAL ANGLE. 209 objects AB and GH which are equal, being situated at different distances from the eye, GH will appear under the large angle TIV, or as large as an object TV, situated at the same place as the object AB, while AB appears under the smaller angle AIB. Therefore the object GH is appa- rently greater than the object AB, though it is only equal to it. Hence it appears that we have no certain standard of the true magnitude of objects, by our visual perception abstractly con- sidered, but only of the proportions of magnitude. In reference to apparent magnitudes, we scarcely ever judge any object to be so great or so small as it appears to be, or that there is so great a dispa- rity in the visible magnitude of two equal bodies at different distances from the eye. Thus, for example, suppose two men, each six feet 3 inches high, to stand directly before us, one at the dis- tance of a pole, or 5^ yards, and the other at the distance of 100 poles, or 550 yards — we should observe a considerable difference in their apparent size, but we should scarcely suppose, at first sight, that the one nearest the eye appeared a hundred times greater than the other, or that, while the nearest one appeared 6 feet 3 inches high, the remote one appeared only about three fourths of an inch. Yet such is in reality the case ; and not only so, but the visible bulk or area of the one is to that of the other, as the square of these num- bers, namely as 10,000 to 1 ; the man nearest us presenting to the eye a magnitude or surface ten thousand times greater than that of the other. Again, suppose two chairs standing in a large room, the one 21 feet distance from us, and the other 3 feet — the one nearest us will appear 7 times larger both in length and breadth, than the more distant one, and consequently, its visible area 210 THE PRACTICAL ASTRONOMER. 49 times greater. If I hold up my finger at 9 inches distant from my eye, it seems to cover a large town a mile and a half in extent, situated at 3 miles distant ; consequently, the apparent mag- nitude of my finger, at 9 inches distant from the organ of vision, is greater than that of the large town at 3 miles distance, and forms a larger pic- ture on the retina of the eye. When I stand at the distance of a foot from my window, and look through one of the panes to a village less than a quarter of a mile distant, I see, through that pane, nearly the whole extent of the village, com- prehending two or three hundred houses ; conse- quently, the apparent magnitude of the pane is equal to nearly the extent of the village, and all the buildings it contains do not appear larger than the pane of glass in the window, otherwise, the houses and other objects which compose the village could not be seen through that single pane. For, if we suppose a line drawn from one end of the village, passing through the one side of the pane, and another line drawn from the other end, and passing through the other side of the pane to the eye, these lines would form the optical angle under which the pane of glass and the village appears. If the pane of glass be fourteen inches broad, and the length of the village 2640 yards, or half a mile — this last lineal extent is 6,788 times greater than the other, and yet they have the same apparent magnitude in the case supposed. Hence we may learn the absurdity and futility of attempting to describe the extent of spaces in the heavens, by saying, that a certain phenomenon was two or three feet or yards distant from an- other, or that the tail of a comet appeared several yards in length. Such representations can convey THE OPTICAL ANGLE. 211 no definite ideas in relation to such magnitudes, unless it be specified at what distance from the eye, the foot or yard is supposed to be placed. If a rod, a yard in length, be held at nine inches from the eye, it will subtend an angle, or cover a space in the heavens, equal to more than one fourth of the circumference of the sky, or about one hundred degrees. If it be eighteen inches from the eye, it will cover a space equal to fifty degrees ; if at three feet, twenty-five degrees, and so on in proportion to the distance from the eye ; so that we can form no correct conceptions of apparent spaces or distances in the heavens, when we are merely told that two stars, for example, appear to be three yards distant from each other. The only definite measure we can use, in such cases, is that of degrees. The sun and moon are about half a degree in apparent diameter, and the distance between the extreme stars in Orion s belt, three degrees, which measures being made familiar to the eye, may be applied to other spaces of the heavens, and an approximate idea conveyed of the relative distances of objects in the sky. From what has been stated above, it is evident that the magnitude of objects may be considered in different points of view. The true dimensions of an object, considered in itself, give what is called its real or absolute magnitude ; and the opening of the visual angle determines the apparent magni- tude. The real magnitude, therefore, is a constant quantity ; but the apparent magnitude varies con- tinually with the distance, real or imaginary ; and therefore, if we always judged of the dimensions of an object from its apparent magnitude, every thing around us would, in this respect, be under- going very sensible variations, which might lead us into strange and serious mistakes. A fly, near 212 THE PRACTICAL ASTRONOMER. enough to the eye, might appear under an angle as great as an elephant at the distance of twenty feet, and the one be mistaken for the other. A giant eight feet high, seen at the distance of twenty- four feet, would not appear taller than a child two feet in height, at the distance of six feet ; for both would be seen nearly under the same angle. But our experience generally prevents us from being deceived by such illusions. By the help of touch, and by making allowance for the different distances at which we see particular objects, we learn to correct the ideas we might otherwise form from attending to the optical angle alone, espe- cially in the case of objects that are near us. By the sense of touch we acquire an impression of the distance of an object; this impression combines itself with that of the apparent magnitude, so that the impression which represents to us the real magnitude is the product of these two elements. "When the objects, however, are at a great distance, it is more difficult to form a correct estimate of their true magnitudes. The visual angles are so small, that they prevent comparison ; and the estimated bulks of the objects depend in a great measure upon the apparent magnitudes ; and thus an object situated at a great distance, appears to us much smaller than it is in reality. We also estimate objects to be nearer or farther distant according as they are more or less clear, and our perception of them more or less distinct and well defined ; and likewise, when several objects inter- vene between us and the object we are particularly observing. We make a sort of addition of all the estimated distances of intermediate objects, in order to form a total distance of the remote object, which in this case appears to be farther off than if the intervening space were unoccupied. It is THE OPTICAL ANGLE. 213 generally estimated that no terrestrial object can be distinctly perceived, if the visual angle it sub- tends be less than one minute of a degree; and that most objects become indistinct, when the angle they subtend at the pupil of the eye is less than six minutes. We have deemed it expedient to introduce the above remarks on the apparent magnitude of ob- jects, because the principal use of a telescope is to increase the angle of vision, or to represent objects under a larger angle than that under which they appear to the naked eye, so as to ren- der the view of distant objects more distinct, and to exhibit to the organ of vision those objects which would otherwise be invisible. A telescope may be said to enlarge an object just as many times as the angle under which the instrument represents it, is greater than that under which it appears to the unassisted eye. Thus the moon appears to the naked eye under an angle of about half a degree ; consequently a telescope magnifies 60 times if it represents that orb under an angle of 30 degrees ; and if it magnified 180 times, it would exhibit the moon under an angle of 90 degrees, which would make her appear to fill half of the visible heavens, or the space which inter- venes from the horizon to the zenith. 214 THE PRACTICAL ASTRONOMER. CHAPTER IV. ON THE DIFFERENT KINDS OF REFRACTING TELESCOPES. There are two kinds of telescopes, corresponding to two modes of vision, namely, those which per- form their office by refraction through lenses, and those which magnify distant objects by reflection from mirrors. The telescope which is constructed with lenses, produces its effects solely by refracted light, and is called a Dioptric, or refracting teles- cope. The other kind of telescope produces its effects partly by reflection, and partly by refrac- tion, and is composed both of mirrors and lenses ; but the mirrors form the principal part of the telescope ; and therefore such instruments are denominated reflecting telescopes. In this chapter I shall describe the various kinds of refracting telescopes. SECT 1. THE GALILEAN TELESCOPE. This telescope is named after the celebrated Galileo, who first constructed, and probably invented it in the year 1609. It consists of only two glasses, a convex glass next the object, and a THE GALILEAN TELESCOPE. 215 concave next the eye. The convex is called the object-glass, and the concave to which the eye is applied, is called the eye-glass. Let C (fig. 43.) figure 43 represent the convex object-glass, presented to any object in the direction DEI, so that the rays fall parallel upon it ; — if these rays, after passing through it, were not intercepted by the concave lens K, they would pass on, and cross each other in the focus F, where an inverted image of the object would be formed. But the concave lens K, the virtual focus of which is at F, being inter- posed, the rays are not suffered to converge to that point, but are made less convergent,^ and enter the pupil almost parallel, as GH, and are converged by the humours of the eye to their proper foci on the retina. The object, through this telescope, is seen upright, or in its natural position, because the rays are not suffered to come to a focus, so as to form an inverted picture. The concave eye-glass is placed as far within the focus of the object-glass, as is equal to its own virtual focus ; and the magnifying power is as the focal * It is one of the properties of concave lenses to render convergent rays less convergent, and when placed as here supposed, to render them parallel ; and it is parallel rays that produce distinct vision. 216 THE PRACTICAL ASTRONOMER. length of the object-glass to that of the eye-glass, that is, as CF to BF. Thus, suppose the focus of the object-glass to be 10 inches, and the focus of the eye-glass to be 1 inch, the magnifying power will be 10 times — which is always found by dividing the focal length of the object-glass by that of the eye-glass. The interval between the two glasses, in this case, will be 9 inches, which is the length of the telescope, and the objects seen through it will appear under an angle nine times greater than the}' do to the naked eye. These propositions might be proved mathematically ; but the process is somewhat tedious and intricate, and might not fully be understood by general readers. I shall therefore only mention some of the general properties of this telescope, which is now seldom used, except for the purpose of opera-glasses. 1. The focal distance of the object-glass must be greater than that of the eye-glass, otherwise it would not magnify an object : if the focal distance of the eye-glass were greater than that of the object-glass, it would diminish objects, instead of magnifying them. 2. The visible area of the object is greater, the nearer the eye is to the glass ; and it depends on the diameter of the pupil of the eye, and on the breadth of the object-glass ; consequently the field of view in this telescope is very small. 3. The distinctness of vision in this construction of a telescope exceeds that of almost any other. This arises from the rays of light pro- ceeding from the object directly through the lenses, without crossing or intersecting each other ; whereas in the combination of convex lenses, they intersect one another to form an image in the focus of the object-glass, and this image is magni- fied by the eye-glass with all its imperfections and distortions. The thinness of the centre of the THE GALILEAN TELESCOPE. 217 concave lens also contributes to distinctness. 4. Although the field of view in this telescope is very small, yet where no other telescope can be pro- cured, it might be made of such a length as to show the spots on the Sun, the crescent of Venus, the satellites of Jupiter, and the ring of Saturn ; and, requiring only two glasses, it is the cheapest of all telescopes. It has been found that an ob- ject-lens 5 feet focal distance, will bear a concave eye-glass of only 1 inch focal distance, and will consequently magnify the diameters of the planets 60 times, and their surfaces 3600 times, which is sufficient to show the phenomena now stated. And, although only a small portion of the sun and moon can be seen at once, yet Jupiter and all his satellites may sometimes be seen at one view ; but there is some difficulty in finding objects with such telescopes. 5. Opera-glasses, which are always of this construction, have the object-lens generally about 6 inches focus and 1 inch diame- ter, with a concave eye-glass of about 2 inches focus. These glasses magnify about 3 times in diameter, have a pretty large field, and produce very distinct vision. When adjusted to the eye, they are about 4 inches in length. To the object end of an opera-glass there is sometimes attached a plane mirror, placed at an angle of 45 degrees, for the purpose of viewing objects on either side of us. By this means, in a theatre or assembly, we can take a view of any person without his having the least suspicion of it, as the glass is directed in quite a different direction. The instrument with this appendage is sometimes called a Polemoscope. L 218 THE PRACTICAL ASTRONOMER. SECT. 2. — THE COMMON ASTRONOMICAL REFRACTING TELESCOPE. The astronomical telescope is the most simple construction of a telescope, composed of convex lenses only, of which there are but two essentially necessary, though a third is sometimes added to the eye-piece for the purpose of enlarging the field of view. Its construction will be easily understood from a description of the following figure. Its two essential parts are, an object- glass AD, and an eye-glass EY, so combined in a tube that the focus F of the object-glass is exactly coincident with the focus of the eye- glass. Let OB (fig. 44.) represent a distant ob- ject, from which rays nearly parallel proceed to figure 44. the object-lens AD. The rays passing through this lens will cross at F, and form an image of the object at 1M. This image forms as it were an object to the eye-glass EY, which is of a short focal distance, and the eye is thus enabled to con- template the object as if it were brought much nearer than it is in reality. For the rays, which after crossing proceed in a divergent state, fall upon the lens EY, as if they proceeded from a real object situated at F. All that is effected there- fore, by such a telescope is, to form an image of REFRACTING TELESCOPE. 219 a distant object by means of the object-lens, and then to give the eye such assistance as is necessary for viewing that image as near as possible, so that the angle it shall subtend at the eye shall be very large compared with the angle which the object itself would subtend in the same situation. Here it may be expedient to explain, 1. how this arrangement of glasses shows distant objects distinctly, and 2. the reason why objects appear magnified when seen through it. As to the first particular, it may be proved as follows : — The rays OA and BD, which are parallel before they fall upon the object-glass, are by this glass re- fracted and united at its focus: In order, then, to distinct vision, the eye-glass must re-establish the parallelism of the rays, — which is effected by placing the eye-glass so that its focus may be at F, and consequently the rays will proceed from it parallel to each other and fall upon the eye in that direction. For distinct vision is produced by parallel rays. 2. The reason why the object appears magnified will appear, if we consider that, if the eye viewed the object from the centre of the object-glass, it would see it under the angle OCB ; let OC and BC then be produced to the focus of the glass, they will then limit the image IM formed in the focus. If then, two parallel rays are supposed to proceed to the eye-glass EY, they will be converged to its focus H, and the eye will see the image under the angle EHY. The apparent magnitude of the object, therefore, as seen by the naked eye, is to the magnitude of the image as seen through the telescope, as OCB to EHY, or as the distance CF to the distance FG, in other words, as the focal length of the object- glass to that of the eye-glass. It is obvious from the figure, that, through this L 2 220 THE PRACTICAL ASTRONOMER. telescope, all objects will appear inverted ; since the object OB is depicted by the object-glass in an inverted position at IM, and in this position is viewed by the eye-glass EY ; and, therefore this kind of telescope is not well adapted for view- ing terrestrial objects, since it exhibits the tops of trees, houses, and other objects as undermost, and the heads of people as pointing downwards. But this circumstance is of no consequence with respect to the heavenly bodies, since they are round, and it can make little difference to an ob- server which side of a globular body appears uppermost or undermost. All astronomical refracting telescopes invert objects ; but they are preferred to any other telescopes, because they have few glasses, and consequently more light. This telescope however, can be transformed into a common day telescope for land objects, by the addition of two other eye-glasses, as we shall afterwards explain ; but in this case a quantity of light is lost by refraction at each lens ; for there is scarcely any transparent substance that trans- mits all the rays of light that fall upon it. The magnifying power of this telescope is found by dividing the focal distance of the object- glass by the focal distance of the eye- glass : the quotient gives the magnifying power, or the num- ber of times that the object seen through the telescope, appears larger or nearer than to the naked eye. Thus, for example, if the focal dis- tance of the object-glass be 28 inches, and the focal distance of the eye-glass 1 inch, the magnify- ing power will be 28 times. If we would enlarge the telescope and select an object-glass 10 feet, or 120 inches focus, an eye-glass of 2 inches focal length might be applied, and then the diameter of objects would be magnified 60 times, and their REFRACTING TELESCOPE. 221 surfaces 3600 times. If we would use an object- glass of 100 feet, it would be necessary to select an eye-glass about 6 inches focus, and the magni- fying power would be 200 times, equal to 1200 inches divided by 6. Since, then, the power of magnifying depends on the proportion of the focal length of the object and eye-glasses, and this pro- portion may be varied to any degree, it may seem strange to some that a short telescope of this kind will not answer that purpose as well as a long one. For instance, it may be asked why an object-glass of 10 feet focus, may not be made to magnify as much, as one of 100 feet focal length, by using an eye-glass of half an inch focus, in which case, the magnifying power would be 240 times ? But it is to be considered, that if the power of magnifying be increased, while the length of the telescope remains the same, it is necessary to diminish the focal length of the eye-glass in the same proportion, and this cannot be done on account of the great distortion and colouring which would then appear in the image, arising both from the deep convexity of the lens and the different refrangibility of the rays of light. It is found that the length of com- mon refracting telescopes must be increased in proportion to the square of the increase of their magnifying power ; so that in order to magnify twice as much as before, with the same light and distinctness, the telescope must be lengthened four times ; to magnify 3 times as much, 9 times ; and to magnify four times as much, sixteen times ; that is — suppose a telescope of 3 feet to magnify 33 times, — in order to procure a power four times as great, or 132 times, we must extend the teles- cope to the length of 48 feet, or 16 times the length of the other. Much likewise depends upon the breadth or aperture of the object-glass. If it 222 THE PRACTICAL ASTRONOMER. be too small, there will not be sufficient light to illuminate the object ; and if it be too large, the redundance of light will produce confusion in the image. The following table, constructed originally by Huygens, and which I have re-calculated and cor- rected, shows the linear aperture, the focal dis- tance of the eye-glass, and the magnifying power of astronomical telescopes of different lengths, which may serve as a guide to those who wish to construct telescopes of this description. Focal distance of the object- glass. Linear aperture of the object- glass. Focal distance of the eye-glass. Magnifying power. Feet. Inch. Dec. Inch. Dec. 1 0. 545 0. 605 20 2 0. 76 0. 84 28.5 3 0. 94 1. 04 34.6 4 08 1. 18 40 5 21 1. 33 45 6 32 1. 45 50 7 43 1. 58 53 8 53 1. 69 56.8 9 62 1. 78 60.6 10 71 1. 88 63.8 15 2*. 10 2. 30 78 20 2. 43 2. 68 89.5 30 3. 00 3. 28 109 40 3. 43 3. 76 127 50 3. 84 4. 20 142 60 4. 20 4. 60 156 70 4. 55 5. 00 168 80 4. 83 5. 35 179 90 5. 15 5. 65 190 100 5. 40 5. 95 200 120 5. 90 6. 52 220 In the above table, the first column expresses the focal length of the object-glass in feet ; the second column, the diameter of the aperture* of the * The word aperture as applied to object-glasses, signifies the open- ing to let in the light, or that part of the object-glass which is left uncovered. An object-glass may be 3 inches in diameter, but if one inch of this diameter be covered, its aperture is said to be only 2 inches. REFRACTING TELESCOPE* 223 object-glass, the third column, the focal distance of the eye-glass, and the fourth, the magnifying power, which is found by reducing the feet in the first column to inches, and dividing by the num- bers in the third column. From this table it appears that, in order to obtain a magnifying power of 168 times, by this kind of telescope, it is requisite to have an objecfglass of 70 feet focal distance, and an eye-glass five inches focus, and that the aperture of the object-glass ought not to be more than about 4^- inches diameter. To obtain a a power of 220 times requires a length of 120 feet. The following is a summary view of the pro- perties of this telescope. 1. The object is always inverted. 2. The magnifying power is always in the proportion of the focal distance of the object- glass to the eye-glass. 3. As the rays emerging from the eye-glass, should be rendered parallel for every eye, there is a small sliding tube next the eye, which should be pushed out or in till the object appears distinct, When objects are pretty near, this tube requires to be pulled out a little. These circumstances require to be attended to in all telescopes. 4. The apparent magnitude of an object is the same wherever the eye be placed, but the visible area, or field of view, is the great- est when the eye is nearly at the focal distance of the eye-glass. 5. The visual angle depends on the breadth of the eye-glass ; for it is equal to the angle which the eye-glass subtends at the object-glass; but the breadth of the eye-glass cannot be increased beyond a certain limit, with- out producing colouring and distortion. If the general principles on which this teles- cope is constructed be thoroughly understood, it will be quite easy for the reader to understand the 224 THE PRACTICAL ASTRONOMER, construction of all the other kinds of telescopes, whether refracting or reflecting. A small astro- nomical telescope can be constructed in a few moments, provided one has at hand the following lenses: — 1. A common reading-glass, eight or ten inches focal distance ; 2. A common magnifying lens, such as watchmakers or botanists use, of about li or 2 inches focus. Hold the reading- glass — suppose of ten inches focus — in the left hand opposite any object, and the magnifying lens of two inches focus, in the right hand near the eye, at twelve inches distance from the other in a direct line, and a telescope is formed which mag- nifies five times. I have frequently used this plan, when travelling, when no other telescope was at hand. SECT. 3. — THE AERIAL TELESCOPE. The Aerial is a refracting telescope of the kind we have now described, intended to be used without a tube in a dark night ; for the use of a tube is not only to direct the glasses, but to make the place dark where the images are formed. It appears from the preceding table inserted above, that we cannot obtain a high magnifying power, with the common astronomical telescope, without making it of an extreme length, in which case the glasses are not manageable in tubes — which are either too slight and apt to bend, or too heavy and unwieldy if made of wood, iron or other strong materials. The astronomers of the seventeenth century, feeling such inconveniences in making celestial observations with long tubes, contrived a method of using the glasses without tubes. Hart- socker, an eminent optician, contrived to fix them THE AERIAL TELESCOPE. 225 at the top of a tree, a high wall, or the roof of a house ; but the celebrated Huygens, who was not only an astronomer, but also an excellent mecha- nic, made considerable improvements in the method of using an object-glass without a tube. He placed it at the top of a very long pole, having previously enclosed it in a short tube, which was made to turn in all directions by means of a ball and socket. The axis of this tube he could com- mand with a fine silken string, so as to bring it into a line with the axis of another short tube which he held in his hand, and which contained the eye-glass. The following is a more particular description of one of these telescopes. On the top of a long pole or mast ab (fig. 45), is fixed a board moveable up and down in the channel cd : e is a perpendicular arm fixed to it, and ff is a transverse board that supports the object glass enclosed in the tube i, which is raised or lowered by means of the silk cord rl ; gg is an endless rope with a weight A, by which the apparatus of the object-glass is counterpoised ; hi is a stick fastened to the tube i ; m the ball and socket, by means of which the object-glass is moveable every way : and to keep it steady, there is a weight n suspended by a wire ; I is a short wire to which the thread rl is tied ; o is the tube which holds the eye-glass ; q the stick fixed to this tube, s a leaden bullet, and t a spool to wind the thread on ; u is pins for the thread to pass through ; x the rest for the observer to lean upon, and y the lan- tern. Fig. 46 is an apparatus contrived by M. de la Hire for managing the object-glass; but which it would be too tedious particularly to describe. To keep off the dew from the object-glass, it was sometimes included in a pasteboard tube, made of spongy paper, to absorb the humidity of the air. L 5 226 THE PRACTICAL ASTRONOMER. figure 45. And to find an object more readily, a broad annu- lus of white pasteboard was put over the tube that carried the eye-glass ; upon which the image of the object being painted, an assistant who perceived it, might direct the tube of the eye-glass into its place. Such was the construction of the telescopes w T ith which Hevelius, Huygens, Cassini, and other eminent astronomers of the seventeenth century made their principal discoveries. With such telescopes, Huygens discovered the fourth satellite THE AERIAL TELESCOPE. 227 of Saturn, and determined that this planet was surrounded with a ring ; and with the same kind of instrument Cassini detected the first, second, third, and fifth, satellites of Saturn, and made his other discoveries. When the night was very dark, they were obliged to make the object-glass visible, by means of a lantern so constructed as to throw th erays of light up to it in a parallel direction. In making such observations, they must have taken incredible pains, endured much cold and fatigue, and subjected themselves to very great labour and expense — which almost makes us wonder at the discoveries they were instrumental in bringing to light — and should make modern philosophers sensible of the obligations they are under to such men as Newton and Dollond, through whose inventions such unwieldy instruments are no longer necessary. Telescopes of the descrip- tion now stated were made of all sizes, from 30 to above 120 feet in length. Divini at Rome, and Campani at Bologna, were famed as makers of the object-glasses of the long focal distance to which we have alluded, who sold them for a great price, and took every method to keep the art of making them a secret. It was with telescopes made by Campani, that Cassini made his discove- ries. They were made by the express order of Louis XIV, and were of 86, 100, and 136 Paris feet in focal length. M. Auzoutmade one object- glass of 600 feet focus ; but he was never able to manage it, so as to make any practical observa- tions with it. Hartsocker is said to have made some of a still greater focal length. The famous aerial telescope of Huygens was 123 feet in focal length, with six inches of aperture. At his death, he bequeathed it to the Royal Society of London, in whose possession it still remains. It required 228 THE PRACTICAL ASTRONOMER. a pole of more than a hundred feet high, on which to place the object-glass for general observations. It was with this glass, that Dr. Derham made the observations to which he alludes in his preface to his ' Astro-Theology.' When this glass was in the possession of Mr. Cavendish, it was compared with one of Mr. Dollond's forty-six inch treble object-glass Achromatics, and the gentlemen who were present at the trial, said that 1 the Dwarf was fairly a match for the Giant.' It magnified 218 times, and the trouble of managing it, was said to be extremely tiresome and laborious. SECT. 4. THE COMMON REFRACTING TELESCOPE FOR TERRESTRIAL OBJECTS. This telescope is constructed on the same prin- ciple as the astronomical telescope already de- scribed, with the addition of two or three glasses. In fig. 47, OB represents a distant object, LN, figure 47. the object glass, which forms the image IM in its focus, which is, of course, in an inverted position, and, if the eye were applied at the lens EE, the object would appear, exactly as through the astro- nomical telescope, every object being apparently turned upside down. To remedy this incon- venience, there are added two other glasses FF and GG, by which a second image is formed from the first, in the same position as the object. In THE COMMON REFRACTING TELESCOPE. 229 order to effect this, the first of these two glasses, namely FF, is placed at twice its focal distance from the former glass EE, and the other lens GG, next the eye, is placed at the same distance from FF. For all the three glasses are supposed to be of the same focal distance. Now, the lens FF, being placed at twice the focal distance for parallel rays from EE, receives the pencils of parallel rays after they have crossed each other at X, and forms an image at i m similar to that at IM and equal to it, but contrary in position, and con- sequently erect; which last image is viewed by the lens GG, in the same manner as the first image IM would be viewed by the lens EE. In this case, the image IM is considered as an ob- ject to the lens FF of which it forms a picture in its focus, in a reverse position from that of the first image, and of course, in the same position as the object. The magnifying power of this telescope is deter- mined precisely in the same way as that of the astronomical telescope. Suppose the object-glass to be thirty inches focal distance, and each of the eye-glasses 1^ inch focal distance, the magnifying power is in the proportion of 30 to li, or 20 times, and the instrument is, of course, considerably longer than an astronomical telescope of the same power. The distance, in this case, between the object-glass and the first eye-glass EE is 31^ inches ; the distance between EE, and the second glass FF, is 3 inches, and the distance between FF and the glass GG next the eye, 3 inches ; in all 37^ inches, the whole length of the telescope. Although it is usual to make use of three eye- glasses in this telescope, yet two will cause the object to appear erect, and of the same magnitude. For suppose the middle lens FF taken away, if 230 THE PRACTICAL ASTRONOMER. the first lens EE be placed at X, which is double its focal distance from the image 1M, it will at the same distance X m, on the other side, form a secondary image i m equal to the primary image IM, and also in a contrary position. But such a combination of eye-glasses produces a great degree of colouring in the image, and therefore is seldom used. Even the combination now described, con- sisting of three lenses of equal focal distances, is now almost obsolete, and has given place to a much better arrangement consisting of four glasses, of different focal distances — which shall be afterwards described. The following figures, 48, 49, 50 represent the manner in which the rays of light are refracted through the glasses of the telescopes we have now described. Fig. 48 represents the rays of light as they pass from the object to the eye in the Gali- lean telescope. After passing in a parallel direc- tion to the object-glass, they are refracted by that glass, and undergo a slight convergence in passing towards the concave eye-glass, where they enter the eye in a parallel direction, but no image is formed previous to their entering the eye, till they arrive at the retina. Fig. 49 represents the rays as they pass through the glasses of the astronomi- cal telescope. The rays, after entering the object- glass, proceed in a converging direction, till they arrive at its focus, about A, where an image of the object is formed ; they then proceed diverging to the eye-glass, where they are rendered parallel, and enter the eye in that direction. Fig. 50 re- presents the rays as they converge and diverge in passing through the four glasses of the common day-telescope described above. After passing through the object-glass, they converge towards B, where the first image is formed. They then THE COMMON REFRACTING TELESCOPE. 231 fig. 48. fig. 49. fig. 50. diverge towards the first eye-glass where they are rendered parallel ; and passing through the second eye-glass, they again converge and form a second image at C ; from which point they again diverge, and passing through the first eye-glass enter the eye in a parallel direction. If the glasses of these telescopes were fixed on long pieces of wood, at their proper distances from each other, and placed in a darkened room, when the sun is shining, the beam of the sun's light would pass through them in the same manner as here represented. 232 THE PRACTICAL ASTRONOMER. SECT. 5. — TELESCOPE FORMED BY A SINGLE LENS. This is a species of telescope altogether un- noticed by optical writers, so far as I know ; nor has the property of a single lens in magnifying distant objects been generally adverted to or re- cognised. It may not therefore be inexpedient to state a few experiments which I have made in relation to this point. When we hold a spectacle- glass of a pretty long focal distance — say, from 20 to 24 inches — close to the eye, and direct it to distant objects, they do not appear sensibly mag- nified. But if we hold the glass about 12 or 16 inches from our eye, we shall perceive a sensible degree of magnifying power, as if distant objects were seen at less than half the distance at which they are placed. This property of a spectacle- glass I happened to notice when a boy, and, on different occasions since that period have made several experiments on the subject, some of which I shall here relate. With the object-glass of a common refracting telescope 4^ feet focal distance, and 2\ inches diameter, I looked at distant objects — my eye being at about 3^ feet from the lens, or about 10 or 12 inches within its focus — and it produced nearly the same effect as a telescope which mag- nifies the diameters of objects 5 or 6 times. With another lens 11 feet focal distance and 4 inches dia- meter — standing from it at the distance of about 10 feet, I obtain a magnifying power of about 12 or 14 times, which enables me to read the letters on the sign-posts of a village half a mile distant. Having some time ago procured a very large lens 26 feet focal distance, and \\\ inches TELESCOPE FORMED BY A SINGLE LENS. 233 diameter, I have tried with it various experiments of this kind upon different objects. Standing at the distance of about 25 feet from it, I can see distant objects through it magnified about 26 times in diameter, and consequently 676 times in surface, and remarkably clear and distinct, so that I can distinguish the hour and minute hands of a public clock in a village two miles distant. This single lens, therefore answers the purpose of an ordinary telescope with a power of 26 times. In making such experiments our eye must always be within the focus of the lens, at least 8 or 10 inches. The object will, indeed, be seen at any distance from the glass within this limit ; but the magnify- ing power is diminished in proportion as we ap- proach nearer to the glass. Different eyes, too, will require to place themselves at different dis- tances, so as to obtain the greatest degree of mag- nifying power with distinctness, according as indi- viduals are long or short-sighted. This kind of telescope stands in no need of a tube, but only of a small pedestal on which it may be placed on a table, nearly at the height of the eye, and that it be capable of a motion in a per- pendicular or parallel direction, to bring it in a line with the eye and the object. The principle on which the magnifying power, in this case, is produced, is materially the same as that on which the performance of the Galilean telescope depends. The eye of the observer serves instead of the con- cave lens in that instrument ; and as the concave lens is placed as much within the focus of the object-glass, as is equal to its own focal distance, so the eye, in these experiments, must be placed at least its focal distance within the focus of the lens with which we are experimenting ; and the magnifying power will be nearly in the 234 THE PRACTICAL ASTRONOMER. proportion of the focal distance of the lens to the focal distance of the eye. If, for example, the focal distance of the eye, or the distance at which we see to read distinctly, be 10 inches, and the focal distance of the lens, 11 feet, the magnifying power will be as 11 feet, or 132 inches to 10, that is, about 13 times. Let A (fig. 51.) represent the lens placed on a pedestal ; the rays of light passing through this lens from distant objects will converge towards a focus at F. If a person then, place his eye at E, a certain distance within the focal point, he will see distant objects magni- fied nearly in the proportion of the focal distance of the lens to that of the eye ; and when the lens is very broad — such as the 26 feet lens mentioned above — two or three persons may look through it at once, though they will not all see the same object. I have alluded above to a lens made by M. Azout of 600 feet focal distance. Were it possible to use such a lens for distant objects, it might represent them as magnified 5 or 600 times, without the application of any eye-glass. In this way the aerial telescope of Huygens would mag- figure 51. F THE ACHROMATIC TELESCOPE. 235 nify objects above 100 times, which is about half the magnifying power it produced with its eye-piece. Suppose Azout's lens had been fitted up as a telescope, it would not have magnified above 480 times, as it would have required an eye- glass of 14 or 15 inches focal distance, whereas, without an eye-glass, it would have magnified objects considerably above 500 times. It is not unlikely that the species of telescope to which I have now adverted, constituted one of those in- struments for magnifying distant objects which were said to have been in the possession of certain persons long before their invention in Holland, and by Galileo in Italy — to which I have referred in p. 182. Were this kind of telescope to be applied to the celestial bodies, it would require to be elevated upon a pole in the manner represented, fig. 45, p. 226. SECT. 6. THE ACHROMATIC TELESCOPE. This telescope constitutes the most important and useful improvement ever made upon telescopic instruments ; and, it is probable, it will, ere long, supersede the use of all other telescopes. Its importance and utility will at once appear when we consider, that a good achromatic telescope of only 4 or 5 feet in length will bear a magnifying power as great, as that of a common astronomical telescope 100 feet long, and even with a greater degree of distinctness, so that they are now come into general use both for terrestrial and celestial observations. There are, indeed, certain obstruc- tions which prevent their being made of a very large size ; but from the improvement in the manufacture of achromatic glass which is now 236 THE PRACTICAL ASTRONOMER. going forward, it is to be hoped that the difficul- ties which have hitherto impeded the progress of opticians will soon be removed. In order to un- derstand the nature of this telescope, it will be necessary to advert a little to the imperfections connected with common refracting telescopes. The first imperfection to which I allude is this, that spherical surfaces do not refract the rays of light accurately to a point ; and hence the image formed by a single convex lens is not perfectly accurate and distinct. The rays which pass near the extremities of such a lens meet in foci more distant from the lens than those which pass nearly through the centre, which may be illustrated by the following figure. Let PP (fig. 52) be a convex figure 52. lens and ~Ee an object, the point E of which cor- responds with the axis, and sends forth the rays EM, EN, EA, &c, all of which reach the surface of the glass, but in different parts. It is manifest that the ray EA which passes through the middle of the glass, suffers no refraction. The rays EM, EM, likewise, which pass through near to EA, will be converged to a focus at F, which we gene- rally consider as the focus of the lens. But the rays EN, EN, which are nearer to the edge of the THE ACHROMATIC TELESCOPE. 237 glass will be differently refracted, and will meet about G, nearer to the lens, where they will form another image Gg. Hence, it is evident, that the first image Ff, is formed only by the union of those rays which pass very near the centre of the lens ; but as the rays of light proceeding from every point of an object are very numerous, there is a succession of images formed, according to the parts of the lens where they penetrate, which necessarily produces indistinctness and confusion. This is the imperfection which is distinguished by the name of spherical aberration, or the error arising from the spherical form of lenses. The second and most important imperfection of single lenses, when used for the object-glasses of telescopes, is, that the rays of compounded light being differently refrangible, come to their respective foci at different distances from the glass ; the more refrangible rays, as the violet, converg- ing sooner than those which are less refrangible, as the red. I have had occasion to illustrate this circumstance, when treating on the colours pro- duced by the prism, (see p. 128, and figures 32 and 33,) and it is confirmed by the experiment of a paper painted red, throwing its image, by means of a lens, at a greater distance than another paper painted blue. From such facts and experiments, it appears, that the image of a white object con- sists of an indefinite number of coloured images, the violet being nearest, and the red farthest from the lens, and the images of intermediate colours at intermediate distances. The aggregate, or image itself, must therefore be in some degree confused ; and this confusion being much increased by the magnifying power, it is found necessary to use an eye glass of a certain limited convexity to a given object glass. Thus, an object glass of 34 238 THE PRACTICAL ASTRONOMER. inches focal length will bear an eye-glass of only 1 inch focus, and will magnify the diameters of objects 34 times ; one of 50 feet focal distance will require an eye-glass of 4i inches focus, and will magnify only 142 times ; whereas, could we apply to it an eye-glass of only 1 inch focus, as in the former case, it would magnify no less than 600 times. And were we to construct an object- glass of 100 feet focal length, we should require to apply an eye-glass, not less than 6 inches focus, which would produce a power of about 200 times ; so that there is no possibility of producing a great power by single lenses, without extending the telescope to an immoderate length. Sir Isaac Newton, after having made his dis- coveries respecting the colours of light, considered the circumstance we have now stated as an insu- perable barrier to the improvement of refracting telescopes ; and therefore turned his attention to the improvement of telescopes by reflection. In the telescopes which he constructed and partly invented, the images of objects are formed by reflection from speculums or mirrors ; and being free from the irregular convergency of the various coloured rays of light, will admit of a much larger aperture and the application of a much greater degree of magnifying power. The reflector which Newton constructed was only 6 inches long, but it was capable of bearing a power equal to that of a 6 feet refractor. It was a long time, however, after the invention of these telescopes before they were made of a size fitted for making celestial observations. After reflecting telescopes had been some time in use, Dollond made his famous discovery of the principle which led him to the construction of the achromatic telescope. This invention consists of a compound object glass THE ACHROMATIC TELESCOPE. 239 formed of two different kinds of glass, by which both the spherical aberration and the errors arising from the different refrangibility of the rays of light are, in a great measure corrected. For the explanation of the nature of this compound object glass and of the effects it produces ; it may be expedient to offer the following remarks respecting the dispersion of light and its refraction by different substances. The dispersion of light is estimated by the variable angle formed by the red and violet rays which bound the solar spectrum ; — or rather, it is the excess of the refraction of the most refrangible ray above that of the least refrangible ray. The dispersion is not proportional to the refraction — that is, the substances which have an equal mean refraction, do not disperse light in the same ratio. For example, if we make a prism with plates of glass, and fill it with oil of Cassia, and adjust its refracting angle ACB, (fig. 31, p. 127,) so that the middle of the spectrum which it forms falls exactly at the same place where the green rays of a spec- trum formed by a glass prism would fall — then we shall find that the spectrum formed by the oil of Cassia prism will be two or three times longer than that of the glass prism. The oil of Cassia, therefore, is said to disperse the rays of light more than the glass, that is, to separate the extreme red and violet rays at O and P more than the mean ray at green, and to have a greater disper- sive power. Sir I. Newton appears to have made use of prisms composed of different substances, yet, strange to tell, he never observed that they formed spectrums, whose lengths were different, when the refraction of the green ray was the same ; but thought that the dispersion was pro- portional to the refraction. This error continued 240 THE PRACTICAL ASTRONOMER. to be overlooked by philosophers for a consider- able time, and was the cause of retarding the invention of the achromatic telescope for more than 50 years. Dollond was among the first who detected this error. By his experiments it appears, that the different kinds of glass differ extremely with respect to the divergency of colours produced by equal refractions. He found that two prisms, one of white flint glass, whose refracting angle was about 25 degrees, and another of crown glass whose refracting angle was about 29 degrees, refracted the beam of light nearly alike ; but that the divergency of colour in the white flint was considerably more than in the crown glass ; so that when they were applied together, to refract contrary ways, and a beam of light transmitted through them, though the emergent continued parallel to the incident part, it was, notwithstand- ing, separated into component colours. From this he inferred, that, in order to render the emergent beam white, it is necessary that the refracting angle of the prism of crown glass should be increased, and by repeated experiments he dis- covered the exact quantity. By these means he obtained a theory in which refraction was per- formed without any separation or divergency of colour ; and thus the way was prepared for apply- ing the principle he had ascertained to the con- struction of the object glasses of refracting teles- copes. For the edges of a convex and concave lens, when placed in contact with each other, may be considered as two prisms which refract contrary ways ; and if the excess of refraction in the one be such as precisely to destroy the divergency of colour in the other, a colourless image will be formed. Thus, if two lenses are made of the THE ACHROMATIC TELESCOPE. 241 same focal length, the one of flint glass and the other of crown, the length or diameter of the coloured image in the first will be to that produced by the crown glass, as 3 to 2 nearly. Now, if we make the focal lengths of the lenses in this proportion, that is, as 3 to 2, the coloured spec- trum produced by each will be equal. But if the flint lens be concave, and the crown convex — when placed in contact — they will mutually cor- rect each other, and a pencil of white light refracted by the compound lens will remain colourless. The following figure may perhaps illustrate what has been now stated. Let LL (fig. 53.) figure 53. represent a convex lens of crown glass, and // a concave lens of flint glass. A ray of the sun S, falls at F on the convex lens which will refract it exactly as the prism ABC, whose faces touch the two surfaces of the lens at the points where the ray enters and quits it. The solar ray, SF, thus refracted by the lens LL, or prism ABC, would have formed a spectrum PT on the wall, had there been no other lens, the violet ray F crossing the M 242 THE PRACTICAL ASTRONOMER. axis of the lens at V, and going to the upper end P of the spectrum ; and the red ray FR, going to the lower end T. But as the flint-glass lens 11, or the prism AaC which receives the rays FV, FR, at the same points, is interposed, these rays will be united at/, and form a small circle of white light ; the ray SF of the sun being now refracted without colour from its primitive direction SFY into the new direction Ff. In like manner the corresponding ray SM will be refracted to/, and a white and colourless image of the sun will be there formed by the two lenses. In this combina- tion of lenses it is obvious that the spherical aber- ration of the flint lens corrects to a considerable degree that of the crown-glass, and by a proper adjustment of the radii of the surfaces, it may be almost wholly removed. This error is still more completely corrected in the triple achromatic object-glass, which consists of three lenses — a concave flint lens placed between convexes of crown glass. Fig. 54 shows the double achromatic lens, and fig. 55, the triple object-glass, as they figure 54. figure 55. THE ACHROMATIC TELESCOPE. 243 are fitted up in their cells, and placed at the object end of the telescope. In consequence of their producing a focal image free of colour they will bear a much larger aperture and a much greater magnifying power than common refracting teles- copes of the same length. While a common telescope whose object-glass is 3£ feet focal dis- tance will bear an aperture of scarcely 1 inch, the 3i feet Achromatic will bear an aperture of Sc- inches, and consequently transmits 101 times the quantity of light. While the one can bear a mag- nifying power of only about 36 times, the other will bear a magnifying power for celestial objects of more than 200 times. The theory of the achromatic telescope is some- what complicated and abstruse, and would require a more lengthened investigation than my limits will permit. But what has been already stated may serve to give the reader a general idea of the principle on which it is constructed, which is all I intended. The term achromatic by which such instruments are now distinguished was first given to them by Dr. Bevis. It is compounded of two Greek words which signify, s free of colour.' And, were it not that even philosophers are not altogether free of that pedantry which induces us to select Greek words which are unintelligible to the mass of mankind, they might have been con- tented with selecting the plain English word colourless, which is as significant and expressive as the Greek word achromatic. The crown-glass, of which the convex lenses of this telescope are made, is the same as good common window-glass ; and the flint-glass is that species of glass of which wine-glasses, tumblers, decanters and similar articles are formed, and is sometimes distinguished by the name of crystal-glass. Some opticians m 2 244< THE PRACTICAL ASTRONOMER. have occasionally formed the concave lens of an achromatic object-glass from the bottom of a broken tumbler. This telescope was invented and constructed by Mr. John Dollond, about the year 1758. When he began his researches into this subject, he was a silk weaver in Spitalflelds, London. The attempt of the celebrated Euler to form a colourless tele- scope, by including water between two meniscus glasses, attracted his attention, and, in the year 1753, he addressed a letter to Mr. Short, the opti- cian, which was published in the Philosophical Transactions of London, 6 concerning a mistake in Euler's theorem for correcting the aberrations in the object glasses of refracting telescopes. After a great variety of experiments on the re- fractive and dispersive powers of different sub- stances, he at last constructed a telescope in which an exact balance of the opposite dispersive powers of the crown and flint lenses made the colours disappear, while the predominating refraction of the crown lens disposed the achromatic rays to meet at a distant focus. In constructing such object glasses, however, he had several difficulties to encounter. In the first place, the focal dis- tance as well as the particular surfaces must be very nicely proportioned to the densities or re- fractive powers of the glasses, which are very apt to vary in the same sort of glass made at different times. In the next place, the centers of the two glasses must be placed truly in the com- mon axis of the telescope, otherwise the desired effect will be in a great measure destroyed. To these difficulties is to be added— that there are four surfaces (even in double achromatic object glasses) to be wrought perfectly spherical; and every person practised in optical operations will THE ACHROMATIC TELESCOPE. 245 allow, that there must be the greatest accuracy throughout the whole work. But these and other difficulties were at length overcome by the judg- ment and perseverance of this ingenious artist. It appears, however, that Dollond was not the only person who had the merit of making this discovery — a private gentleman, Mr. Chest, of Chest-hall, a considerable number of years be- fore, having made a similar discovery, and applied it to the same purpose. This fact was ascertained in the course of a process raised against Dollond at the instance of Watkins, optician at Charing- cross, when applying for a patent. But as the other gentleman had kept his invention a secret, and Dollond had brought it forth for the benefit of the public, the decision was given in his favour. There was no evidence that Dollond borrowed the idea from his competitor, and both were, to a certain extent^ entitled to the merits of the invention. One of the greatest obstructions to the con- struction of large achromatic telescopes is, the difficulty of procuring large discs of flint glass of an uniform refractive density — of good colour, and free from veins. It is said that, fortunately for Mr. Dollond, this kind of glass was procura- ble when he began to make achromatic telescopes, though the attempts of ingenious chemists have since been exerted to make it without much suc- cess. It is also said, that the glass employed by Dollond in the fabrication of his best telescopes, was of the same melting, or made at the same time, and that, excepting this particular treasure, casually obtained, good dense glass for achromatic purposes, was always as difficult to be procured as it is now. The dispersion of the flint glass, too, is so variable, that, in forming an achromatic 246 THE PRACTICAL ASTRONOMER. lens, trials on each specimen require to be made before the absolute proportional dispersion of the substances can be ascertained. It is owing, in a great measure, to these circumstances, that a large and good achromatic telescope cannot be procured unless at a very high price. Mr. Tulley of Isling- ton — who has been long distinguished as a maker of excellent achromatic instruments — showed me, about six years ago, a rude piece of flint glass about five inches diameter, intended for the con- cave lens of an achromatic object glass, for which he paid eight guineas. This was before the piece of glass was either figured or polished, and, con- sequently, he had still to perform the delicate operation of figuring, polishing, and adjusting this concave to the convex lenses with which it was to be combined ; and during the process some veins or irregularities might be detected in the flint glass which did not then appear. Some years be- fore, he procured a disc of glass from the conti- nent about seven or eight inches diameter, for which he paid about thirty guineas, with which an excellent telescope, twelve feet focal length, was constructed for the Astronomical Society of London. It is obvious therefore, that large achro- matic telescopes must be charged at a pretty high price. In order to stimulate ingenious chemists and opticians to make experiments on this subject, the Board of Longitude, more than half a century ago, offered a considerable reward for bringing the art of making good flint glass for optical purposes to the requisite perfection. But considerable diffi- culties arise in attempting improvements of this kind ; as the experiments must all be tried on a very large scale, and are necessarily attended with a heavy expence. And although government has THE ACHROMATIC TELESCOPE. 247 been extremely liberal in voting money for warlike purposes, and in bestowing pensions on those who stood in no need of them, it has hitherto thrown an obstruction in the way of such experiments, by the heavy duty of excise, which is rigorously ex- acted, whether the glass be manufactured into saleable articles or not ; and has thus been instru- mental in retarding the progress of improvement and discovery. It would appear that experiments of this kind have been attended with more success iu France, Germany, and other places on the continent, than in Britain ; as several very large achromatic telescopes have been constructed in those countries by means of flint glass which was cast for the purpose in different manufactories, and to which British artists have been considerably indebted ; as the London opticians frequently pur- chase their largest discs of flint glass from Pari- sian agents. Guinaud, a continental experimenter, and who was originally a cabinet maker, appears to have had his labours in this department of art crowned with great success. Many years were employed in his experiments, and he too fre- quently, notwithstanding all his attention, dis- covered his metal to be vitiated by striae, spects or grains, with cometic tails. He constructed a fur- nace capable of melting two cwt of glass in one mass, which he sawed vertically, and polished one of the sections, in order to observe what had taken place during the fusion. From time to time, as he obtained blocks, including portions of good glass, his practice was to separate them by sawing the blocks into horizontal sections, or perpendi- cular to their axes. A fortunate accident con- ducted him to a better process. While his men were one day carrying a block of this glass, on a hand- barrow, to a saw mill which he had erected at the 248 THE PRACTICAL ASTRONOMER. Fall of the Doubs, the mass slipped from its bearers, and, rolling to the bottom of a steep and rocky declivity, was broken to pieces. Guin- aud having selected those fragments which appeared perfectly homogeneous, softened them in circular moulds, in such a manner, that on cooling, he ob- tained discs that were afterwards fit for working. To this method he adhered, and contrived a way for clearing his glass while cooling, so that the fractures should follow the most faulty parts. When flaws occurred in the large masses, they were removed by cleaving the pieces with wedges ; then smelting them again in moulds, which give them the form of discs. The Astronomical Society of London have made trial of discs made by Guinaud, and have found them entirely homo- geneous and free from fault. Of this ingenious artist's flint glass, some of the largest achromatic telescopes on the continent have been constructed. But, it is more than twenty years since this expe- rimenter took his flight from this terrestrial scene, and it is uncertain whether his process be still carried on with equal success. Notices of some large Achromatic telescopes on the Continent and in Great Britain. 1. The Dorpat Telescope. — This is one of the largest and most expensive Refracting telescopes ever constructed. It was made by the celebrated Fraunhofer of Munich for the observatory of the Imperial University of Dorpat, and was received into the observatory by Professor Struve in the year 1825. The aperture of the object glass of this telescope is 9^ English inches, and its solar focal length about fourteen feet, the main tube NOTICES OF ACHROMATIC TELESCOPES, 249 being thirteen French feet exclusive of the tube which holds the eye pieces. The smallest of the four magnifying powers it possesses, is 175, and the largest 700, which, in favourable weather, is said to present the object with the utmost pre- cision. ' This instrument/ says Struve, 'was sold to us by Privy-Counsellor Von Utzchneider, the chief of the optical establishment at Munich, for 10,500 florins, (about £950 sterling), a price which only covers the expenses which the estab- lishment incurred in making it/ The frame work of the stand of this telescope is of oak inlaid with pieces of mahogany in an ornamental manner, and the tube is of deal veneered with mahogany and highly polished. The whole weight of the tele- scope and its counterpoises is supported at one point, at the common center of gravity of all its parts; and though these weigh 3000 Russian pounds, yet, we are told that this enormous tele- scope may be turned in every direction towards the heavens with more ease and certainty than any other hitherto in use. When the object end of the telescope is elevated to the zenith, it is sixteen feet four inches, Paris measure, above the floor, and its eye end in this position is two feet nine inches high. This instrument is mounted on an Equatorial stand, and clock work is applied to the Equatorial axis, which gives it a smooth and regular sidereal motion, which, it is said, keeps a star in the exact center of the field of view, and produces the appearance of a state of rest in the starry regions, which motion can be made solar, or even lunar, by a little change given to the place of a pointer, that is placed as an index on the dial plate. Professor Struve considers the optical powers of this telescope superior to those of Schroeter's twenty-five feet reflector, from m 5 250 THE PRACTICAL ASTRONOMER. having observed tt Orionis with fifteen companions, though Schroeter observed only twelve, that he could count with certainty. Nay, he seems dis- posed to place it in competition with the late Sir W. Herschel's forty feet reflector. The finder of this telescope has a focal distance of 30 French inches, and 2-42 aperture. 2. Sir James South 9 s Telescope. — About the year 1829, Sir J. South, President of the London Astronomical Society, procured of M. Cauchoix of Paris, an achromatic object glass of 11^ inches, clear aperture, and of 19 feet focal length. The flint glass employed in its construction was the manufacture of the late Guinaud le Pere, and was found to be absolutely perfect. The first ob- servation was made with this telescope, while on a temporary stand, on Feb. 13, 1830, when Sir J. Herschel discovered with it a sixth star in the trapezium in the nebula of Orion, whose brightness was about one third of that of the fifth star dis- covered by Struve, which is as distinctly seen as the companion to Polaris is in a five feet achro- matic. Sir James gives the following notices of the performance of this instrument on the morning of May 14, 1830. < At half past two, placed the £0 feet achromatic on the Georgium Sidus, saw it with a power of 346, a beautiful planetary disc ; not the slightest suspicion of any ring, either per- pendicular or horizontal ; but the planet three hours east of the meridian, and the moon within three degrees of the planet. At a quarter before three, viewed Jupiter with 252 and 346, literally covered with belts, and the diameters of his satel- lites might have been as easily measured as him- self. One came from behind the body, and the contrast of the colour with that of the planet's limb was striking. At three o'clock viewed Mars. NOTICES OF ACHROMATIC TELESCOPES. 251 The contrast of light in the vicinity of the poles very decided. Several spots on his body well and strongly marked — 'hat about the south pole seems to overtake the body of the planet, and gives an appearance not unlike that afforded by the new moon, familiarly known as ' the old moon in the new moon's arms/ Saturn has been repeatedly seen with powers from 130 to 928 under circum- stances the most favourable ; but not any thing anomalous about the planet or its ring could even be suspected. This telescope is erected on an Equatorial stand at Sir J. South's observatory, Kensington. 3. Captain Smyth's Telescope in his private observatory at Bedford* — This Achromatic teles- cope is 8£ feet focal length, with a clear aperture of 5| inches worked by the late Mr. v Tulley, Senior, from a disk purchased by Sir James South at Paris. It is considered by Captain Smyth to be the finest specimen of that eminent optician's skill, and, it is said, will bear with distinctness, a magnifying power of 1200. Its distinctness has been proved by the clear vision it gives of the obscure nebulae, and of the companions of Polaris, Rigel, a Lyrse, and the most minute double stars — the lunar mountains, cavities and shadows under all powers — the lucid polar regions of Mars — the sharpness of the double ring of Saturn — the gib- bous aspect of Venus — the shadows of Jupiter's satellites across his body, and the splendid contrast of colours in a Hercules, y Andromedae and other superb double stars. Other large Achromatics. — Besides the above, the following, belonging to public observatories and private individuals, may be mentioned. In the Royal observatory at Greenwich, there is an Achromatic of 10 feet focal distance, having a 252 THE PRACTICAL ASTRONOMER. double object glass 5 inches diameter, which was made by Mr. Peter Dollond, and the only one of that size he ever constructed. There is also a 46 inch achromatic, with a triple object glass 3f inches aperture, which is said to be the most per- fect instrument of the kind ever produced. It was the favourite instrument of Dr. Maskelyne, late Astronomer Royal, who had a small room fitted up in the observatory for this telescope. The observatory, some years ago erected near Cambridge, is perhaps the most splendid structure of the kind in Great Britain. It is furnished with several very large achromatic telescopes on Equa- torial machinery : but the Achromatic telescope, lately presented to it by the Duke of Northum- berland, is undoubtedly the largest instrument of this description which is to be found in this country. The object glass is said to be 25 feet focal distance, and of a corresponding diameter, but as there was no access to this instrument at the time I visited this observatory, nearly six years ago, I am unable to give a particular de- scription of it. In the Royal Observatory at Paris, which I visited in 1837, I noticed, among other instruments, two very large Achromatic telescopes which, measuring them rudely by the eye — 1 estimated to be from 15 to 18 feet long, and the aperture at the object end, from 12 to 15 inches diameter. They were the largest achro- matics I had previously seen ; but I could find no person in the observatory at that time, who could give me any information as to their history, or to their exact dimensions, or powers of magni- fying* * An achromatic telescope is said to be in possession of Mr. Cooper, M.P. for Sligo, which is 26 feet long, and the diameter of the object glass 14 inches. NOTICES OF ACHROMATIC TELESCOPES. 253 The Rev. Dr. Pearson, Treasurer to the Astro- nomical Society of London, is in possession of the telescope formerly alluded to, made by Mr. Tulley, of twelve feet focal distance and seven inches aperture, which is said to be a very fine one. The small star which accompanies the pole star, with a power of a 100, appears through this telescope, as distinct and steady as one of Jupi- ter's satellites. With a single lens of 6 inches focus, which produced a power of 24 times, according to the testimony of an observer who noticed it — the small star appeared as it does in an achromatic of 3 inches aperture, which shows the great effect of illuminating power in such instruments. Mr. Lawson, a diligent astronomical observer in Hereford, possesses a most beautiful achromatic telescope of about 7 inches aperture, and 12 feet focal distance, which was made by one of the Dollonds, who considered it as his chief d'ouvre. It is said to bear powers as high as 1100 or 1400 ; and has been fitted up with mechanism devised by Mr. Lawson himself, so as to be perfectly easy and manageable to the observer, and which displays this gentleman's in- ventive talent. In several of his observations with this instrument, he is said to have had a view of some of the more minute subdivisions of the ring of Saturn. A very excellent achro- matic telescope was fitted up some years ago by my worthy friend William Bridges, Esq, Black- heath. Its object glass is 5\ inches diameter, and about 5^ feet focal length. It is erected upon Equatorial machinery, and placed in a circular observatory which moves round with a slight touch of the hand. The object glass of this in- strument cost about 200 Guineas, the equatorial machinery on which it is mounted cost 150 254 THE PRACTICAL ASTRONOMER. Guineas, and the circular observatory in which it is placed about 100 Guineas ; in all 450 Guineas. Its powers vary from 50 to 300 times.* Achromatic telescopes of a moderate size. Such telescopes as I have alluded to above, are among the largest which have yet been made on the achromatic principle ; they are, of course, comparatively rare, and can be afforded only at a very high price. Few of the object glasses in the telescopes to which I have referred, would be valued at less than 200 Guineas, independently of the tubes, eye pieces and other apparatus with which they are fitted up. It is so difficult to pro- cure large discs of flint glass for optical purposes, to produce the requisite curves of the different lenses, and to combine them together with that extreme accuracy which is requisite, that when a good compound lens of this description is found perfectly achromatic, the optician must necessarily set a high value upon it; since it may happen that he may have finished half a dozen before he has got one that is nearly perfect. The more common sizes of achromatic telescopes for astro- nomical purposes, which are regularly sold by the London opticians, are the following : — 1. The 2% feet Achromatic. — This telescope has an object glass 30 inches in focal length, and 2 inches clear aperture. It is generally furnished with two eye pieces, one for terrestrial objects, magnifying about 30 or 35 times, and one for celestial objects with a power of 70 or 75 times. * This telescope, which was made by Dollond, with a power of 240 times, gives a beautiful view of the belts of Jupiter and the double ring of Saturn, and with a power of 50, the stars in the milky way and some of the nebulas appear very numerous and brilliant. Its owner is a gentleman who unites science with Christianity. NOTICES OF ACHROMATIC TELESCOPES. 255 It might be furnished with an additional astro- nomical eye-piece — if the object glass be a good one, so as to produce a power of 90 or 95 times. With such a telescope, the belts and satellites of Jupiter, the phases of Venus and the ring of Saturn may be perceived; but not to so much advantage as with larger telescopes. It is gene- rally fitted up either with a mahogany or a brass tube, and is placed upon a tripod brass stand, with a universal joint which produces a horizontal and vertical motion. It is packed, along with the eye-pieces, and whatever else belongs to it, in a neat mahogany box. Its^price varies, accord- ing as it is furnished with an elevating rack or other apparatus. The following are the prices of this instrument as marked in the catalogue of Mr. Tulley, Ter- rett's Court, Islington, London. £ s. d. 2} feet telescopes, brass mounted on plain pillar and claw stand, with one eye piece for astronomical purposes, and one for land objects, to vary the magnifying power, packed in a mahogany box - - - - - -10100 Ditto, ditto, brass mounted on pillar and claw stand, with elevating rack, 1 eye piece for astronomical purposes, and 1 for land objects, to vary the magnifying power, packed in a mahogany box - - - - - - - -12 120 The following prices of the same kind of teles- cope are from the catalogue of Messrs. W. and, S. Jones, 30, Lower Holborn, London. £ s. d. The improved 2 J feet achromatic refractor, on a brass stand, mahogany tube, with three eye pieces, two magnify- ing about 40 and 50 times for terrestrial objects, and the other about 75 times for astronomical purposes, in a mahogany case - - - 10100 Ditto, ditto, the tube all brass, with three eye pieces - 1111 0 Ditto, ditto, with vertical and horizontal rack work motions - - 15 15 0 2. The 3i feet Achromatic Telescope. — The 256 THE PRACTICAL ASTRONOMER. object glass of this telescope is from 44 to 46 inches focal length, and 2f inches diameter. It is generally furnished with four eye-pieces, two for terrestrial and two for celestial objects. The lowest power for land objects is generally about 45, which affords a large field of view, and ex- hibits the objects with great brilliance. The other terrestrial power is usually from 65 to 70. The astronomical powers are about 80 and 130 ; but such a telescope should always have another eye-piece, to produce a power of 180 or 200 times, which it will bear with distinctness, in a serene state of the atmosphere, if the object glass be truly achromatic. The illuminating power in this telescope is nearly double that of the 2i feet telescope, or in the proportion of 7, 56 to 4 ; and therefore it will bear about double the magnifying power with nearly equal distinctness. This teles- cope is fitted up in a manner somewhat similar to the former, with a tripod stand which is placed upon a table. Sometimes, however, it is mounted on a long mahogany stand which rests upon the floor, (as in fig. 58.), and is fitted with an equato- rial motion ; and has generally a small telescope fixed near the eye end of the large tube, called a finder, which serves to direct the telescope to a particular object in the heavens when the higher powers are applied. It is likewise eligible that it should have an elevating rack and sliding tubes, for supporting the eye end of the instrument, to keep it steady during astronomical observations, and it would be an advantage, for various purposes which shall be afterwards described, to have fitted to it a Diagonal Eye Piece magnifying 40 times or upwards. The prices of this instrument, as marked in Mr. Tulley's Catalogue, are as follows : — NOTICES OF ACHROMATIC TELESCOPES. 257 £ s. d. The feet achromatic telescope 2§ inches aperture, on plain pillar and claw stand, 2 eye pieces for astronomical purposes, and 1 for land objects to vary the magnifying power, packed in a mahogany box - - - - - 2100 Ditto, ditto, with elevating rack and achromatic finder, 2 eye pieces for astronomical purposes, and 1 for day ob- jects to vary the magnifying power, packed in a mahogany box - 26 5 0 The following are the prices as marked in Messrs. W. and S. Jones' Catalogue. £ s. d. The 3 h feet achromatic, plain mahogany tube - - 18 18 0 Ditto, "ditto, brass tube - - - - - -21 00 Ditto, all in brass, with rack work motions, &c. - - 26 5 0 Ditto, the object glass of the largest aperture, and the rack motions on an improved principle - from 37/. 1 6s. to 42 0 0 Ditto, fitted up with Equatorial motion, framed maho- gany stand, divided altitude, and azimuth arches, or decli- nation and right ascension circles, &c. &c. - from 601 to 80 0 0 This is the telescope which I would particularly recommend to astronomical amateurs, whose pecu- niary resources do not permit them to purchase more expensive instruments. When fitted up with the eye pieces and powers already mentioned, and with a finder and elevating rack, — price 25 guineas — it will serve all the purposes of general observation. By this telescope, satisfactory views may be obtained of most of the interesting phe- nomena of the heavens, such as the spots of the sun — the mountains, vales, and caverns on the lunar surface — the phases of Mercury and Venus — the spots on Mars — the satellites and belts of Jupiter — the ring of Saturn —many of the more interesting nebulae, and most of the double stars of the second and third classes. When the object glass of this telescope is accurately figured and per- fectly achromatic, a power of from 200 to 230 may be put upon it, by which the division of Saturn's ring might occasionally be perceived. It is more 258 THE PRACTICAL ASTRONOMER. easily managed and represents objects considerably brighter than reflecting telescopes of the same price and magnifying power, and it is not so apt to be deranged as reflectors generally are. A telescope of a less size would not in general be found satisfactory for viewing the objects I have now specified, and for general astronomical pur- poses. It may not be improper for the informa- tion of some readers, to explain what is meant in Mr. Tulley's catalogue, when it is stated that this instrument has ( one eye piece for day objects, to vary the magnifying power' The eye piece alluded to is so constructed, that by drawing out a tube next the eye, you may increase the power at pleasure, and make it to vary, say from 40 to 80 or 100 times ; so that such a construction of the terrestrial eye piece (to be afterwards explained) serves in a great measure, the purpose of separate eye-pieces. The whole length of the 3f feet teles- cope, when the terrestrial eye piece is applied, is about 4^ feet from the object glass to the first eye glass. "When the aperture of the object glass of this telescope exceeds 2f inches its price rapidly ad- vances. The following is Mr. Tulley's scale of prices, proportionate to the increase of aperture : — £ s. d. 3| feet telescopes 3 J inches aperture, with vertical and horizontal rack work motions, achromatic finder, 3 eye pieces for astronomical purposes, and one for day objects to vary the magnifying power, packed in a mahogany box 42 0 0 Ditto, ditto, 3| inches diameter, mounted as above - 68 5 0 Ditto, with universal Equatorial, instead of pillar and claw stand - - 84 00 Here, in the one case, the increase of half an inch in the diameter of the object-glass, adds about £16. to the expense ; and in the other case NOTICES OF ACHROMATIC TELESCOPES. 259 no less than £26. 5s. The proportion of light in those two telescopes, compared with that of 2f inches aperture, is as follows : — The square of the M object-glass is 7.56; that of 3i, 10.56, and that of the 3f , 14.06 ; so that the light admitted by the 3} compared with the 2f aperture, is nearly as 10 to 7 ; and the light admitted by the 3| object- glass is nearly double that of the 2f aperture, and will bear nearly a proportional increase of magni- fying power. 3. The 5 feet Achromatic telescope. The focal length of the object-glass of this telescope is 5 feet 3 inches, and the diameter of its aperture 3j5 inches. The usual magnifying powers applied to it are, for land objects 65 times ; and for celes- tial objects, 110,190, 250, and sometimes one or two higher powers. The quantity of light it possesses is not much larger than that of the 3i feet telescope, with 3f inches aperture; but the larger focal length of this telescope is considered to be an advantage ; since the longer the focus of the object-glass, the less will be its chromatic and spherical aberrations, and the larger may be the eye-glasses, and the flatter the field of view. The following are the prices of these telescopes as marked in Mr. Tulley's catalogue. £ s. d. 5 feet telescopes 31 inches aperture, on an universal equa- torial stand, with achromatic finder, 4 eye pieces for astro- nomical purposes, and 1 for day objects to vary the magni- fying power, packed in a mahogany box - 100 guineas to 157 10 0 7 feet ditto, 5 inches aperture, on a newly improved universal equatorial stand, 6 eye pieces for astronomical purposes, and 1 for day objects to vary the magnifying power, with achromatic finder, and Troughton's Micrometer 207 5 0 The above are all the kinds of achromatic telescopes generally made by the London opti- cians. Those of the larger kind, as 5 and 7 feet telescopes, and the 3Jfeet with 3f inches aperture, 260 THE PRACTICAL ASTRONOMER. are generally made to order, and are not always to be procured. But the 2% and 3i feet achromatics of 2f inches aperture, are generally to be found ready-made at most of the optician's shops in the metropolis. The prices of these instruments are nearly the same in most of the optician's shops in London. Some of them demand a higher price, but few of them are ever sold lower than what has been stated above, unless in certain cases, where a discount is allowed. The stands for these telescopes, and the manner in which they are fitted up for observation are re- presented in figures 57, 58, and 59. Fig. 57 re- figure 57. NOTICES OF ACHROMATIC TELESCOPES. 261 presents either the 2i or the 3i feet telescopes mounted on a plain brass stand, to be placed on a table. A is the long eye-piece for land objects, and B the small eye-piece for astronomical obser- vation, which is composed of two lenses, and re- presents the object in an inverted position. These eye-pieces are screwed on, as occasion re- quires, at E, the eye-end of the telescope. The shorter of the two astronomical eye-tubes which accompany this telescope, produces the highest magnifying power. For adjusting the telescope to distinct vision, there is a brass knob or button at a, which moves a piece of rack-work connected with the eye-tube, which must be turned either one way or the other till the object appears dis- tinctly; and different eyes frequently require a different adjustment. Fig. 58, represents a 5 feet telescope fitted up for astronomical observations. It is mounted on a mahogany stand, the three legs of which are made to close up together by means of the brass frame aaa, which is composed of three bars, con- nected with three joints in the centre, and three other joints, connected with the three mahogany bars. It is furnished with an apparatus for equa- torial motions. The brass pin is made to move round in the brass socket b, and may be tightened by means of the finger screw d, when the telescope is directed nearly to the object intended to be viewed. This socket may be set perpendicular to the horizon, or to any other required angle ; and the quantity of the angle is ascertained by the divided arc, and the instrument made fast in that position by the screw e. If this socket be set to the latitude of the place of observation, and the plane of this arc be turned so as to be in the plane of the meridian, the socket b being fixed to the in- 262 THE PRACTICAL ASTRONOMER. figure 58. clination of the pole of the earth, the telescope when turned in this socket, will have an equatorial motion, so that celestial objects may be always kept in view, when this equatorial motion is per- formed. The two handles at k are connected with rack-work, intended to move the telescope in any required direction. The two sets of brass sliding rods ii are intended to render the telescope as steady as possible, and to elevate and depress it at pleasure, and are so constructed as to slide into each other with the utmost ease. The Finder is placed at AE, either on the top or the left side of the tube of the telescope. NOTICES OF ACHROMATIC TELESCOPES. 263 When high magnifying powers are applied to any telescope, it is sometimes difficult, on account of the smallness of the field of view, to direct the main tube of the telescope to the object. But the Finder, which is a telescope with a small power, and consequently has a large field of view — when directed to any object, it is easily found, and being brought to the centre of the field, w 7 here two cross hairs intersect each other, it will then be seen in the larger telescope. B is the eye-tube for terrestrial objects, containing 4 glasses, and C, one of the astronomical eye-pieces. A socket is represented at g, containing a stained glass, which is screwed to any of the eye-pieces, to protect the eye from the glare of light, when viewing the spots of the sun. The brass nut above f } is intended for the adjustment of the eye- piece to distinct vision. The ?>\ feet telescope is sometimes mounted in this form. Fig. 59, represents a 5 or 6 feet telescope, mounted on a stand of a new construction by Dollond. It possesses the advantage of support- ing the telescope in two places, which renders it extremely steady — a property of great importance when viewing celestial objects with high magnify- ing powers. It possesses likewise, the advantage of enabling the observer to continue seated at the same height from the floor, although the telescope be raised to any altitude — the elevation being en- tirely at the object end, although it may be changed from the horizon to the zenith. The frame-work is composed of bars of mahogany, and rests on three castors, two of which are made fast to their respective legs in the usual way, and the third stands under the middle of the lower horizontal bar that connects the two opposite legs, so that the frame has all the advantages of a tripod. As 264 THE PRACTICAL ASTRONOMER. figure 59. B it becomes very inconvenient to stoop to the eye end of a telescope, when the altitude of an object is considerable, and the centre of motion at the middle of the tube, this construction of a stand serves to remedy such inconvenience, NOTICES OF ACHROMATIC TELESCOPES. 265 Proportions of curvature of the lenses which form an achromatic object-glass. As some ingenious mechanics may feel a desire to attempt the construction of a compound achro- matic object-glass, I shall here state some of the proportions of curvature of the concave and con- vex lenses, which serve to guide opticians in their construction of achromatic instruments. These proportions are various ; and even when demon- strated to be mathematically correct, it is some- times difficult to reduce them to practice, on account of the different powers of refraction and dispersion possessed by different discs of crown and flint-glass, and of the difficulty of producing by mechanical means, the exact curves which theory requires. The following table shows the radii of curvature of the different surfaces of the lenses necessary to form a double achromatic object-glass — it being supposed that the sine of refraction in the crown-glass is as 1.528 to 1, and in the flint as 1.5735 to 1 ; the ratio of their dis- persive powers being as 1 to 1.524. It is also assumed that the curvatures of the concave lens are as 1 to 2, that is, that the one side of this lens is ground on a tool, the radius of which is double that of the other. The 1st column expresses the compound focus of the object-glass in inches ; the 2nd column states the radius of the anterior surface of the crown, and column 3rd, its posterior side. Column 4th expresses the radius of the anterior surface of the concave lens, and column 5th its posterior surface, which, it will be observed, is exactly double that of the other. N 266 THE PRACTICAL ASTRONOMER. A UlUb JtvclCllUS 01 clllLclUJI BUI face, convex. -tvaQrus oi nnc^on at* jJUSLcllOI surface. xvaciius 01 anterior sur- face 9 concave. Jtvauius 01 posterior surface. JLnc. Uec. inc. dqc. T Inc. Dec. 12 3 4. 652 4. 171 24 6 9! 304 8. 342 16. 684 30 7. 5 11. 063 10. 428 20. 856 36 9 13. 956 12. 513 25. 027 48 12 18. 608 16. 684 33. 369 60 15 23. 260 20. 856 41. 712 120 30 46. 520 41 712 83. 424 From the above table it will be seen, that to construct, for example, a 30 inch compound object* glass, the radius of the anterior side of the crown must be 7\ inches, and that of the posterior side 11.63 inches; the radius of the anterior surface of the concave 10.428, and that of the posterior 20.856 inches. It may be proper to observe, that in these computations, the radius of the anterior surface of the concave is less than the posterior side of the convex, and consequently admits of its approach, without touching in the centre — a circumstance which always requires to be guarded against in the combination of achromatic glasses. The following table shows the radii of curvature of the lenses of a triple object-glass, calculated from formula deduced by Dr. Robison of Edinburgh. Focal Convex lens of j Concave lens of Convex lens of length. crown glass. flint glass. crown glass. Inches Inc. Dec. Inc. Dec. Inc. Dec. Inc. Dec. Inc. Dec Inc. Dec. 6 4. 54 3. 03 3. 03 6. 36 6. 36 0. 64 9 6. 83 4. 56 4. 56 9. 54 9. 54 0. 92 12 9. 25 6. 17 6. 17 12. 75 12. 75 1. 28 18 13. 67 9. 12 9. 12 19. 08 19. 08 1. 92 24 18. 33 12. 25 12. 25 25. 50 25. 50 2. 56 30 22. 71 15. 16 15. 16 31. 79 31. 79 3. 20 36 27. 33 18. 25 18. 25 38. 17 38. 17 3. 84 42 31. 87 2]. 28 21. 28 44. 53 44. 53 4. 48 48 36. 42 24. 33 24. 33 50. 92 50. 92 5. 12 54 40. 98 27. 36 27. 36 57. 28 57. 28 5. 76 60 45. 42 30. 33 30. 33 63. 58 63. 58 6. 40 NOTICES OF ACHROMATIC TELESCOPES. 267 The following table contains the proportions of curvature, said to be employed by the London opticians. XT' „„1 t ocal length. Convex of crown glass. Radius of both the surfaces of the concave of flint glass. Convex lens of crown glass. T 1, incnes. Inc. Dec. Inc. Dec. Inc. Dec. Inc. Dec. Inc. Dec. 6 3. 77 4. 49 3. 47 3. 77 4. 49 9 5. 65 6. 74 5. 21 5. 65 6. 74 12 7. 54 8. 99 6. 95 7. 54 8. 99 18 11. 30 13. 48 10. 42 11. 30 13. 48 24 15. 08 17. 98 13. 90 15. 08 17. 98 36 22. 61 26. 96 20. 84 22. 61 26. 96 42 26. 38 31. 45 24. 31 26. 38 31. 45 48 30. 16 35. 96 27. 80 30. 16 35. 96 54 33. 91 40. 45 31. 27 33. 91 40. 45 60 37. 68 44. 94 34. 74 37. 68 44. 94 From this table it appears, that the two convex lenses, have the same radii of their respective sides and that the concave flint lens has its two surfaces equally concave, so that a triple object- glass formed according to these proportions, would require only three pair of grinding tools. The following are the curves of the lenses of one of the best of Dollond's achromatic telescopes, the focal length of the compound object-glass being 46 inches. Reckoning from the surface next the object — the radii of the crown-glass were 28 and 40 inches : the concave lens 20.9 inches, and the inner crown-glass lens, 28.4 and 28.4 inches. This telescope carried magnifying powers of from 100 to 200 times. Although I have inserted the above tables, which might in some measure guide an ingenious artist, yet on the whole, a private amateur has little chance in succeeding in such attempts. The diversity of glasses, and the uncertainty of an unpractised workman's producing the precise curvatures he intends, is so great, that the object- N 2 268 THE PRACTICAL ASTRONOMER. glass, for the most part, turns out different from his expectations. The great difficulty in the con- struction is to find the exact proportion of the dispersive powers of the crown and flint glass. The crown is pretty constant, but there are hardly two pots of flint glass which have the same dispersive power. Even if constant, it is difficult to measure it accurately; and an error in this greatly affects the instrument ; because the focal distances of the lenses must be nearly as their dispersive powers. In the two preceding tables, the sine of incidence, in the crown glass, is sup- posed to be to the sine of refraction as 1.526 to 1 ; and in the flint glass, as 1.604 to !. Opticians who make great numbers of lenses both of flint and crown glass, acquire, in time, a pretty good guess of the nature of the errors which may remain after they have finished an object-glass ; and having many lenses intended to be of the same form, but unavoidably differing a little from it, they try several of the concaves with the two convexes, and finding one better than the rest, they make use of it to complete the set. In this way some of the best achromatic telescopes are frequently formed. I have sometimes found, when supplying a concave flint glass to a telescope where it happened to be wanting, that, of four or five concave lenses which appeared to be the same as to curvature and other properties, only one was found to produce a distinct and colourless image. Should any one, however, wish to attempt the construction of an achromatic lens, the best way for preventing disappointments in the result is, to procure a variety of tables of the respective curvatures founded on different condi- tions, and which, of course, require the surfaces of the several lenses to be of different curves* NOTICES OF ACHROMATIC TELESCOPES. 269 Having lenses of different radii at his command, and having glass of different refractive or dispersive powers, when one combination does not exactly suit, he may try another, and ultimately may suc- ceed in constructing a good achromatic telescope ; for, in many cases, it has been found that chance, or a happy combination of lenses by trial, has led to the formation of an excellent object-glass. Achromatic telescopes composed of fluid lenses. The best achromatic telescopes, when minutely examined, are found to be in some respects defec- tive, on account of that slight degree of colour which, by the aberration of the rays, they give to objects, unless the object-glass be of small diame- ter. "When we examine with attention a good achromatic telescope we find that it does not show white or luminous objects perfectly free from colour, their edges being tinged on one side with a claret-coloured fringe, and on the other with a green fringe. This telescope, therefore, required farther improvement, to get rid of these secondary colours, and Father Boscovich, to whom every branch of optics is much indebted, displayed much ingenuity in his attempts to attain this object. But it is to Dr. Blair, professor of astronomy in Edinburgh, that we are chiefly indebted for the first successful experiments by which this end was accomplished. By a judicious set of experiments, he proved that the quality of dispersing the rays in a greater degree than crown-glass, is not con- fined to a few mediums ; but is possessed by a great variety of fluids, and by some of these in a most extraordinary degree. Having observed that when the extreme red and violet rays were perfectly united, the green were left out, he con- 270 THE PRACTICAL ASTRONOMER. ceived the idea of making an achromatic concave lens which should refract the green less than the united red and violet, and an achromatic convexlens which should do the same, and as the concave lens refracted the outstanding green to the axis, while the concave one refracted them from the axis, it followed, that, by a combination of these two opposite effects, the green would be united with the red and violet. By means of an ingenious prismatic apparatus, he examined the optical properties of a great variety of fluids. The solutions of metals and semi- metals proved in all cases more dispersive than crown glass. Some of the salts, such as sal- ammoniac, greatly increased the dispersive power of water. The marine acid disperses very consi- derably, and this quality increases with its strength. The most dispersive fluids were accord- ingly found to be those in which this acid and the metals were combined. The chemical preparation called causticum an timoniale, or butter of antimony, in its most concentrated state, when it has just attracted sufficient humidity to render it fluid, possesses the quality of dispersing the rays in an astonishing degree. The great quantity of the semi- metal retained in solution, and the highly concen- trated state of the marine acid, are considered as the cause of this striking effect. Corrosive subli- mate of mercury, added to a solution of sal- ammoniacum in water, possesses the next place to the butter of antimony among the dispersive fluids, which Dr. Blair examined. The essential oils were found to hold the next rank to metallic solu- tions, among fluids which possess the dispersive quality, particularly those obtained from bitumi- nous minerals, as native petrolea, pit coal, and amber. The dispersive power of the essential oil NOTICES OF ACHROMATIC TELESCOPES. 271 figure 60. of sassafras, and the essential oil of lemons, when genuine, were found to be not much inferior to any of these. But of all the fluids fitted for optical purposes, Dr. Blair found that the muriatic acid mixed with a metallic solution, or, in other words, a fluid in which the marine acid and metalline particles, hold a due proportion, most accurately suited his purpose. In a spectrum formed by this fluid the green were among the most refran- gible rays, and when its dispersion was corrected by that of glass, there was pro- duced an inverted secondary spectrum, that is, one in which the green was above, when it would have been below with a common medium. He there- fore placed a concave lens of muriatic acid with a metallic solution between the two lenses, as in fig. 60, where AB is the concave fluid lens, CF a plano- convex lens, with its plane side next the object, and ED, a meniscus. With this object- glass the rays of different colours were bent from their rectilineal course with the same equality and regularity as in reflection. Telescopes constructed with such object-glasses were examined by the late Dr. Robison and pro- fessor Play fair. The focal distance of the object- glass of one of these did not exceed 17 inches, and yet it bore an aperture of 3i inches. They viewed some single and double stars and some common objects with this telescope; and found, that, in magnifying power, brightness, and dis- 272 THE PRACTICAL ASTRONOMER. tinctness, it was manifestly superior to one of Mr. Dollond of 42 inches focal length. They had most distinct vision of a star, when using an erect- ing eye-piece, which made this telescope magnify more than a 100 times ; and they found the field of vision as uniformly distinct as with Dollond's 42 inch telescope magnifying 46 times ; and were led to admire the nice figuring and centering of the very deep eye-glasses which were necessary for this amplification. They saw double stars with a degree of perfection which astonished them. These telescopes, however, have never yet come into general use ; and one reason perhaps, is, that they are much more apt to be deranged, than telescopes constructed of object-glasses which are solid. If any species of glass, or other solid transparent substance could be found with the same optical properties, instruments might perhaps be constructed of a larger size, and considerably superior to our best achromatic telescopes.* It is said that Mr. Blair, the son of Dr. Blair, some years ago, was engaged in prosecuting his father's views, but I have not heard any thing respecting the result of his investigations. Barlow's refracting telescope with a fluid concave lens. Professor Barlow, not many years ago, sug- gested a new fluid telescope, which is deserving of attention ; and, about the year 1829 constructed one of pretty large dimensions. The fluid he employs for this purpose is the sulphuret of * For a more particular account of Dr. Blair's instruments and experiments, the reader is referred to his Dissertation on this subject in Vol. II. of the c Transactions of the Royal Society of Edinburgh," which occupies 76 pages — or to Nicholson's 'Journal of Natural Philosophy,' &c. Quarto Series, Vol. I., April, September, 1797. NOTICES OF ACHROMATIC TELESCOPES. 273 Carbon, which he found to be a substance which possessed every requisite he could desire. Its index is nearly the same as that of the best flint glass, with a dispersive power more than double. It is perfectly colourless, beautifully transparent, and although very expansible, possesses the same, or very nearly the same optical properties under all circumstances to which it is likely to be ex- posed in astronomical observations — > except perhaps, direct observations on the solar disc, which will probably be found inadmissible. Mr. Barlow first constructed an object-glass with this fluid of 3 inches aperture, with which he could see the small star in Polaris with a power of 46, and with the higher powers several stars which are considered to require a good telescope, for exam- ple 70, p Ophinchi, 39 Bootis, the quadruple star 6 Lyrae, $ Aquarii, a Herculis, &c. He next constructed a 6 inch object-glass. "With this instrument the small star in Polaris is so distinct and brilliant, with a power of 143, that its transit might be taken with the utmost certainty. As the mode of constructing these telescopes is some- what novel, it may be expedient to enter some- what into detail. In the usual construction of achromatic tele- scopes, the two or three lenses composing the object-glass are brought into immediate contact ; and in the fluid telescope of Dr. Blair, the con- struction was the same, the fluid having been enclosed in the object-glass itself. But in Mr. Barlow's telescope, the fluid correcting lens is placed at a distance from the plate lens equal to half its focal length ; and it might be carried still farther back, and yet possess dispersive power to render the object-glass achromatic. By this means the fluid lens — which is the most difflcult N 5 274 THE PRACTICAL ASTRONOMER. power part of the construction — is reduced to one half or to less than one half of the size of the plate lens ; consequently, to construct a telescope of 10 or 12 inches aperture involves no greater diffi- culty in the manipulation, than in making a teles- cope of the usual description of 5 or 6 inches aperture, except in the simple plate lens itself ; and, hence, a telescope of this kind, of 10 or 12 feet length, will be equivalent in its focal to one of 16 or 20 feet. By this means, the tube may be shortened figure 6i. several feet and yet possess a focal ^ power more considerable than could be conveniently given to it on the usual principle of construction. This ® will be better understood from the annexed diagram, (fig. 61.) In this figure ABCD represent the tube of the 6 inch telescope, CD, the plate object-glass, F the first focus of rays, de the fluid concave lens, distant from the former 24 inches. The focal length MF being 48, and consequently, as 48 : 6 : : ' j |J| 24: 3 inches, the diameter of the fluid lens. The resulting compound focus is 62.5 inches. It is obvious, therefore, that the rays df, ef, arrive at the focus under the same conver- gency, and with the same light as if they proceeded from a lens of 6 inches diameter, placed at a distance ; j \ beyond the object-glass CD (as GH,) determined by producing those rays till they meet the sides of the tube in GH, namely at 62.5 inches be- yond the fluid lens. Hence, it is to NOTICES OF ACHROMATIC TELESCOPES. 275 obvious, the rays will converge as they would do from an object-glass GH of the usual kind with a focus of 10 feet 5 inches. We have thus, there- fore, shortened the tube 38.5 inches, or have at least the advantage of a focus 38.5 inches longer than our tube ; and the same principle may be carried much farther, so as to reduce the usual length of refracting telescopes nearly one half without increasing the aberration in the first glass beyond the least that can possibly belong to a telescope of the usual kind of the whole length. It should likewise be observed that the adjust- ment for focus may be made either in the usual way, or by a slight movement of the fluid lens, as in the Gregorian Reflectors, by means of the small speculum. Mr. Barlow afterwards constructed another and a larger telescope on the same principle, the clear aperture of which is 7.8 inches. Its tube is 11 feet, which, together with the eye-piece, makes the whole length 12 feet, but its effective focus is on the principle stated above, 18 feet. It carries a power of 700 on the closest double stars in South's and HerschePs catalogue, and the stars are, with that power, round and defined, although the field is not then so bright as could be desired. The telescope is mounted on a revolving stand, which works with considerable accuracy as an azimuth and altitude instrument. To give steadi- ness to the stand it has been made substantial and heavy ; its weight by estimation being 400 pounds, and that of the telescope 130 pounds, yet its motions are so smooth, and the pow T er so arranged, that it may be managed by one person with the greatest ease, the star being followed by a slight touch, scarcely exceeding that of the keys of a piano-forte. The focal length of the plate lens is 276 THE PRACTICAL ASTRONOMER. 78 inches, and of the fluid lens 59.8 inches — which at the distance of 40 inches produce a focal length of 104 inches, a total length of 12 feet, and an equivalent focus of 18 feet. The curves of the parallel meniscus checks for con- taining the fluid are — 30 inches, and 144 inches, the latter towards the eye. The curves for the plate lens are 56.4 and 144. There is an interior tube 5 inches diameter, and 3 feet 6 inches long, which carries the cell in which the fluid is en- closed, and an apparatus by which it may be moved backwards and forwards, so that the proper adjustment may be made for colour, in the first instance, and afterwards the focus is obtained by the usual rack-work motion. The following is the mode by which the fluid was enclosed. After the best position has been determined practically for the checks forming the fluid lens, these, with the ring between them ground and polished accu- rately to the same curves, are applied together, and taken into an artificial high temperature, ex- ceeding the greatest at which the telescope is ever expected to be used. After remaining here with the fluid some time, the space between the glasses is completely filled, immediately closed, cooled down by evaporation, and removed into a lower temperature. By this means a sudden condensa- tion takes place, an external pressure is brought on the checks, and a bubble formed inside, which is of course filled wuth the vapour of the fluid ; the excess of the atmospheric pressure beyond that of the vapour being afterwards always acting externally to prevent contact. The extreme edges are then sealed with the serum of human blood, or by strong fish-glue, and some thin pliable metal surface. By this process, Mr. Barlow says, 6 1 have every reason to believe the NOTICES OF ACHROMATIC TELESCOPES. 277 lens becomes as durable as any lens of solid glass. At all events I have the satisfaction of stating, that my first 3 inch telescope has now been com- pleted more than fifteen months, and that no change whatever has taken place in its perform- ance, nor the least perceptible alteration either in the quantity or the quality of the fluid.' The following are some of the observations which have been made with this telescope, and the tests to w r hich it has been subjected. The very small star which accompanies the pole-star is generally one of the first tests applied to teles- copes. This small point of light appeared bril- liant and distinct ; it was best seen with a power of 120, but was visible with a power of 700. The small star in Aldebaran was very distinct with a power of 120. The small star a Lyrae was dis- tinctly visible with the same power. The small star called by Sir J. Herschel Debilissima, between 4 e and 5 Lyras, whose existence, he says, could not be suspected in either the 5 or 7 feet equato- rial, and invisible also with the 7 and 10 feet reflectors of six and 9 inches aperture, but seen double with the 20 feet reflector, is seen very sa- tisfactorily double with this telescope, y Persei, marked as double in South and Herschel's cata- logue, at the distance of 28", with another small star at the distance of 3' 67", is seen distinctly sixfold, four of the small stars being within a con- siderably less distance than the remote one of y marked in the catalogue. And, rejecting the remote star, the principal, and the four other stars, form a minature representation of Jupiter and his satellites, three of them being nearly in a line on one side, and the other on the opposite. Castor, is distinctly double with 120, and well opened and stars perfectly round with 360 and 278 THE PRACTICAL ASTRONOMER. 700 : y Leonis and a Piscium are seen with the same powers equally round and distinct. In e Bootis, the small star is well separated from the larger, and its blue colour well marked with a power of 360. vj Coronae Borealis is seen double with a power of 360 and 700. 52 Orionis, £ Orionis, and others of the same class are also well defined with the same powers. In regard to the planets which happened to be visible — Venus appeared beautifully white and well defined with a power of 120, but showed some colour with 360. Saturn with the 120 power, is a very brilliant object, the double ring and belts being well and satisfactorily defined, and with the 360 power, it is still very fine. The moon also is remarkably beautiful, the edges and the shadows being well marked, while the quantity of light is such as to bring to view every minute distinction of figure and shade. The principal objections that may be made to this construction of a telescope are such as these : — Can the fluid be permanently secured? Will it preserve its transparency and other optical pro- perties ? Will it not act upon the surface of the glass and partially destroy it ? &c. To such enquiries Mr. Barlow replies, that experience is the only test we have ; our spirit levels, spirit thermometers, &c, show that some fluids at least may be preserved for many years, without experi- encing any change, and without producing any in the appearance of the glass tubes containing them. But should any of these happen, except the last, nothing can be more simple than to supply the means of replacing the fluid at any time, and by any person, without disturbing the adjustment of of the telescope. He expresses his hope that, should these experiments be prosecuted, an achro- matic telescope may ultimately be produced which NOTICES OF ACHROMATIC TELESCOPES. 279 shall exceed in aperture and power, any instru- ments of the kind hitherto attempted. If the prejudice against the use of fluids could be re- moved, he feels convinced that well-directed practice would soon lead to the construction of the most perfect instruments, on this principle, at a comparatively small expense. ' I am convinced,' he says, ' judging from what has been paid for large object-glasses, that my telescope, telescope stand, and the building for observation, with every other requisite convenience, have been constructed for a less sum than would be demanded for the object-glass only, if one could be produced of the same diameter of plate and flint-glass ; and this is a consideration which should have some weight, and encourage a perseverance in the principle of construction.'* ROGERS* ACHROMATIC TELESCOPE ON A NEW PLAN. The object of this construction is to render a small disc of flint-glass available to perform the office of compensation to a much larger one of crown-glass, and thus to render possible the con- struction of telescopes of much larger aperture than are now common, without hindrance from the difficulty at present experienced in procuring large discs of flint-glass. It is well known to * A more detailed account of the processes connected with the construction of this telescope, will be found in a paper presented to the Royal Society, in 1827, and published in the Philosophical Trans- actions of that Society, for 1828, and likewise another paper, pub- ished in the Transactions for 1829. From these documents, chiefly, the preceding account has been abridged. See also the 6 Edinburgh New Philosophical Journal for Jan., — April, 1 828, and Brewster's 6 Edinburgh Journal of Science,' for October, 1829. 280 THE PRACTICAL ASTRONOMER. those who are acquainted with telescopes, that in the construction of an ordinary achromatic object- glass, in which a single crown lens is compensated by a single one of flint, the two lenses admit of being separated only by an interval too small to afford any material advantage, in diminishing the diameter of the flint lens, by placing it in a narrower part of the cone of rays — the actual amount of their difference in point of dispersive power being such as to render the correction of the chromatic aberration impossible, when their mutual distance exceeds a certain limit. This inconvenience Mr. Rogers proposes to obviate, by employing, as a correcting lens — not a single lens of flint, but a compound one consisting of a convex crown and concave flint, whose foci are such as to cause their combination to act as a plain glass on the mean refrangible rays. Then it is evident, that by means of the greater dispersive power of flint than of crown glass, this will act as a concave on the violet, and as a convex on the red rays, and that the more powerfully, according as the lenses separately have greater powers or curvature. If then, such a compound lens be interposed between the object-glass of a telescope — supposed to be a single lens of plate or crown-glass — and its focus, it will cause no alteration in the focus for mean rays, while it will lengthen the focus for violet, and shorten it for red rays. Now this is precisely what is wanted to produce an achromatic union of all the rays in the focus ; and as nothing in this construction limits the powers of the individual correcting lenses, they may therefore be applied any where that convenience may dictate ; and thus, theoretically speaking, a disc of flint-glass, however small, may be made to correct the colour of one of crown however large. NOTICES OF ACHROMATIC TELESCOPES. 281 This construction, likewise, possesses other and very remarkable advantages. For, first, when the correcting lens is approximately constructed on a calculation founded on its intended aperture, and on the refractive and dispersive indices of its materials, the final and complete dispersion of colour may be effected, not by altering the lenses by grinding them anew, but by shifting the com- bination nearer to, or farther from, the object- glass, as occasion may require, along the tube of a telescope, by a screw motion, till the condition of achromaticity is satisfied in the best manner possible. And secondly, the spherical aberration may in like manner be finally corrected, by slightly separating the lenses of the correcting glass, whose surfaces should for this purpose be figured to curvatures previously determined by calculation, to admit of this mode of correction — a condition which Mr. Rogers finds to be always possible. The following is the rule he lays down for the determination of the foci of the lenses of the correcting glass : — 6 The focal length of either lens of the correcting lens is to that of the object- glass, in a ratio compounded of the ratio of the square of the aperture of the correcting lens to that of the object-glass, and of the ratio of the difference of the dispersive indices of the crown and flint glass, to the dispersive index of crown/ For example, to correct the colour of a lens of crown or plate glass of 9 inches aperture, and 14 feet focal length (the dimensions of the telescope of Fraunhofer at Dorpat) by a disc of flint glass 3 inches in diameter, the focus of either lens of the correcting lens will require to be about 9 inches. To correct it by a 4 inch disc will require a focus of about 16 inches each. Mr. Roger remarks, that it is not indispensable 282 THE PRACTICAL ASTRONOMER. to make the correcting glass act as a plane lens. It is sufficient if it be so adjusted as to have a shorter focus for red rays than for violet. If, pre- serving this condition, it be made to act as a con- cave lens, the advantage procured by Mr. Barlow's construction of reducing the length of the teles- cope with the same focal power, is secured, and he considers, moreover, that by a proper adapta- tion of the distances, foci, &c, of the lenses, we might hope to combine with all these advantages that of the destruction of the secondary spectrum, and thus obtain a perfect telescope. The above is an abstract of a paper read to the 6 Astronomical Society of London/ in April 1828, by A. Rogers, Esq. The reader will easily perceive that the princi- ple on which Mr. Rogers proposes to construct his telescope is very nearly similar to that of pro- fessor Barlow, described above, with this difference, that the correcting lens of the Professor's telescope is composed of a transparent fluid, while that of Mr. Rogers is a solid lens consisting of a convex crown and concave flint. The general object intended to be accomplished by both is the same, namely, to make a correcting lens of a compara- tively small diameter serve the purpose of a large disc of flint glass, which has hitherto been very expensive, and very difficult to be procured ; and likewise to reduce the length of the telescope while the advantage of a long focal power is secured. — A telescope, on this principle, was con- structed 7 or 8 years ago by Mr. Wilson, lecturer on Philosophy and Chemistry, Glasgow, before he was aware that Mr. Rogers had proposed a similar plan. I have had an opportunity of particularly inspecting Mr. Wilson's telescope, and trying its effects on terrestrial objects with high powers, and NOTICES OF ACHROMATIC TELESCOPES. 283 was on the whole highly pleased with its perform- ance. It appeared to be almost perfectly achro- matic, and produced a distinct and well-defined image of minute distant objects, such as small letters on sign-posts, at 2, 3 and 4 miles distant. But I had no opportunity of trying its effects on double stars or any other celestial objects. The instrument is above 6 feet long ; the object lens is a plano-convex of crown glass 4 feet focal dis- tance, and 4 inches diameter, the plain side next the object. At 26 inches distant from the object lens is the compound lens of 2 inches in diameter ; and the two lenses of which it is composed are both ground to a radius of 3f inches. That made of crown glass is plano-convex, the other, made of flint glass, is piano-concave, and are placed close toge- ther, the convex side being next the object, and the concave side next the eye. The greater refractive power of the flint glass renders the com- pound one slightly concave in its effect (although the radius of curvature is similar in both), and lengthens the focus to 6 feet from the object-glass; and this is consequently the length of the instru- ment. The compound corrector so placed inter- cepts all those rays which go to form the image in the field of view, producing there an achromatic image. The concave power of the corrector renders the image larger than if directly produced by a convex lens of the same focus. The concavity of the corrector is valuable also in this respect, that a very slight alteration in its distance from the object-glass, changes the focal distance much more than if it were plain, and enables us to adjust the instrument to perfect achromatism with great precision. 284 THE PRACTICAL ASTRONOMER. CHAPTER V. ON REFLECTING TELESCOPES. SECT. 1. HISTORY OF THE INVENTION, AND A GENERAL DESCRIPTION OF THE CONSTRUCTION OF THESE INSTRUMENTS. Reflecting telescopes are those which represent the images of distant objects by reflection, chiefly from concave mirrors. Before the achromatic telescope was invented, there were two glaring imperfections in refracting telescopes, which the astronomers of the 17th century were anxious to correct. The first was its very great length when a high power was to be applied, which rendered it very unwieldy and difficult to use. The second imperfection was the incorrectness of the image as formed by a single lens. Mathematicians had demonstrated that a pencil of rays could not be collected in a single point by a spherical lens, and also that the image transmitted by such a lens would be in some degree incurvated. After several attempts had been made to correct this imperfection by grinding lenses to the figure of one of the conic sections, Sir I. Newton happened to commence an examina- tion of the colours formed by a prism ; and having, ON REFLECTING TELESCOPES. 285 by the means of this simple instrument, discovered the different refrangibility of the rays of light — to which we have several times adverted in the pre- ceding descriptions — he then perceived that the errors of telescopes, arising from that cause alone, were some hundred times greater than such as were occasioned by the spherical figure of lenses ; which induced this illustrious philosopher to turn his attention to the improvement of telescopes by reflection. It is generally supposed that Mr. James Gregory — a son of the Rev. John Gregory, minister of Drumoak in the county of Aberdeen — was the first who suggested the construction of a reflecting telescope. He was a young man of uncommon genius, and an eminent mathema- tician ; and in the year 1 663, at the age of only 24, he published in London, his treatise entitled ' Optica Promota,' in which he explained the theory of that species of reflecting telescope which still bears his name, and which he stated as being his own invention. But as Gregory, according to his own account, was endowed with no mechanical dexterity, and could find no workman capable of realizing his invention — after some fruitless attempts to form proper specula, he was obliged to give up the pursuit ; so that this telescope remained for a considerable time neglected. It was several years after Gregory suggested the construction of reflecting telescopes, till Newton directed his attention fully to the subject. In a letter addressed to the secretary of the Royal Society, dated in February, 1672, he says, 1 Find- ing reflections to be regular, so that the angle of reflection of all sorts of rays was equal to the angle of incidence, I understood that, by their mediation, optic instruments might be brought to 286 THE PRACTICAL ASTRONOMER. any degree of perfection imaginable, providing a reflecting substance could be found which would polish as finely as glass, and reflect as much light as glass transmits, and the art of communicating to it a parabolic figure be also obtained. Amidst these thoughts I was forced from Cambridge by the intervening plague, and it was more than two years before I proceeded further.' It was towards the end of 1668, or in the beginning of the following year, when New T ton, being obliged to have recourse to reflectors, and not relying on any artificer for making the specula, set about the work himself, and early in the year 1672, completed two small reflecting telescopes. In these he ground the great speculum into a spherical concave, although he approved of the parabolic form, but found himself unable to accomplish it. These telescopes were of a con- struction somewhat different from what Gregory had suggested, and though only 6 inches long, were considered as equal to a 6 feet common refracting telescope. It is not a little singular, however, that we hear no more about the con- struction of reflectors till more than half a century afterwards. It was not till the year 1723, that any reflectors were known to have been made, adapted to celestial observations. In that year, Mr. Hadley, the inventor of the reflecting quad- rant, which goes by his name, published in No. 376 of the Philosophical Transactions, an account of a large reflector on Newton's plan, which he had just then constructed, the performance of which left no room to doubt that this invention would remain any longer in obscurity. The large speculum of this instrument was 62f inches focal distance and 5 inches diameter, was furnished with magnifying powers of from 190 to 230 times, ON REFLECTING TELESCOPES. 287 and equalled in performance the famous aerial telescope of Huygens of 123 feet in length.* Since this period, the reflecting telescope has been in general use among astronomers in most countries of Europe, and has received numerous improvements, under the direction of Short, Mudge, Edwards and Herschel — the last of whom constructed reflectors of 7, 10, 20, and even 40 feet in focal length, which far surpassed, in bright- ness and magnifying power, all the instruments of this description, which had previously been attempted. I shall now proceed to give a brief sketch of the nature of a reflecting telescope, and the different forms in which they have been proposed to be constructed. Fig. 62 represents the reflecting telescope as originally proposed by Gregory. ABEF repre- sents a tube open at AF towards the object ; at the other end is placed a concave speculum BE, with a hole CD in its centre, the focus of which is at e. A little beyond this focus, towards the object end of the telescope AF, is placed another small concave mirror G, having its polished face turned towards the great speculum, and is sup- ported by an arm GH fastened to a slider con- nected with the tube. At the end of the great tube BE is screwed in a small tube CDK1, con- taining a small plano-convex lens IK. Such are the essential parts of this instrument and their relative positions. It will be recollected in our description of the properties of concave mirrors (see page 92), that, when rays proceed from a * A particular description of this telescope, with the machinery for Gray's 'Abridgement of the Philosophical Transactions.' — Vol. vi. Parti, for 1723, pp. 147—152. moving it, illustrated with be seen in Reid and THE PRACTICAL ASTRONOMER. distant object, and fall upon a concave-speculum, they paint an image or representation of the object in its focus before the speculum. Now suppose two parallel rays ab falling on the speculum BE, in cd ; they are reflected to its focus e where an inverted image of the object is formed. This image is formed at a little more than the focal distance of the small speculum from its surface, and serves as it were for an object on which the small mirror may act. By the action of this mirror this first image is reflected to a point about /, where a second image is formed very large and erect. This image is magnified in the proportion of fG to eG, the rays from which are transmitted to the eye glass IK, through which the eye per- ceives the object clear and distinct, after the proper adjustments nave been made. B figure 66. ON REFLECTING TELESCOPES, 289 Suppose the focal distance of the great mirror was 9 inches, and the focal distance of the small mirror 1^ inch — were we to remove the eye piece of this telescope, and look through the hole of the great mirror, we should see the image of the object depicted upon the face of the small specu- lum, and magnified, in the proportion of 9 to 1^, or, 6 times, on the same principle as a common convex object glass 9 inches focal length, with an eye glass whose focus is li inch magnifies 6 times. This may be regarded as the first part of the magnifying power. If now, we suppose the small speculum placed a little more than 14 inch from the image formed by the great speculum, a second image is formed about /, as much exceeding the first in its dimensions as it exceeds it in distance from the small speculum, on the principle on which the object glass of a compound microscope forms a large image near the eye glass. Suppose this distance to be 9 times greater, then the whole magnifying power will be compounded of 6 multi- plied by 9, or 54 times. As a telescope it magni- fies 6 times, and in the microscope part 9 times. — Such is a general idea of the Gregorian telescope, the minute particulars and structure of which can only be clearly perceived by a direct inspection of the instrument. The Newtonian Reflector. — This instrument is somewhat different both in its form and in its mode of operation from that of Gregory. It is repre- sented in fig. 63, where BAEF is the tube, and BE, the object concave mirror, which reflects the parallel rays ab to a plane speculum G, placed 45o, or half a right angle to the axis of the concave speculum. This small plane reflector must be of an oval form, the length of the oval should be to the breadth as 7 to 5, on account of the obliquity o 290 THE PRACTICAL ASTRONOMER. of its position. It is supported on an arm fixed to the side of the tube ; an eye-glass is placed in a small tube, moveable in the larger tube, so as to be perpendicular to the axis of the large reflector, the perpendicular line passing through the centre of the small mirror. The small mirror is situated between the large mirror and its focus, that its distance from this focal point may be equal to the distance from the centre of the mirror to the focus of the eye-glass. When the rays ab from a distant object fall upon the large speculum at cd> they are reflected towards a focus at h ; but being- intercepted by the plane mirror G. they are re- flected perpendicularly to the eye-glass at I, in the side of the tube, and the image formed near that position at e is viewed through a small plano- convex lens. The magnifying power of this tele- scope is in the proportion of the focal distance of the speculum to that of the eye-glass. Thus, if the focal distance of the speculum be 36 inches, and that of the eye-glass ^ of an inch, the magni- fying power will be 108 times. It was this form of the reflecting telescope, that Newton invented, which Sir. W. Herschel adopted, and with which he made most of his observations and discoveries. The Cassegrainian Reflector. — This mode of the reflecting telescope, suggested by M. Casse- grain, a Frenchman, is represented in fig. 64. It is constructed in the same way as the Gregorian, with the exception of a small convex speculum G being substituted in the room of the small concave in Gregory's construction. As the focus of a convex mirror is negative, it is placed at a dis- tance from the large speculum equal to the dif- ference of their foci, that is, if the focal length of the large speculum be 18 inches, and that of the small convex 2 inches, they are placed at 16 ON REFLECTING TELESCOPES. 291 inches distant from each other, on a principle similar to that of the Galilean telescope, in which the concave eye-glass is placed within the focus of the object-glass by a space equal to the focal length of the eye-glass. In this telescope, like- wise, instead of two there is only one image formed, namely that in the focus of the eye-glass ; and, on this account some are of opinion that the distinctness is considerably greater than in the Gregorian. Mr. Ramsden was of opinion that this construction is preferable to either of the former reflectors, because the aberrations of the two metals have a tendency to correct each other, whereas in the Gregorian both the metals being concave, any error in the specula will be doubled. It is his opinion that the aberrations in the Casse- grainian construction to that of the Gregorian is as 3 to 5. The length of this telescope is shorter than that of a Gregorian of equal focal length, by twice the focal length of the small mirror, and it shows every thing in an inverted position, and consequently is not adapted for viewing terrestrial objects. Dr. Hook's Reflector. — Before the reflecting telescope was much known, Dr. Hook contrived one, the form of which is represented, fig. 65, which differs in little or nothing from the Grego- rian, except that the eye-glass I is placed in the hole of the great speculum BE. Martins Reflector. — Mr. Bengamin Martin, a distinguished writer on optical and philosophical science, about a century ago, described a new form of the reflecting telescope, approximating to the Newtonian structure, which he contrived for his own use. It is represented in fig. 66. ABEF is the tube, in which there is an opening or aper- ture OP, in the upper part. Against this hole o 2 292 THE PRACTICAL ASTRONOMER. within the tube is placed a large plane speculum GH, at half a right angle with the axis or sides of the tubes, with a hole CD perforated through its middle. The parallel rays a b falling on the inclined plane GH are reflected perpendicularly and parallel on the great speculum BE in the bottom of the tube. From thence they are re- flected converging to a focus e through the hole of the plane mirror CD, which being also the focus of the eye-glass IK, the eye will perceive the object magnified and distinct. In the figures referred to in the above descrip- tions, only one eye-glass is represented to avoid complexity ; but in most reflecting telescopes, the eye-piece consists of a combination of two plano- convex glasses, as in fig. 67, which produces a more correct and a larger field of view than a single lens. This combination is generally known by the name of the Huygenian eye-piece which shall be described in the section on the eye-pieces of telescopes. The following rule has been given for finding the magnifying power of the Gregorian telescope : — Multiply the focal distance of the great mirror by the distance of the small mirror from the image next the eye ; and multiply the focal distance of the small mirror by the focal distance of the eye- glass ; then divide the product of the former mul- tiplication by the product of the latter, and the quotient will express the magnifying power. The following are the dimensions of one of the reflect- ing telescopes constructed by Mr. Short — who was long distinguished as the most eminent maker of such instruments, on a large scale, and whose large reflectors are still to be found in various ob- servatories throughout Europe. The focal distance of the great mirror 9.6 inches ; or P m, fig. 67, its breadth FD 2.3 ; the ON REFLECTING TELESCOPES. 293 focal distance of the small mirror L n 1.5 — or 1| inch — its breadth g h 0.6 — or ^ of an inch ; the breadth of the hole in the great mirror UV, 0.5 — or half an inch — the distance between the small mirror and the next eye-glass LR, 14.2 ; the dis- tance between the two eye-glasses SR, 2.4; the focal distance of the eye-glass next the metal, 3.8. ; and the focal distance of the eye-glass next the eye, S a 1.1, or one inch and one tenth. The mag- nifying power of this telescope was about 60 times. figure 67. Taking this telescope as a standard, the following table of the dimensions and magnifying powers of Gregorian reflecting telescopes, as constructed by Mr. Short, has been computed. Focal distance of the great mirror. Breadth of the great mirror. Focus of the small speculum. i Breadth of the hole in the great speculum. Distance between the small speculum and the first eye-glass. Focal distance of the glass next the metals. Focal distance of the glass next the eye. Distance between the plain sides of the two glasses. Magnifying power. Distance between the second glass and the small eye-hole. P m D F L n U V L R R S R S 6 . . o d £ & £ « £ a £ m £ O S3 5. 65 1. 54 1. 10 0. 31 8. 54 2. 44 0. 81 1. 68 39 0. 41 9. 60 2. 30 1. 50 0. 39 14. 61 3. 13 1. 04 2. 09 60 0. 52 15. 50 3. 30 2. 14 0. 50 23. 81 3. 94 1. 31 2. 63 86 0. 66 36. 00 6. 26 3. 43 0. 65 41. 16 5. 12 1. 71 3. 41 165 0. 85 60. 00 9. 21 5. 00 0. 85 68. 17 6. 43 2. 14 4. 28 243 1. 07 294 THE PRACTICAL ASTRONOMER. Mr. Short — who was born in Edinburgh in 1710, and died near London, 1768 — was considered as the most accurate constructor of reflecting telescopes, during the period which intervened from 1732, to 1768. In 1743, he constructed a reflector for Lord Thomas Spencer, of 12 feet focal length, for which he received 600 guineas. He made several other telescopes of the same focal distance, with greater improvements and higher magnifiers ; and in 1752, finished one for the king of Spain, for which, with its whole ap- paratus, he received £1200. This was considered the noblest instrument of its kind that had then been constructed, and perhaps it was never sur- passed, till Herschel constructed his twenty and forty feet reflectors. High as the prices of large telescopes now are, Mr. Short charged for his in- struments at a much higher rate than opticians now do, although the price of labour, and every other article required in the construction of a telescope, is now much dearer. But he had then scarcely any competitor, and he spared neither trouble nor expense to make his telescopes perfect, and put such a price upon them as properly repaid him. The following table contains a state- ment of the apertures, powers, and prices of Gregorian telescopes, as constructed by Mr. James Short.* * Miss Short, who has erected and who superintends an observa- tory on the Calton hill, Edinburgh, is the descendant of a brother of Mr. Short. She is in possession of a large Gregorian reflector, about 12 feet long, made by Mr. Short, and mounted on an Equatorial axis. It was originally placed in a small observatory erected on the Calton hill, about the year 1776, but for many years past it has been little used. ON REFLECTING TELESCOPES. 295 Number. Focal length in inches. Diameter of aperture in inches. Magnifying powers. Prices in guineas. i i o o l.l 1 Power of 18 times Q o 2 4h 1.3 I 25 4 3 7 1.9 1 „ 40 „ 6 4 9h 2.5 2 Powers 40 and 60 „ 8 12 3.0 2 „ 55 and 85 , 10 5} 12 3.0 4 „ 35, 55, 85, and 110 " 14 7 18 3.8 4 „ 55, 95, 130, and 200 „ 20 8 24 4.5 4 „ 90, 150,230, and 300 „ 35 9 36 6.3 4 „ 100, 200, 300, and 400 „ 75 10 48 7.6 4 „ 120, 260, 380, and 500 „ 100 11 72 12.2 4 „ 200, 400, 600, and 800 „ 300 12 144 18.0 4 „ 300, 600, 900, and 1200 „ 800 From this table, it appears that Mr, Short charged 75 guineas for a 3 feet reflector, whereas such an instrument is now marked in the London opticians' catalogues at £23, when mounted on a common brass stand, and £39. 18s., when accom- panied with rack-work motions and other appara- tus. It is now generally understood that in the above table, Short always greatly overrated the higher powers of his telescopes. By experiment they were generally found to magnify much less than here expressed. General remarks on Gregorian Reflectors. — 1. In regard to the hole UV, of the great speculum — its diameter should be equal, or nearly so, to that of the small speculum L, fig. 67. For if it be less, no more parallel rays will be reflected than if it were equal to g h, and it may do harm in contracting the visible area within too narrow limits. Nor must it be larger than the mirror L, because some parallel rays will then be lost, and those of most consequence as being nearest the centre. 2. The small hole at e to which the eye is applied, must be nicely adjusted to the size of the 296 THE PRACTICAL ASTRONOMER. cone of rays proceeding from the nearest lens S. If it be larger, it will permit the foreign light of the sky or other objects to enter the eye, so as to pre- vent distinct vision ; for the eye should receive no light, but what comes from the surface of the small mirror L. If the hole be smaller than the cylinder of rays at e then some of the necessary light will be excluded, and the object rendered more obscure. The diameter of this hole may be found by dividing the aperture of the telescope in inches by its magnifying power. Thus, if we divide the diameter of one of Short's telescopes, the diameter of whose large speculum is 2. 80, by 60, the magnifying power, the quotient will be .0383, which is nearly the ~ of an inch. Some- times this hole is made so small as the & of an inch. When this hole is, by any derangement, shifted from its proper position, it sometimes re- quires great nicety to adjust it, and, before it is accurately adjusted, the telescope is unfit for accu- rate observation. 3. It is usual to fix a plate with a hole in it, at a b, the focus of the eye glass S, of such a diameter as will circumscribe the image, so as to exhibit only that part of it which appears distinct, and to exclude the superfluous rays. 4. There is an adjusting screw on the out- side of the great tube, connected with the small speculum, by which that speculum may be pushed backwards or forwards to adjust the instrument to distinct vision. The hand is applied for this pur- pose at T. Newtonian Telescopes. — These telescopes are now more frequently used for celestial observa- tions than during the last century, when Gregorian reflectors were generally preferred. Sir W. Her- schel was chiefly instrumental in introducing this form of the reflecting telescope to the more par- ON REFLECTING TELESCOPES. 297 ticular attention of astronomers, by the splendour and extent of the discoveries which it enabled him to make. In this telescope there is no hole required in the middle of the great speculum, as in the Gregorian construction, which circumstance secures the use of all the rays which flow from the central parts of the mirror. The following table contains a statement of the apertures and magnifying powers of Newtonian Telescopes, and the focal distances of their eye- glasses. The first column contains the focal length of the great speculum in feet ; the second, its linear aperture in inches ; the third, the focal dis- tance of the single glass in decimals, or in lOOOths of an inch, and the fourth column, contains the magnifying power. This portion of the table was constructed by using the dimensions of Mr. Had- ley's Newtonian Telescope, formerly referred to, as a standard — the focal distance of the great mirror being Q2\ inches, its medium aperture 5 inches, and power 208. The fifth, sixth, and seventh columns contains the apertures of the concave speculum, the focal lengths of the eye-glasses and the magnifying powers, as calculated by Sir D. Brewster, from a telescope of Mr. Hauksbee, taken as a standard ; whose focal length was 3 feet 3 inches, its aperture about 4 inches, and magnifying power 226 times. 298 THE PRACTICAL ASTRONOMER. Focal distance of con- cave metal. Aperture of concave metal. Focal distance of single eye- glass. Magnifying power. Sir D. Bre Aperture of the concave speculum. wster's Nu Focal length of the eye- glass. 3 Magnify- g ing power 5 Feet. Inch. Dec. in. Dec. Inch. Dec. In. Dec. Oj 0. 86 A u. 1 £7 10/ OO I. 34 0. 107 56 1 1. 44 A u. i no Cft 2. 23 0. 129 93 2 2. 45 A u. ZOO 1 ftO 1UZ 3. 79 0. 152 158 3 3. 31 A u. zol 1 QQ loo 5. 14 0. 168 214 4 4. 10 A U. Zol 17 1 1/ 1 6. 36 0. 181 265 5 4. 85 ft u. Oft7 7. 51 0. 192 313 6 5. 57 A u. ol 1 8. 64 0. 200=J 360 7 6. 24 A U. O^ft 9. 67 0. 209 403 8 6. 89 ft u. OQ7 10. 44 0. 218 445 9 7. 54 0. 344 314 11 69 0. 222 10 8. 16 0. 353 340 12.' 65 0. 228 527 11 8, 76 0. 362 365 13. 58 0. 233 566 12 9. 36 0. 367 390 14. 50 0. 238 604 13 9. 94 0. 377 414 15. 41 0. 243 642 14 10. 49 0. 384 437 16. 25 0. 248 677 15 11. 04 0. 391 460 17. 11 0. 252 713 16 11. 59 0. 397 483 17. 98 0. 256 749 17 12. 14 0. 403 506 18. 82 0. 260 784 18 12. 67 0. 409 528 19. 63 0. 264 818 19 13. 20 0. 414 550 20. 45 0. 268 852 20 13. 71 0. 420 571 21. 24 0. 271 885 One great advantage of reflecting telescopes above common refractors, is, that they will admit of eye glasses of a much shorter focal distance, and consequently, will magnify so much the more, for the rays are not coloured by reflection from a concave mirror, if it be ground to a true figure, as they are by passing through a convex glass though figured and polished with the utmost exactness. It will be perceived from the above table, that the focal length of the eye glasses is very small, the lowest there stated being only about of an inch, and the highest little more than £ of an inch focal distance. Sir W. Herschel obtained the high powers which he sometimes put upon his telescopes, by using small double convex ON REFLECTING TELESCOPES. 299 lenses for eye glasses, some of which did not ex- ceed the one fiftieth of an inch in focal length* When the focal length of the concave speculum, and that of the eye glass are given, the magnifying power is found by dividing the former by the latter, after having reduced the focal length of the concave speculum to inches. Thus the 6 feet speculum, multiplied by 12, produces 72 inches, which, divided by Brewster's number for the focus of the eye glass = 200, or \ of an inch, pro- duces a quotient of 360 as the magnifying power. It has been calculated that, if the metals of a Newtonian telescope be worked as exquisitely as those in Sir W. Herschel's 7 feet reflector, the highest power that such a telescope should bear with perfect distinctness, will be found by multi- plying the diameter of the great speculum in inches, by 74, and the focal distance of the single eye glass may be found by dividing the focal dis- tance of the great mirror by the magnifying power. Thus 6. 25 — the aperture in inches of Herschel's 7 feet Newtonian — multiplied by 74 is 462i, the magnifying power ; and 7 multiplied by 12, and divided by 462, 5 is 0.182 of an inch, the focal distance of the single eye glass required. But it is seldom that more than one half of this power can be applied with effect to any of the planetary bodies. For general purposes the power produced by multiplying the diameter of the speculum by 30, or 40, will be found most satisfactory. The following are the general prices of reflect- ing telescopes as made by the London opticians. £ s. A four feet, seven inch aperture, Gregorian Reflector ; with the vertical motions upon a new invented principle, as well as apparatus to render the tube more steady in observation ; according to the additional apparatus of small speculums, eye-pieces, micrometers, &c. from - - - - 80 to 120 0 300 THE PRACTICAL ASTRONOMER. £ s. Three feet long, mounted on a plain brass stand - - 23 2 Ditto, with rack- work motions, improved mounting, and metals ----- 39 18 Two feet long without rack -work, and with 4 magnifying powers, improved - - - - - - -1515 Ditto with rack- work motion - - - - - 22 1 Eighteen inch on a plain stand - - - 9 ,9 Twelve inch Ditto ------- 6 6 The above are the prices stated in Messrs, W. and S. Jones's catalogue. The following list of prices of the various kinds of reflecting telescopes is from Messrs, Tulley's (of Islington) catalogue. £ &. 1 foot Gregorian reflector, an pillar and claw stand, metal 2^ inches diameter, packed in a mahogany box - -66 11 foot ditto, on pillar and claw stand, metal 3 inches diameter, packed in mahogany box - - - - 11 13 2 feet ditto, metal 4 inches diameter - - - - 16 16 Ditto, ditto, with rack- work motions - - - - 25 4 3 feet ditto, metal 5 inches diameter, with rack-work motions - -- -- -- -42 0 Ditto, metal 6 inches diameter, on a tripod stand, with cen- tre of gravity motion - - - - - -68 5 4 feet ditto, metal 7 inches diameter, as above - - 105 0 6 feet ditto, metal 9 inches diameter, on an improved iron stand - 210 0 7 feet Newtonian reflectors, 6 inches aperture, mounted on a new and improved stand 105 0 Ditto, ditto, metal 7 inches diameter - - - - 126 0 9 feet ditto, metal 9 inches diameter - 210 0 10 feet ditto, metal 10 inches diameter - 315 0 12 feet ditto, metal 12 inches diameter - 525 0 Comparative brightness of achromatic and re- flecting telescopes. The late astronomer royal, Dr. Maskelyne, from a comparison of a variety of telescopes, was led to the following conclusion, — 6 that the aperture of a common reflecting teles- cope, in order to show objects as bright as the achromatic must be to that of an achromatic telescope as 8 to 5/ — in other words, an achro- matic whose object glass is 5 inches diameter, ON REFLECTING TELESCOPES. 301 will show objects with as great a degree of bright- ness as a reflector whose large speculum is 8 inches in diameter. This result, if correct, must be owing to the small number of rays reflected from a speculum compared with the number transmitted through an achromatic object glass. SECT. 2. THE HERSCHELIAN TELESCOPE. Soon after Sir William Herschel commenced his astronomical career, he introduced a new era in the history of reflecting telescopes. After he had cast and polished an immense variety of specula for telescopes of different sizes — he, at length, in the year 1782, finished a 20 feet reflector with a large aperture. Being sensible of the vast quantity of light which is lost by a second reflection from the small speculum, he determined to throw it aside altogether, and mounted this 20 feet reflec- tor on a stand that admitted of being used without a small speculum in making front observations — that is, in sitting with his back to the object, and looking directly towards the surface of the specu- lum. Many of his discoveries and measurements of double stars were made with this instrument, till, at length, in the year 1785 he put the finish- ing hand to that gigantic speculum, which soon be- came the object of universal astonishment, and which was intended for his forty feet reflecting telescope ; he had succeeded so well in construct- ing reflecting telescopes of comparatively small aperture, that they would bear higher magnifying powers than had ever previously been applied ; but he found that a deficiency of light could only be remedied by an increased diameter of the large speculum, which therefore was his main 302 THE PRACTICAL ASTRONOMER. object, when lie undertook to accomplish a work which to a man less enterprising, would have appeared impracticable. The difficulties he had to overcome were numerous ; particularly in the operative department of preparing, melting, annealing, grinding, and polishing a mass of metal that was too unwieldly to be moved without the aid of mechanical powers. At length, how- ever, all difficulties having been overcome, this magnificent instrument was completed with all its complicated apparatus, and erected for observa- tion, on the 28th of August, 1789, and on the same day the sixth satellite of Saturn was detected, as a prelude of still farther discoveries which were afterwards made by this instrument, in the celes- tial regions. It would be too tedious to attempt a description of all the machinery and apparatus connected with this noble instrument. The reader who wishes to peruse a minute description of the stairs, ladders, platform, rollers, and of every circumstance relat- ing to joiner's work, carpenter's work, smith's work, and other particulars connected with the formation and erection of this telescope, will find the details recorded in the 85th volume of the Philosophical Transactions of the Royal Society of London, for 1795, in which there are sixty- three pages of letter press, and eighteen plates illustrative of the subject. I shall content myself with giving a short outline of the essential parts belonging to this instrument. The tube of this telescope is made of rolled or sheet iron, joined together without rivets; the thickness of the sheets is somewhat less than 3 ^ part of an inch, or 14 pounds weight for a square foot ; great care was taken that the cylindrical form should be secured, and the whole was coated ON REFLECTING TELESCOPES. 303 over three or four times with paint, inside and outside, to secure it against the damp. This tube was removed from the place in which it was formed by twenty-four men, divided into six sets ; so that two men on each side, with a pole of 5 feet long in their hands, to which was affixed a piece of course cloth, 7 feet long going under the tube, and joined to a pole 5 feet long, in the hands of two other men, assisted in carrying the tube. The length of this tube is 39 feet 4 inches, the diame- ter 4 feet 10 inches; and, on a moderate compu- tation, it was ascertained, that a wooden tube of proper dimensions would have exceeded an iron one in weight by at least 3000 pounds. Reckon- ing the circumference of the tube 15 feet, its length 39^ feet, and 14 lib. for the weight of a square foot, it must have contained 590 square feet, and weighed 8,260 pounds. Various hoops were fixed within the tube, and longitudinal bars of iron connecting some of them are attached to the two ends of the tube, by way of bracing the sheets, and preserving the shape perfect, when the pulleys are applied to give the necessary elevation at the upper end, and that the speculum may be kept secure at the lower end. The lower end of the tube is firmly supported on rollers that are capable of being moved forwards or backwards by a double rack, connected with a set of wheels and pinions. By an adjustment at the lower extre- mity of the tube, the speculum is turned to a small inclination, so that the line of collimation may not be coincident with the longitudinal axis of the tube, but may cross the tube diagonally, and meet the eye in the air at about two inches from the edge of the tube, which is the peculiarity of the construction, that supersedes the necessity of applying a second reflector. Hence no part 304 THE PRACTICAL ASTRONOMER. of the head of the observer intercepts the incident rays, and the observation is taken with the face looking at the speculum, the back being turned to the object to be observed. The large speculum is enclosed in a strong iron ring, braced across with bars of iron, and an enclosure of iron and ten sheets makes a case for it. It is lifted by three handles of iron attached to the sides of the ring, and is put into and taken out of its proper place in the tube by the help of a moveable crane, running on a carriage, which operation requires great care. The speculum is made of a metallic composition, and is 492 inches in diameter ; but the concave polished surface is only 48 inches, or 4 feet in diameter. Its thick- ness is 3\ inches ; and when it came from the cast its weight was 2118 pounds. The metals for its formation were procured at a warehouse in Thames Street, London, where they kept ingots of two kinds ready made, one of white, and the other of bell-metal ; and it was composed of two ingots of bell-metal for one of white. It was not to be expected that a speculum of such large dimensions, could have a perfect figure imparted to its surface, nor that the curve, whatever it might be, would remain identically the same in changes of temperature ; therefore we are not surprised when we are told, that the magnifying powers used with this telescope seldom exceeded 200 ; the quantity of light collected by so large a surface being the principal aim of the maker. The raising of the balcony, on which the observer stands, and the sliding of the lower end of the tube, in which the speculum rests, are effected by separate tackles, and require only occasional mo- tions ; but the elevation of the telescope requires the main tackle to be employed, and the motion ON REFLECTING TELESCOPES. 305 usually given in altitude at once was two degrees ; the breadth of the zone in which the observations were made, as the motion of the sphere in right ascension brought the objects into view. A star, however, could be followed for about a quarter of an hour. Three persons were employed in using this telescope, one to work the tackle, another to observe, and a third to mark down the observa- tions. The elevation was pointed out by a small quadrant fixed to the main tube, near the lower end, but the polar distance was indicated by a piece of machinery, worked by a string, which continually indicated the degree and minute on a dial in the small house adjoining, while the time was shown by a clock in the same place, Miss Herschel performing the office of Registrar. At the upper end the tube is open, and directed to the part of the heavens intended for observa- tion, and the observer, standing on the foot board, looks down the tube, and perceives the object by rays reflected from the speculum, through the eye glass at the opening of the tube. When the telescope is directed to any objects near the zenith, the observer is necessarily at an elevation at least 40 feet from the ground. Near the place of the eye glass is the end of a tin pipe, into which a mouth-piece may be placed, so that, during an observation, a person may direct his voice into this pipe, while his eye is at the glass. This pipe, which is 1^ inch in diameter runs down to the bottom of the tube, where it goes into a turning joint, thence into a drawing tube, and out of this into another turning joint, from whence it proceeds, by a set of sliding tubes towards the front of the foundation timber. Its use is to convey the voice of the observer to his assistants, for at the last place, it divides itself into two branches, one going into the observatory, the other into the workman's 306 THE PRACTICAL ASTRONOMER. room, ascending in both places through the floor, and terminates in the usual shape of speaking trumpets. Though the voice passes in this manner through a tube, with many inflections, and through not less than 115 feet, it requires very little exer- tion to be well understood. To direct so unwieldy a body to any part of the heavens at pleasure, many mechanical contrivances were evidently necessary. The whole apparatus rests upon rollers, and care was previously taken of the foundation in the ground. This consists of concentrical brick walls, the outermost 42 feet, the innermost 21 feet in diameter, 2 feet 6 inches deep under ground, 2 feet 3 inches broad at the bottom, and 1 foot 2 inches at the top, capped with paving stones 3 inches thick, and 12f inches broad. In the centre is a large post of oak, framed together with braces under ground, and walled fast to brick-work to make it steady. Round this centre the whole frame is moved horizontally by means of 20 rollers, 12 upon the outer, and 8 upon the inner wall. The vertical motion is given to the instrument by means of ropes and pullies, passing over the main beam supported by the ladders. These ladders are 49 feet long, and there is a moveable gallery with 24 rollers to ease its motion. There is a stair-case intended for persons who wish to ascend into the gallery, without being obliged to go up the ladder. The ease with which the horizontal and vertical motions may be communicated to the tube may be conceived, from a remark of Sir W. Herschel, that, in the year 1789, he several times observed Saturn, two or three hours before and after its meridian passage with one single person to con- tinue, at his directions, the necessary horizontal and vertical motions. ON REFLECTING TELESCOPES. 807 By this telescope the sixth and seventh satellites of Saturn were discovered, only one of which is within the reach of the 20 feet reflector, or even of a 25 feet instrument. The discovery of the satellites of the planet Uranus, however, was made by the 20 feet reflector, but only after it had been converted from the Newtonian to the Herschelian construction — which affords a proof of the supe- riority of the latter construction over the former when the same speculum is used. Never had the heavens before been observed with so extraordinary an instrument as the forty feet reflector. The nebulosities which are found among the fixed stars, in various regions of the heavens, appeared almost all to resolve themselves into an innumera- ble multitude of stars ; others, hitherto impercep- tible, seemed to have acquired a distinct light. On the entrance of Sirius into the field of the telescope, the eye was so violently affected, that stars of less magnitude could not immediately after be perceived ; and it was necessary to wait for 20 minutes before these stars could be observed. The ring of Saturn had always before ceased to be visible when its plane was directed towards the earth ; but the feeble light which it reflects in that position was enough for Herschel's instru- ment, and the ring, even then, still remained visible to him. It has been generally considered that this teles- cope was capable of carrying a power of 6000 times ; and perhaps for the purpose of an experi- ment, and for trying its effect on certain objects, such a power may have been applied, — in which case the eye-glass must have been only | 5 of an inch focal distance, or somewhat less than one twelfth of an inch. But such a power could not be generally applied, with any good effect, to the 308 THE PRACTICAL ASTRONOMER. planetary bodies ; and I question much whether any power above 1000 times was ever generally used. For, it is the quantity of light which the telescope collects, more than the magnifying power, that enables us to penetrate, with effect, into the distant spaces of the firmament : and hence, as above stated, the power seldom exceeded 200, which on account of the large diameter of the speculum, would enable the instrument to penetrate into the distant celestial spaces perhaps further than if a power of as many thousands of times had been applied. Sir John Herschel, who inherits all the science, skill, and industry of his father, some time ago ground and polished a new speculum for the 20 feet tube, formerly noticed, which is connected with a stand, pulleys and other appendages, similar to those above described, though of smaller di- mensions. This telescope shows the double stars exceedingly well defined, and was one of the principal instruments used informing his catalogue of these objects which was presented to the Royal Society, in conjunction with that of Sir James South, about the year 1828. I suppose, it is likewise the same telescope with which Sir John lately made his Sidereal observations at the Cape of Good Hope. sect. 3. — ramage's large reflecting telescope. The largest front view reflecting telescope in this country — next to Herschel's 40 feet instru- ment—is that which was erected at the Royal Observatory at Greenwich, in the year 1820, by Mr. Ramage of Aberdeen. The diameter of the concave reflector is 15 inches, and its focal length ON REFLECTING TELESCOPES. 309 25 feet. It is erected on machinery which bears a certain resemblance to that of HerschePs, which we have now described ; but the mechanical ar- rangements are greatly simplified, so that the in- strument is manageable by an observer without an assistant. The tube is composed of a twelve- sided prism of deal f inch thick. At the mouth is a double cylinder of different diameters on the same axis ; around this a cord is wound by a winch, and passes up from the small cylinder, over a pulley, and down through another pulley on to the large cylinder. When the winch, therefore, is turned to raise the telescope, the endless cord is unwound from the smaller cylinder, and wound on to the larger, the difference of the size of the two cylinders will be double the quantity raised, and a mechanical force to any extent may thus be obtained, by duly proportioning the diameters of the two cylinders : by this contrivance the neces- sity of an assistant is superseded. The view through this instrument first astonished those ob- servers who had not been accustomed to examine a heavenly body with a telescope possessing so much light ; and its performance was deemed quite extra- ordinary. But when the first impression had sub- sided, and different trials had been made in differ- ent states of the atmosphere, it was discovered that the central portion of the speculum was more perfectly figured than the ring bordering on the extreme edges. When the aperture was limited to ten or twelve inches, the performance as to the distinctness in its defining power, was greatly improved, and the light was so brilliant, that the Astronomer Royal was disposed to enter- tain an opinion, that it might equal that of a good achromatic refractor of the same dimensions. When, however, very small and obscure objects 310 THE PRACTICAL ASTRONOMER. are to be observed, the whole light of the entire aperture may be used with advantage on favourable evenings. The eye-pieces adapted to this telescope have powers which magnify the object linearly from 100 to 1500 times, which are competent to fulfil all the purposes of vision when cleared of aber- ration. When the telescope is placed in the plane of the meridian and elevated together with the gallery, into any required altitude, the meridional sweeps, formerly practised by Sir W. Herschel, and continued by Sir John with great success, in the examination of double stars and nebula, may be managed with great ease. Mr. Ramage had a telescope of about the same size, erected in an open space in Aberdeen, which I had an opportunity of inspecting when I paid a visit to that gentleman in 1833 ; but cloudy weather prevented my obtaining a view of any celestial bodies through it. He showed me at that time two or three large speculums, from 12 to 18 inches in diameter, which he had finished some time before, and which appeared most beautifully polished. He told me, too, that he had ground and polished them simply with his hand, without the aid of any machinery or mechanical power — a circumstance which, he said, astonished the opti- cians of London, when it was stated, and which they considered as almost incredible. His expe- rience in casting and polishing metals of various sizes, during a period of 15 or 16 years, qualified him to prepare specula of great lustre, and with an unusually high polish. It has been asserted that a fifty feet telescope by Ramage of 21 inches aperture was intended to be substituted for the 25 feet instrument erected at Greenwich, and the speculum it is understood, was prepared, and ON REFLECTING TELESCOPES. 311 ready for use, provided the Navy Board was dis- posed to defray the expense of carrying the plan into execution. But, unfortunately, this ingeni- ous artist was unexpectedly cut off in the midst of his career, about the year 1835. SECT. 4. — THE AERIAL REFLECTOR CON- STRUCTED BY THE AUTHOR. A particular description of this telescope was given in the 6 Edinburgh New Philosophical Journal' for April — July, 1826, conducted by Professor Jameson, the greater part of which was copied in the 6 London Enclyclopedia/ under the article Telescope. From this description I shall endeavour to condense a brief account of this in- strument with a few additional remarks. About the year 1822, an old speculum 27 inches in focal length, very imperfectly polished happened accidentally to come into my possession ; and feeling no inclination to fit it up in the Gregorian form, I formed the resolution of throwing aside the small speculum, and attempting the front view notwithstanding the uniform assertion of opticians, that such an attempt in instruments of a small size is impracticable. I had some ground for ex- pecting success in this attempt, from several ex- periments I had previously made, particularly from some modifications I had made in the construction of astronomical eye-pieces, which have a tendency to correct the aberration of the rays of light, when they proceed somewhat obliquely from a lens or speculum. In the first instance, I placed the speculum at the one end of a tube of the form of a segment of a cone — the end next the eye being somewhat wider than that at which the speculum 312 THE PRACTICAL ASTRONOMER, was fixed, and its length about an inch shorter than the focal distance of the mirror. A small tube for receiving the different eye-pieces was fixed in the inside of the large tube at the end next the eye, and connected with an apparatus by which it could occasionally be moved either in a vertical or horizontal direction. With the instru- ment fitted up in this manner, I obtained some interesting views of the moon, and of terrestrial objects. But finding that one side of the tube intercepted a considerable portion of light from the object, I determined to throw aside the tube altogether, and to fit up the instrument on a dif- ferent plan. A short mahogany tube, about 3 inches long, was prepared, to serve as a socket for holding the speculum. To the side of this tube an arm was attached, about the length of the focal distance of the mirror, at the extremity of which a brass tube for receiving the eye-pieces, was fixed, connected with screws and sockets, by which it might be raised or depressed, and turned to the right hand or to the left, and with adjusting apparatus by which it might be brought nearer to or farther from the speculum. Fig. 69 exhibits a general repre- sentation of the instrument in profile. AB is the short tube which holds the speculum ; CD the arm which carries the eye-tubes, which consists of two distinct pieces of mahogony ; the part D being capable of sliding along the under side of C, through the brass sockets EF. To the under part of the socket F is attached a brass nut with a female screw, in which the male screw ah acts by applying the hand to the knob e, which serves for adjusting the instrument to distinct vision. G is the brass tube which receives the eye-pieces. It is supported by a strong brass wire de, which ON REFLECTING TELESCOPES. 313 figure 69. passes through a nut connected with another strong wire, which passes through the arm D. By means of the nut /this tube may be elevated or depressed, and firmly fixed in its proper position ; and by the nut d it may be brought nearer to or further from, the arm D. By the same apparatus, it is also rendered capa- ble of being moved either in a vertical or horizon- tal direction : but when it is once adjusted to its proper position, it must be firmly fixed, and requires no further attention. The eye-piece re- presented in this figure is the one used for terres- trial objects, which consists of the tubes belonging to a pocket achromatic telescope. When an as- tronomical eye-piece is used, the length of the instrument extends only to the point I. In look- ing through this telescope, the right eye is applied at the point H, and the observer's head is under- 314 THE PRACTICAL ASTRONOMER. stood to be uncovered, or, at least, tightly covered with a thin cap. For those who use only the left eye, the arm would require to be placed on the opposite side of the tube, or the arm, along with the tube, be made to turn round 180 degrees. Fig. 70 represents a front, or rather an oblique figure 70. view of the instrument, in which the position of the speculum may be seen. All the specula which I fitted up in this form, having been originally intended for Gregorian reflectors, have holes in their centres. The eye-piece is therefore directed to a point nearly equi-distant from the hole to the left hand edge of the speculum, that is, to the point a. In one of these instruments fitted up with a four feet speculum, the line of vision is ON REFLECTING TELESCOPES. 315 directed to the point b on the opposite side of the speculum, but, in this case, the eye-tube is re- moved farther from the arm, than in the former case. The hole in the centre of the speculum is obviously a defect in this construction of a reflec- ing telescope, as it prevents us from obtaining the full advantage of the rays which fall near the centre of the mirror ; yet the performance of the instruments, even with this disadvantage, is superior to what we should previously have been led to expect. The principal nicety in the construction of this instrument, consists in the adjustment and proper direction of the eye-tube. There is only one position in which vision will be perfectly distinct. It must be neither too high nor too low, — it must be fixed at a certain distance from the arm, — and must be directed to a certain point of the specu- lum. This position must be ultimately determined by experiment, when viewing terrestrial objects. A person unacquainted with this construction of the telescope, would, perhaps, find it difficult, in the first instance, to make this adjustment ; but were it at any time deranged, through accident or otherwise, I can easily make the adjustment anew, in the course of a minute or two. In pointing this telescope to the object intended to be viewed, the eye is applied at K, fig, 69, and looking along the arm, towards the eye-piece, till it nearly coincide with the object, it will, in most cases, be readily found. In this way I can easily point this instrument to Jupiter or Saturn, or to any of the other planets, visible to the naked eye. even when a power of 160 or 170 times is applied. "When high magnifying powers, however, are used, it may be expedient to fix, on the upper part of the short tube in which the speculum rests, a p 2 316 THE PRACTICAL ASTRONOMER. Finder, such as that which is used in Newtonian telescopes. When the moon is the object intended to be viewed, she may be instantly found by moving the instrument till her reflected image be seen from the eye-end of the telescope on the face of the mirror. I have fitted up several instruments of the above description with specula of 16, 27, 35, and 49 inches focal distance. One of these having a speculum of 27 inches focal length, and an astro- nomical eye-piece, producing a magnifying power of about 90 times, serves as a good astronomical telescope. By this instrument the belts and satel- lites of Jupiter, the ring of Saturn, and the moun- tains and cavities of the moon, may be contem- plated with great ease and distinctness. With a magnifying power of 35 or 40 times, terrestrial objects appear remarkably bright and well-defined, When compared with a Gregorian, the quantity of light upon the object appears nearly doubled, and the image is equally distinct — although the specu- lum has several blemishes, and its surface is but imperfectly polished. It represents objects in their natural colours, without that dingy and yel- lowish tinge which appears when looking through a Gregorian. Another of these instruments is about four feet long. The speculum which belongs to it is a very old one : when it came into my possession, it was so completely tarnished, as scarcely to reflect a ray of light. After it was cleaned, it appeared to be scarcely half polished, and its surface is covered with yellowish stains which cannot be erased. Were it fitted up upon the Gregorian plan, it would, I presume, be of very little use, unless when a very small magnify- ing power was applied. Yet, in its present form, it bears, with distinctness, a magnifying power of ON REFLECTING TELESCOPES. 317 130 times, and is equal in its performance to a 3i feet achromatic. It exhibits distinct and interest- ing views of the diversities of shade, and of the mountains, vales, cavities, and other inequalities of the moon's surface. With a power of about 50 times, and a terrestrial eye-piece, it forms an excellent telescope for land objects, and exhibits them in a brilliant and novel aspect. The smallest instrument I have attempted to construct on this plan, is only 5j inches focal distance, and If inch diameter. With a magnifying power of about 15 times, it shows terrestrial objects with distinctness and brilliancy. But I should deem it inexpedient to fit up any instrument of this description with specula of a shorter focal distance than 20 or 24 inches. The longer the focal distance the more distinctness may be expected, although the aper- ture of the speculum should be comparatively small. The following are some of the properties and advantages peculiar to this construction of the reflecting telescope. 1. It is extremely simple, and may be fitted up at a comparatively small expense. Instead of large and expensive brass tubes, such as are used in the Gregorian and Newtonian construction, little more is required than a short mahogany tube, two or three inches long, to serve as a socket for the speculum, with an arm connected with it about the focal length of the speculum. The ex- pense of small specula, either plain or concave, is saved, together with the numerous screws, springs, &c, for centering the two specula, and placing the small mirror parallel to the large one. The only adjustment requisite in this construction, is that of the eye-tube to the speculum ; and, by means of the simple apparatus above described, it 318 THE PRACTICAL ASTRONOMER. can be effected in the course of a few minutes. Almost the whole expense of the instrument con- sists in the price of the speculum and the eye- pieces. The expense of fitting up the four feet speculum, alluded to above — exclusive of specu- lum and eye-piece — but including mahogany tube and arm, brass sockets, screws, eye-tube, brass joint, and a cast-iron stand painted and varnished, did not amount to £1 : 8s. A Gregorian of the same size would have required a brass tube at least 4^ feet in length, which would cost 5 or 6 guineas, besides the apparatus connected with the small speculum, and the additional ex- pense connected with the fitting up of the joint and stand requisite for supporting and steadying so unwieldy an instrument. While the one in- strument would require two persons to carry it from one room to another, and would occupy a considerable space in an ordinary apartment, the other can be moved, with the utmost ease, with one hand, to any moderate distance, and the space it occupies is extremely small. 2. It is more convenient for viewing celestial objects at a high altitude, than other telescopes. When we look through a Gregorian reflector or an achromatic telescope of 4 or 5 feet in length, to an object elevated 50 or 60 degrees above the horizon, the body requires to be placed in an uneasy and distorted position, and the eye is somewhat strained, while the observation is con- tinued. But when viewing similar objects by the Aerial Reflector, we can either stand perfectly erect, or sit on a chair, with the same ease as we sit at a desk when reading a book or writing a letter. In this way, the surface of the moon or any of the planets, may be contemplated for an hour or two, without the least weariness or fatigue. ON REFLECTING TELESCOPES. 319 A delineation of the lunar surface may be taken with this instrument with more ease and accuracy than with any other instrument, as the observer can sketch the outline of the object by one eye on a tablet placed a little below the eye-piece, while the other eye is looking at the object. For the purpose of accommodating the instrument to a sitting or standing posture a small table was constructed, capable of being elevated or depressed at pleasure, on which the stand of the telescope is placed. When the telescope is 4 or 5 feet long, and the object at a very high elevation, the instru- ment may be placed on the floor of the apartment, and the observer will stand in an erect position. 3. This instrument is considerably shorter than a Gregorian telescope whose mirror is of the same focal length. When an astronomical eye- piece is used, the whole length of the instrument is nothing more than the focal length of the specu- lum. But a Gregorian whose large speculum is 4 feet focus, will be nearly 5 feet in length, in- cluding the eye-piece. 4. The Aerial Reflector far excels the Grego- rian in brightness. The deficiency of light in the Gregorians is owing to the second reflection from the small mirror ; for it has been proved by expe- riment that nearly the one half of the rays of light which fall upon a reflecting surface is lost by a second reflection. The image of the object may also be presumed to be more correct, as it is not liable to any distortion by being reflected from another speculum. 5. There is less tremor in these telescopes than in Gregorian Reflectors. One cause, among others, of the tremors complained of in Grego- rians is, I presume, the formation of a second image at a great distance from the first, besides 320 THE PRACTICAL ASTRONOMER. that which arises from the elastic tremor of the small speculum, when carried by an arm supported only at one end. But as the image formed by the speculum in the aerial telescope is viewed directly, without being exposed to any subsequent reflection, it is not so liable to the tremors which are so frequently experienced in other reflectors. Notwithstanding the length of the arm of the 4 feet telescope above mentioned, a celestial object appears remarkably steady, when passing across the field of view, especially when it is at a mode- rate degree of altitude ; and it is easily kept in the field by a gentle motion applied to the arm of the instrument. In prosecuting my experiments in relation to these instruments, I wished to ascertain what effect might be produced by using a part of a speculum instead of the whole. For this purpose, I cut a speculum, three feet in focal length, through the centre, so as to divide it into two equal parts, and fitted up each part as a distinct teles- cope ; so that I obtained two telescopes from one speculum. In this case I found that each half of the speculum performed nearly as well as the whole speculum had done before, at least there appeared to be no very sensible diminution in the brightness of the object, when viewed with a moderate power, and the image was equally accu- rate and distinct ; so that if economy were a par- ticular object aimed at in the construction of these instruments, two good telescopes might be obtained from one speculum ; or if a speculum happened to be broken accidentally into large fragments, one or more of the fragments might be fitted up on this principle to serve as a tolerably good telescope. From the experiments I have made in reference to these instruments, it is demonstrable, that a ON REFLECTING TELESCOPES. 321 tube is not necessary in the construction of a re- flecting telescope — at least on the principle now stated — whether it be used by day or by night for terrestrial or celestial objects ; for I have fre- quently used these telescopes in the open air in the day time, without any inconvenience from ex- traneous light. Therefore, were a reflecting telescope of 50 or 60 feet in length to be con- structed, it might be fitted up at a comparatively small expence, after the expense of the metallic substances, and of casting, grinding, and polishing the speculum is defrayed. The largest instrument of this description which has hitherto been con- structed is the 40 feet reflector of Sir W. Herschel. This complicated and most unwieldy instrument had a tube of rolled or sheet iron 39 feet 4 inches in length, about 15 feet in circumference, and weighed about 8000 pounds. Now, I conceive that such enormous tubes, in instruments of such dimensions, are altogether unnecessary. Nothing more is requisite than a short tube for holding the speculum. Connected with one side of this tube (or with both sides were it found necessary), two strong bars of wood, projecting a few feet beyond the speculum end, and extending in front as far as the focal length of the mirror, and connected by cross bars of wood, iron or brass — would be quite sufficient for a support to the eye-piece, and for directing the motion of the instrument. A telescope of 40 or 50 feet in length, constructed on this plan, would not require one fifth of the expense, nor one fourth of the apparatus and mechanical power for moving it to any required position, which were found necessary in the con- struction of Sir W. Herschel's large reflecting telescope. The idea here suggested will perhaps be more readily appreciated by an inspection of p 5 322 THE PRACTICAL ASTRONOMER. figure 71. C fig. 71, where A is the short tube, BC and DE the two large bars or arms, connected with cross bars, for the purpose of securing strength and steadiness. At I and K, behind the speculum, weights might be applied, if necessary, for coun- terbalancing the lever power of the long arm. F represents the position of the eye-piece, and GH the joint and part of the pedestal on which the instrument is placed. With regard to telescopes of smaller dimensions, as from 5 to 15 feet in focal length — -with the exception of the expense of the specula and eye-pieces — they might be fitted up for a sum not greater than from 3 to 10 or 15 guineas. Were any person to attempt the construction of those telescopes, it is possible he might not succeed in his first attempts without more minute directions than I have yet given. The following directions may perhaps tend to guide the experi- menter in adjusting the eye-tube to the speculum, which is a point that requires to be particularly ON REFLECTING TELESCOPES. 323 attended to, and on which depends the accurate performance of the instrument. After having fixed the eye-piece nearly in the position it should occupy, and directed the instrument to a particular object, look along the arm of the telescope, from K (fig. 69.) to the extremity of the eye-piece at H, and observe, whether it nearly coincides with the object. If the object appear lower than this line of vision, the eye-piece must be lowered, and if higher, it must be raised, by means of the nuts and screws at gd and fe, till the object and the line of vision now stated nearly co-incide. The eye-piece should be directed as nearly perpendicular to the front of the speculum as possible, but so that the reflected image of one's head from the mirror shall not interfere to obstruct the rays from the object. An object may be seen with an approximate degree of distinctness, but not accurately, unless this adjustment be pretty accurately made. The astronomical eye-pieces used for these telescopes are fitted with a brass cap which slides on the end next the eye, and is capable of being brought nearer to or farther from the first eye-glass. In the centre of this cap, next the eye, is a small hole, about the ~ 0 th or ~th of an inch diameter, or about as wide as to admit the point of a pin or a moderate-sized needle. The distance of this hole from the lens next the eye must be adjusted by trial, till the whole field of view appear distinct. A common astronomical eye-piece, without this ad- dition, does not answer well. I find by experience, that terrestrial eye-pieces, such as those used in good achromatic telescopes, are, on the whole, best adapted to this construction of a reflecting telescope. I have sometimes used these instruments for the purpose of viewing perspective prints, which they 324 THE PRACTICAL ASTRONOMER. exhibit in a beautiful and interesting manner. If a coloured perspective be placed at one end of a large room or gallery, and strongly illuminated either by the sun or by two candles, and one of the reflectors furnished with a small magnifying power, placed at the opposite end of the room — the representation of a street or a landscape will be seen in its true perspective, and will appear even more pleasant and interesting than when viewed through the common optical diagonal machine* If an inverting eye-piece be used — which is most eligible in this experiment — the print, of course, must be placed in an inverted position. That reflecting telescopes of the descriptions now stated are original in their construction, appears from the uniform language of optical writers, some of whom have pronounced such attempts to be altogether impracticable. Sir David Brewster, one of the latest and most respectable writers on this subjeet, in the ' Edinburgh Encyclopedia ? art optics, and in the last edition of his appendix to c Ferguson's Lectures/ has the following re- marks : — ' If we could dispense with the use of the small specula in telescopes of moderate length, by inclining the great speculum, and using an oblique, and consequently a distorted reflection, as proposed first by La Maire, we should consider the Newtonian telescope as perfect; and on a large scale, or when the instrument exceeds 20 feet, it has undoubtedly this character, as nothing can be more simple than to magnify, by a single eye-glass, the image formed by a single speculum. As the front view is quite impracticable, and indeed has never been attempted in instruments of a small size, it becomes of great practicable con- sequence to remove as much as possible, the ON REFLECTING TELESCOPES. 325 evils which arise from the use of a small specu- lum,' &c. The instruments now described have effectuated, in some degree, the desirable object alluded to by this distinguished philosopher, and the mode of construction is neither that of Sir W. HerschePs front view, nor does it coincide with that proposed by La Maire, which appears to have been a mere hint that was never realized in the construction of reflecting telescopes of a small size. The simpli- city of the construction of these instruments, and the excellence of their performance, have been much admired by several scientific gentlemen and others to whom they have been exhibited. Prior to the description of them in the Edin. Philos. Journal, they were exhibited in the Calton Hill Observatory, Edinburgh, in the presence of Pro- fessor Wallace, and another gentleman, who com- pared their performance with that of an excellent Gregorian. As this instrument is distinguished from every other telescope, in being used without a tube, it has been denominated ■ The aerial reflector. 9 SECT. 4. — EARL OF ROSSE's REFLECTING TELE- SCOPES. This nobleman, unlike many of his compeers, has, for a considerable number of years past, devoted his attention to the pursuits of science, and particularly to the improvement of reflecting telescopes. He is evidently possessed of high mathematical attainments, combined with an un- common degree of mechanical ingenuity. About 14 or 15 years ago, he engaged in various experi- ments with the view of counteracting the effects 326 THE PRACTICAL ASTRONOMER. of the spherical aberration of the specula of reflect- ing telescopes — which imperfection, if it could be completely remedied, would render the reflecting telescope almost a perfect instrument, as it is not affected by the different refrangibility of the rays of light. His method, we believe, consisted in forming a large speculum of two or three separate pieces of metal, which were afterwards accurately combined into one — a central part which was surrounded by one or two rings ground on the same tool. When the images formed by the separate pieces, were made exactly to coincide, the image of the object towards which the whole speculum was directed, was then found to be as distinct as either image had been when separate. But at the period referred to, a sufficient number of experiments had not been made to determine that his lordship had completely accomplished the object he intended. Great interest, however, has of late been ex- cited by the improvements which his lordship has made in the formation of specula. Sir W. Herschel never made public the means by which he succeeded in giving such gigantic develope- ment to the reflecting telescope : and therefore the construction of a large reflector has been con- sidered as a perilous adventure. But, according to a report of Dr. Robinson of Armagh, to the Irish academy, the Earl of Rosse has overcome the difficulties which have hitherto been met with, and carried to an extent which even Herschel himself did not venture to contemplate, the illuminating power of this telescope, along with a sharpness of definition little inferior to that of the achromatic ; and it is scarcely possible, he observes, to preserve the„ necessary sobriety of language in speaking of the moon's appearance ON REFLECTING TELESCOPES. 327 with this instrument, which Dr. Robinson believes to be the most powerful ever constructed. The difficulty of constructing large specula, and of imparting to them the requisite degree of polish, has hitherto been considered so great, that from 8 to 12 inches diameter has been in general their utmost size. Indeed, except with the greatest reluctance, London opticians would not accept of orders for specula of more than 9 inches in diame- ter. It appears, however, that the Earl of Rosse has succeeded, by a peculiar method of moulding, in casting object-mirrors of true speculum metal of three feet in diameter, and of a weight exceeding 17 cwt. He is about to construct a telescope, the speculum of which is six feet in diameter, fifty feet focal distance, and of the weight of four tons ; and from what he has already accomplished, it is not doubted that he possesses the power to carry his design into effect. These great masses of metal, which, in the hands of all other makers of specula would have been as untractable as so much unannealed flint-glass, the Earl of Rosse has further succeeded in bringing to the highest degree of polish, and the utmost perfection of curvature by means of machinery. The process is conducted under water, by which means those variations of temperature, so fatal to the finest specula hitherto attempted, are effectually guarded against. To convince Dr. Robinson of the efficacy of this machinery, the earl took the three feet speculum out of its telescope, destroyed its polished surface, and placed it under the mechanical polisher. In six hours it was taken out with a perfect new surface as bright as the original. Under the old system of hand-polishing, it might have required months, and even years, to effect this restoration. Even before achieving these extraordinary 328 THE PRACTICAL ASTRONOMER. triumphs on the solid substance, his lordship had constructed a six feet reflector by covering a curved surface of brass with squares of the true speculum metal, which gave an immense quantity of light, though subject to some irregularities, arising from the number of joinings necessary in such a mosaic work. Of the performance of his lordship's great telescope, mounted with this reflector, those who have seen it speak in terms of high admiration ; but in reference to the smaller and more perfect instrument, furnished with the solid three feet speculum, the language of the Armagh astronomer assumes a tone of enthusiasm and even of sublimity. By means of this exquisite instrument, Dr. Robinson and Sir J. South, in the intervals of a rather unfavourable night, saw several new stars, and corrected numerous errors of other observers. For example, the planet Uranus, supposed to possess a ring similar to that of Saturn, was found not to have any such appendage ; and those nebulae, hitherto regarded, from their apparently circular outline, as ' coalescing systems/ appeared, when tested by the three feet speculum, to be very far indeed from presenting a globular appearance ; numerous off-shoots and appendages, invisible by other tele- scopes, appearing in all directions radiating from their edges. Such discoveries, which reflect great honour on the Earl of Rosse, will doubtless have great effect on the interests of astronomical science.* * A particular account of the Earl of Rosse's fifty-feet Reflector, which is now finished, is given in the Appendix. ON REFLECTING TELESCOPES. 329 SECT. 5. — REFLECTING TELESCOPES WITH GLASS SPECULA. After making a variety of experiments with aerial telescopes constructed of metallic specula of different focal lengths, I constructed a telescope on the same plan, with a concave glass mirror. Having obtained a fragment of a very large con- vex mirror which happened accidentally to have been broken, I caused the convex side to be foliated, or silverised, and found its focal length to be about 27 inches. This mirror, which was about 5 inches diameter, I placed in one of the aerial reflectors, instead of the metallic speculum, and tried its effects with different terrestrial eye- pieces. "With a power of about 35 or 40 times, it gave a beautiful and splendid view of distant terrestrial objects — the quantity of light reflected from them, being considerably greater than when a metallic speculum was used, and they appeared on the whole well-defined. The only imperfec- tion—as I had foreseen — consisted in a double image being formed of objects which were remark- ably bright and white, such as a light-house whit- ened on the outside, and strongly illuminated by the sun. One of the images was bright and the other faint. This was obviously owing to the two reflections from the two surfaces of the mirror — one from the convex silverised side, and the other from the concave side next the eye, which pro< duced the faint image — which circumstance has been generally considered as a sufficient reason for rejecting the use of glass specula in telescopes. But although very bright objects exhibited a double image, almost all the other objects in the 330 THE PRACTICAL ASTRONOMER. terrestrial landscape appeared quite distinct and without any secondary image, so that a common observer could scarcely have noticed any imperfec- tion. When the instrument, however, was directed to celestial objects, the secondary image was somewhat vivid, so that every object appeared double. Jupiter appeared with two bodies, at a little distance from each other, and his four satel- lites appeared increased to eight. The moon like- wise appeared as a double orb, but the principal image was distinct and well-defined. Such a tele- scope, therefore, was not well-adapted for celestial observations, but might answer well enough for viewing terrestrial objects. Considering that the injurious effects of the secondary image arose from the images reflected from the two surfaces being formed near the same point, and at nearly the same focal distance, I formed a plan for destroying the secondary image, or at least counteracting its effects, by forming the concavity of the mirror next the eye of a por- tion of a sphere different from that of the convex side which was silverised, and from which the principal image is formed. But, for a long time, I could find no opticians possessed of tools of a sufficient length of radii for accomplishing my design. At length a London working optician undertook to finish a glass speculum, according to my directions, which were, that the convex sur- face of the mirror should be ground on a tool which would produce a focal distance by reflection of about 4 feet; and that the concave surface should have its focal distance at about 3 feet 3 inches, so that the secondary image might be formed at about 9 inches, within the focal dis- tance of the silverised side, and not interfere to dis- turb the principal image. But, either from ON REFLECTING TELESCOPES. 331 ignorance or inattention, the artist mistook the radius for the half radius of concavity, and the speculum turned out to be only 23 inches focal distance by reflection. This mirror was fitted up as a telescope, on the aerial plan, and I found, as I expected, the secondary image completely destroyed. It produced a very beautiful and brilliant view of land objects, and even the bright- est objects exhibited no double image. The mir- ror was nearly 5 inches in diameter, but the image was most accurately defined when the aperture was contracted to about 3 inches. It was fitted with a terrestrial eye-piece which produced a magnifying power of about 25 times. When directed to the moon, it gave a very distinct and luminous view of that orb, without the least appearance of a secondary image. But as the focal distance of the speculum was scarcely half the length I had prescribed, I did not apply to it any high astronomical powers ; as I find, that these can only be applied with effect, in this construc- tion, to a speculum of a considerable focal length. Happening to have at hand a convex lens 10 feet focal length, and 4 inches in diameter — the one side of which had been ground to a certain degree of concavity — I caused the convex side to be foliated, which produced a focus by reflection, at 13^ inches distant. To this mirror I applied terrestrial powers of 15 and 24, with considerable distinctness. The power of 15 produced a very brilliant and distinct view of land objects. Had the mirror been at least 3 times the focal length, it would have formed an excellent telescope, with the same aperture. 332 THE PRACTICAL ASTRONOMER. SECT. 6. A REFLECTING TELESCOPE, WITH A SINGLE MIRROR AND NO EYE-PIECE. On the same principle as that by which a refract- . ing telescope may be constructed by means of a single lens— as represented fig. 51, (page 234) we may form a telescope by reflection with a single mirror, and without an eye-piece. Let AB, fig. 72, represent a large concave speculum, figure 72. and C its focus — if an eye be placed at D, about 8 or 10 inches within the focal point C, all the objects in the direction of C, or behind the spec- tator, will be seen magnified by reflection on the face of the mirror, and strongly illuminated. The magnifying power, in this case, will be nearly in the proportion of the focal length of the mirror to the focal length of the eye for near objects. If for example, the focal distance of the mirror be 8 feet, and the distance from the eye at which we ON REFLECTING TELESCOPES. 333 see near objects most distinctly, be 8 inches — the magnifying power will be in the ratio of 8 to 96, or 12 times. I have a glass mirror of this description, whose focal length is 4 feet 8 inches, and diameter 6 inches, which magnifies distant objects about 7 times, takes in a large field of view, and exhibits objects with great brilliancy. It presents a very distinct picture of the moon, showing the different streaks of light and shade upon her surface ; and, in some cases, shows the larger spots which traverse the solar disc. This mode of viewing objects is extremely easy and pleasant, especially when the mirror is of a large diameter ; and the observer is at first struck and gratified with the novel aspect in which the objects appear. Were a concave mirror of this description — whether of glass or of speculum metal — to be formed to a very long focus, the magnifying power would be considerable. One of 50 feet focal length, and of a corresponding diameter, might produce a magnifying power, to certain eyes, of about 75 times ; and, from the quantity of light with which the object would be seen, its effect would be much greater than the same power applied to a common telescope. Sir W. Her- schel states, that, on one occasion, by looking with his naked eye on the speculum of his 40 feet Reflector, without the interposition of any lens or mirror, he perceived distinctly one of the satel- lites of Saturn, which requires the application of a considerable power to be seen by an ordinary telescope. Such an instrument is one of the most simple forms of a telescope, and would exhibit a brilliant and interesting view of the moon, or of terrestrial objects. 334 THE PRACTICAL ASTRONOMER. PRICES OF REFLECTING TELESCOPES. 1. Prices as stated by Messrs. W. and S. Jones, Holborn, London. £ s, A 4 feet, 7 inch aperture Gregorian reflector, with the vertical motions upon a new invented principle, as well as apparatus to render the tube more steady for observation, according to the additional apparatus of small speculums, eye-pieces, micrometers, &c. - - from 80£. to 120 0 Three feet long, mounted on a plain brass stand - 23 2 Ditto with rack-work motions, improved mountings and metals - - - - - - 39 18 Two feet long without rack- work, and with 4 magnifying powers, improved - - - - - -1515 Ditto improved, with rack-work motions - - - 22 1 Eighteen inch, on a plain stand - - - 9 9 Twelve inch ditto - - - - - -66 2. Prices as stated by Messrs. Tulley, Islington. £ s. 1 foot Gregorian Reflector, on pillar-and-claw stand, metal 2 h inches diameter, packed in a mahogany box - 6 6 1 h foot ditto on pillar and claw stand, metal 3 inches dia- meter, packed in a mahogany box - - - -1111 2 feet ditto, metal 4 inches diameter - * 16 16 Ditto with rack-work motions - - - - 25 4 3 feet ditto, metal 5 inches diameter, rack-work motions - 42 0 4 feet ditto, metal 7 inches diameter, on a tripod stand with centre of gravity motion - - - - 105 0 6 feet ditto, metal 9 inches diameter - - - 21 0 0 7 feet Newtonian, 6 inches aperture - - - 105 0 12 feet ditto, metal 12 inches diameter - 525 0 3. Prices stated by Mr. G. Dollond, St. Paul's Church Yard. £ s. Reflecting telescopes 14 inches long, in a mahogany box - 9 9 Ditto, 18 inches 12 12 Ditto 2 feet - 18 18 Ditto with 4 different powers, and rack- work stand sup- porting the telescope in the centre of gravity - - 36 1 5 Ditto 3 feet, with ditto - - - - - 50 0 4. Prices of single speculums and reflecting telescopes, as made by Mr. Grub, Charlemont Bridgeworks, Dublin. ON THE EYE-PIECES OF TELESCOPES. 335 NEWTONIAN TELESCOPES. | Diameter in inches. Focal length in feet. Price of Mirrors alone. Price of telescope com- plete without stand. £ s. £ s. 7 7 17 10 27 10 9 10 25 0 40 0 12 12 60 0 90 0 15 15 120 0 170 0 18 18 200 0 260 0 GREGORIAN REFLECTORS. Q a 1 . cn +-> cal length in i :ice of Mirrors :-ice of telescoj ste without stj A Ph 6 3 £ s. 17 10 £ s, 25 0 7 3 25 0 34 0 9 35 0 50 0 12 7 70 0 100 0 15 9 150 0 200 0 18 12 240 0 300 0 ON THE EYE-PIECES OF TELESCOPES. Although the performance of telescopes chiefly depends on the goodness of the object-glass, or the object-speculum of the instrument, yet it is of considerable importance, in order to distinct vision, and to obtain a large and uniformly distinct field of view, that the eye-piece be properly con- structed. The different kinds of eye-pieces may be arranged into two general divisions — Astrono- mical and terrestrial. 1. Astronomical eye-pieces. — The most simple astronomical eye-piece is that which consists of a single convex lens; and when the focal distance of this lens, and that of the object-glass of the instrument is accurately ascertained, the magnify- 336 THE PRACTICAL ASTRONOMER. ing power may be nicely determined, by dividing the focal length of the object-lens by that of the eye-glass. But, as the pencil of white light trans- mitted by the object-glass, will be divided by the eye-glass into its component colours, the object will appear bordered with coloured fringes, and the distinctness of vision consequently injured. Besides, the spherical aberration, when a single lens is used, is much greater than when two or more glasses are employed. Hence astronomical eye-pieces are now formed by a combination of at least two lenses. The combination of lenses now generally used for astronomical purposes, is that which is usually denominated the Huygenian eye-piece, having been first proposed by the celebrated Huygens, as a great improvement on the single lens eye-piece. The following figure (73) represents a section of figure 73. A B F f this eye-piece. Let AB be a compounded pencil of white light proceeding from the object-glass ; BF a piano convex field-glass, with its plane side next the eye-glass E. The red rays of the pencil AB, after refraction would cross the axis in R, and the violet rays in V, but meeting the eye- glass E, the red rays will be refracted to O, and ON THE EYE-PIECES OF TELESCOPES. 337 the violet nearly in the same direction, when they will cross each other about the point O, in the axis, and unite. The distance of the two glasses FE, to produce this correction, when made of crown glass, must be equal to half the sum of their focal distances nearly. For example, sup- pose the focal distance of the largest, or field lens, to be 3 inches, and the focal distance of the lens next the eye, 1 inch, the two lenses should be placed exactly at the distance of 2 inches ; the sum of their focal length being 4, the half of which is 2. In other words, the glass next the eye should be placed as much within the focus of the field-glass as is equal to its own focal distance. The focal length of a single lens, that has the same magnifying power as this compound eye- piece — is equal to twice the product of the focal lengths of the two lenses, divided by the sum of the same numbers. Or, it is equal to half the focal length of the field-glass. Thus, in reference to the preceding example, twice the product of the focal length of the two lenses — is equal to 6, and their sum is 4. The former number divided by the latter, produces a quotient of H, which is the focal length of a single lens, which would pro- duce the same magnifying power as the eye-piece; and 1 is just half the focal length of the field- glass. The proportion of the focal lengths of the two lenses to each other, according to Huygens, should be as 3 to 1 ; that is, if the field-glass be 4^ inches, the eye-glass should be 1^ ; and this is the proportion most generally adopted. But some opticians have recommended that the proportions should be as 3 to 2. Boscovich recommended two similar lenses ; and in this case the distance be- tween them was equal to half the sum of their focal distances, as in the Huygenian eye-piece. Q 338 THE PRACTICAL ASTRONOMER. The image is formed at IM, at the focal dis- tance of the lens next the eye, and at the same distance from the lield-glass. When distinct vision is the principal object of an achromatic telescope, the two lenses are usually both plano-convex, and fixed with their curved faces towards the ob^ ject glass, as in the figure. Sometimes, however, they consist of what is called crossed lenses, that is lenses ground on one side to a short focus, and on the other side to a pretty long focus, the sides with the deepest curves being turned towards the object glass. A diaphragm, or aperture of a proper diameter, is placed at the focus of the eye lens, where the image formed by the object-glass falls, for the purpose of cutting off the extreme rays of the field lens, and rendering every part of the field of view equally distinct. This is like- wise the form of the eye -piece generally applied to Gregorian reflectors. In short, when accu- rately constructed, it is applicable to telescopes of every description. This eye-piece, having the image viewed, by the eye behind the inner lens, is generally called the negative eye-piece, and is that which the optical-instrument makers usually supply, of three or four different sizes, for so many magnifying powers, to be applied to different celes- tial objects, according to their nature or the state of the atmosphere in which they are used. Ramsdens eye-piece, — There is another modi- fication of lenses, known by the name of the Positive, or Ramsden's eye-piece, which is much used in Transit instruments, and telescopes which are furnished with micrometers, and which affords equally good vision as the other eye-piece. In this construction the lenses are plano-convex, and nearly of the same focus, but are placed at a dis- tance from each other less than the focal distance ON THE EYE-PIECES OF TELESCOPES. 339 of the glass next the eye, so that the image of the object viewed is beyond both the lenses, when measuring from the eye. The flat faces of the two lenses are turned into contrary directions in this eye-piece — -one facing the object-glass, and the other the eye of the observer ; and as the image formed at the focus of the object-glass, lies paral- lel to the flat face of the contiguous lens, every part of the field of view is distinct at the same adjustment, or, as opticians say, there is a flat field, which, without a diaphragm, prevents distor- tion of the object. This eye-piece is represented in fig. 74, where AB and CD are two plano-con- figure 74. vex lenses, with their convex sides inwards. They have nearly the same focal length, and are placed at a distance from each other, equal to about two thirds of the focal length of either. The focal length of an equivalent single lens is equal to three fourths the focal length of either lens, supposing them to have equal focal distances. This eye- piece is generally applied, when wires of spider s lines are used in the common focus ; as the piece containing the lenses can be taken out without disturbing the lines, and is adjustable for distinct vision ; and whatever may be the measure of any object given by the wire micrometer, at the solar Q 2 340 THE PRACTICAL ASTRONOMER. focus, it is not altered by a change of the magnify- ing power, when a second eye-piece of this con- struction is substituted. Aberration of lenses. — In connection with the above descriptions, the following statements re- specting the spherical aberration of lenses may not be inappropriate. Mr. John Dollond, in a letter to Mr. Short, remarks, that * the aberration in a single lens is as the cube of the refracted angle ; but if the refraction be caused by two lenses, the sum of the cubes of each half will be i of the refracted angle, twice the cube of 1 being i the cube of 2. So three times the cube of 1 is only one ninth of the cube of 3/ &c. Hence the indistinctness of the borders of the field of view of a telescope is diminished by in- creasing the number of lenses in an eye piece. Sir J. Herschel has shown that if two plano-con- vex lenses are put together as in fig. 75, the aber- ration will be only 0.2481, or one fourth of that of a single lens in its best form. The focal length of the first of these lenses, must be to that of the figure 75. figure 76. second as 1 to 2.3. If their focal lengths are equal, the aberration will be 0.603, or nearly one half. The spherical aberration, however, may be entirely destroyed by combining a meniscus and double convex lens, as shown in fig. 76, the convex sides ON THE EYE-PIECES OF TELESCOPES. 341 being turned to the eye when they are used as lenses, and to parallel rays, when they are used as burning glasses. Sir J. Herschel has computed the following curvatures for such lenses. Focal length of the convex lens - - + 10.000 Radius of its first surface - - -f 5.833 Radius of its second surface - - — 35.000 Focal length of the meniscus - - + 17.829 Radius of its first surface - - - + 3.688 Radius of its second surface - - + 6.294 Focal length of the compound lens - + 6.407 On the general principles above stated, a good astronomical eye-piece may be easily constructed with two proper lenses, either according to the plan of Huygens or that of Ramsden ; and, from what has been now stated it is demonstrably cer- tain, that, in all cases where two glasses are pro- perly combined, such an eye-piece is superior to a single lens, both in point of distinctness, and of the enlargement of the field of view. I lately fitted up an eye-piece, on Ramsden's principle, with two lenses, each about 3 inches focal length, and If inch diameter, placed at half an inch dis- tant, with their convex surfaces facing each other as in fig. 74, which forms an excellent eye-piece for an achromatic telescope, 6 feet 8 inches focal distance, and 4 inches aperture, particularly for viewing clusters of stars, the Milky Way, and the large nebulae. The field of view is large, the magnifying power is only between 50 and 60 times, and the quantity of light being so great, every celestial object appears with great brilliancy, and it is in general much preferable, when applied to the stars than any of the higher powers. When applied to Presepe in Cancer, it exhibits that group at one view, as consisting of nearly a 100 stars which exhibit a beautiful and most striking appearance. 342 THE PRACTICAL ASTRONOMER. It may appear a curious circumstance that any eye-piece which is good with a short telescope, is also good with a long one, but that the reverse is not true ; for it is found to be more difficult to make a good eye-piece for a short than for a long focal distance of the object-glass. Celestial eye-pieces are sometimes constructed so as to produce variable powers. This is effected by giving a motion to the lens next the eye, so as to remove it nearer to or farther from the field lens ; for at every different distance at which it is placed from the other lens, the magnifying power will either be increased or diminished. The greatest power is when the two lenses are nearly in contact, and the power diminishes in propor- tion to the distance at which the glass next the eye is removed from the other. The scale of dis- tance, however, between the two lenses, cannot be greater than the focal distance of the field, or inner glass ; for if it were, the lenses would no longer form an eye-piece, but would be changed into an inverting opera-glass. For effecting the purpose now stated, the eye-glass is fixed in a tube which slides upon an interior tube on which is marked a scale of distances, corresponding to cer- tain magnifying powers ; and, in this way an eye- piece may be made to magnify about double the number of times, when the lenses are in one posi- tion than when they are in another — as, for exam- ple, all the powers from 36 to 72 times may be thus applied, merely by regulating the distance between the two lenses. When the glasses are varied in this manner the eye-piece becomes sometimes a positive eye-piece, like Ramsden's, and sometimes a negative one like that of Huygens. Diagonal eye-pieces. The eye-pieces to which ON THE EYE-PIECES OF TELESCOPES. 343 we have now adverted, when adapted to refracting telescopes, both reverse and invert the object, and. therefore are not calculated for showing terrestrial objects in their natural position. But as the heavenly bodies are of a spherical form, this cir- cumstance detracts nothing from their utility. When the celestial object, however, is at a high altitude, the observer is obliged to place his head in a very inconvenient position, and to direct his eye nearly upwards ; in which position he cannot remain long at ease, or observe with a steady eye. To remedy this inconvenience, the diagonal eye- piece has been invented, which admits of the eye being applied at the side — or at the upper part of the eye-piece, instead of the end ; and when such an eye-piece is used, it is of no importance in what direction the telescope is elevated, as the observer can then either sit or stand erect, and look down upon the object with the utmost ease. This object is effected by placing a flat piece of polished speculum-metal at an angle of 45 degrees in respect to the two lenses of the eye- piece, which alters the direction of the converging rays, and forms an image which becomes erect with respect to altitude, but is reversed with respect to azimuth; — that is, in other words, when we look down upon the objects in the field of view, they appear erect; but that part of an object which is in reality on our right hand appears on our left ; and if it be in motion, its apparent is opposite to its real motion ; if it be moving towards the west, it will seem to move towards the east. There are three situations in which the diagonal reflector in this eye-piece may be placed. It may be placed either 1. before the eye-piece, — or 2. behind it, — or 3. between the two lenses of 344 THE PRACTICAL ASTRONOMER. which the eye-piece consists. The most common position of the reflector is between the lenses ; and this may be done both in the negative and the positive eye-pieces ; but as the distance between the two lenses is necessarily considerable, to make room for the diagonal position of the reflector, the magnifying power cannot be great ; otherwise, a diagonal eye-piece of this construction remains always in adjustment, and is useful in all cases where a high power is not required. The follow- ing is a description and representation of a diagonal eye-piece of this kind in my possession. figure 77. In fig. 77, AB represents the plano-convex lens next the object, which is about 2 inches in focal length, and f inch in diameter; CD, a plain metallic speculum of an oval form, well polished, and placed at half a right angle to the axis of the tube ; and EF another plano-convex lens, about \\ inch focal distance. The centre of the specu- lum is about 1^ inch from the lens AB, and about ^ or ^ inch from EF ; so that this eye-piece is a •positive one, on the principle proposed by Rams- den. The rays proceeding from the lens AB, and falling upon the speculum, are reflected in a per- ON THE EYE-PIECES OF TELESCOPES. 345 pendicular direction to the lens EF, where they enter the eye at G, which looks down upon the object through the side of the tube. The real size of this eye-piece is much about the same as that represented in the figure. When applied to an achromatic telescope of 44^ inches focal dis- tance it produces a magnifying power of 36 times, and exhibits a very beautiful view of the whole of the full moon. It likewise presents a very pleas- ing prospect of terrestrial objects, which appear as if situated immediately below us. Another plan of the diagonal eye-piece is repre- sented in fig. 78, where the speculum is fixed figure 78. C JB within the sliding tube which receives the eye- piece, or immediately below it. The part of the tube at AB slides into the tube of the telescope, CD is the speculum placed at half a right angle to the axis of the tube, and EF, the tube con- taining the lenses, which stands at right angles to the position of the telescope, and slides into an exterior tube, and the eye is applied at G. This construction of the diagonal eye-piece may be used with any eye-piece whatever, whether the Huygenian or that of Ramsden. It will admit of Q 5 346 THE PRACTICAL ASTRONOMER. any magnifying power, and if several different eye-pieces be fitted to the sliding tube, they may be changed at pleasure. This form of the diagonal eye-piece, I therefore consider as the best and the most convenient construction, although it is not commonly adopted by opticians. When any of these eye-pieces are applied to a telescope, with the lens E on the upper part of it, we look down upon the object, if it be a terrestrial one, as if it were under our feet. If we turn the eye-piece round in its socket a quarter of a circle towards the left, an object directly before us in the south, will appear as if it were in the west and turned upside down. If, from this position, it is turned round a semicircle towards the right, and the eye applied, the same object will appear as if it were situated in the east, and inverted ; and if it be turned round another quadrant, till it be directly opposite to its first position, and the eye applied from below, the object or landscape will appear as if suspended in the atmosphere above us. This eye-piece, therefore, is capable of ex- hibiting objects in a great variety of aspects, and the use of it is both pleasant and easy for the observer. But there is a considerable loss of light, occasioned by the reflection from the speculum, which is sensibly felt when very high powers are applied ; and therefore when very small stars are to be observed, such as some of those connected with double or triple stars, the observer should not study his own ease so much as the quantity of light he can retain with a high power, which ob- ject is best attained with an ordinary eye-piece and a telescope of large aperture. We have said that a diagonal eye-piece may be constructed with a reflector before the eye-piece. In this case, the speculum is sometimes made to ON THE EYE-PIECES OF TELESCOPES. 347 slide before the eye at the requisite angle of reclination, in which application each eye-piece must necessarily have a grove to receive it, and the eye must be applied without a hole to direct it, but it may be put on and taken off without disturbing the adjustment for distinct vision, and is very simple in its application. But, on the whole, the form represented in fig. 78, is the most convenient, and should generally be preferred, as any common astronomical eye-piece can be applied to it. I have used a diagonal eye-piece of this kind, with good effect, when a power of 180 has been applied to the sun and other celestial objects. Instead of a metallic speculum, a rectangular prism of glass is sometimes substituted ; for the rays of light are then bent by reflection from the second polished surface, which ought to be dry, and undergo two refractions which achromatise them ; and the same effect is thus produced as by polished metal. Ramsden sometimes gave one of the polished faces of a right angled prism a curve, which prism served instead of a lens in an eye- piece, and also performed the office of a reflector. A semi-globe, or what has been called a BulFs eye, has also been used as a diagonal eye-piece, and when the curve is well-formed, and the glass good, it is achromatic, and is said to perform pretty well, but it is not superior to the forms already described. SECT. 2.— TERRESTRIAL EYE-PIECES. When describing the common refracting teles- cope, (p. 228.) I have noticed that three eye- glasses, placed at double their focal distances 348 THE PRACTICAL ASTRONOMER. from each other, formerly constituted the terres- trial eye-piece, as represented in fig. 47. But this construction, especially for achromatic instru- ments, has now become obsolete, and is never used, except in small pocket spy-glasses formed with a single object lens. In its place a four glassed eye-piece has been substituted, which is now universally used in all good telescopes, and which, besides improving the vision and producing an erect position of the images of objects, pre- sents a considerably larger field of view. During the progressive stages of improvement made in the construction of erect eye-pieces by Dollond and Ramsden, three, four, and five lenses were successively introduced; and hence, in some of the old telescopes constructed by these artists, we frequently find five lenses of different descriptions composing the eye-piece. But four lenses, arranged in the manner I am now about to de- scribe, have ultimately obtained the preference. In a telescope having a celestial eye-piece of the Huygenian form, the image that is formed in the focus of the object glass, is that which is seen magnified, and in an inverted position ; but when a four glassed eye-piece is used, which produces an erect view of the object, the image is repeated, and the second image, which is formed by the inner pair of lenses AB on an enlarged scale, is that which the pair of lenses CD at the eye-end render visible on a scale still more enlarged. The modern terrestrial eye-piece, represented in fig. 79, is, in fact, nothing else than a compound microscope, consisting of an object lens, an ampli- fying lens, and an eye-piece composed of a pair of lenses on the principle of the Huygenian eye- piece. Its properties will be best understood by considering the first image of an object, which is ON THE EYE-PIECES OF TELESCOPES. 349 figure 79. formed in the focus of the object glass, as a small luminous object to be rendered visible, in a magni- fied state, by a compound microscope. The object to be magnified may be considered as placed near the point A, and the magnified image at i, which is viewed by the lens D. Hence, if we look through such an eye-piece at a small object placed very near the lens A, we shall find that it acts as a compound microscope of a moderate magnifying power increasing, in some cases, the diameter of the object about 10 times, and 100 times in surface. In order to distinguish the different lenses in this eye-piece, we may call the lens A, which is next to the first image, the object-lens, the next to it B, the amplifying -lens, the third, or C, the field-lens, and the one next the eye, D, the eye- lens. The first image formed a little before A, may be denominated the radiant, or the object from which the rays proceed. Now, it is well known as a principle in optics, that if the radiant be brought nearer to the lens than its principal focus, the emerging rays will diverge, and, on the contrary, if the radiant be put farther from the lens than its principal focal distance, the emerging rays will converge to a point at a distance beyond the lens, which will depend on the distance of the radiant from the first face of the lens. In this place an image of the radiant will be formed hy the concurrence of the converging rays, but in a 350 THE PRACTICAL ASTRONOMER. contrary position ; and the length of the image will exceed the length of the radiant in the same proportion, as the distance of the image from the radiant exceeds that of the radiant from the lens. This secondary image of the radiant at i, is not well-defined, when only one lens, as A, is used, owing to the great spherical aberrations, and therefore the amplifying lens is placed at the dis- tance of the shorter conjugate focus, with an in- tervening diaphragm of a small diameter at the place of the principal focus ; the uses of which lens and diaphragm are, first to cut off the coloured rays that are occasioned by the dispersive property of the object lens, — and secondly, to bring the rays to a shorter conjugate focus for the place of the image, than would have taken place with a single lens having only one refraction. As the secondary image is in this way much better de- fined and free from colouration, the addition of this second lens is a great improvement to vision. For this reason I am clearly of opinion, that the object glass of a compound microscope, instead of consisting of a small single lens, should be formed of two lenses on the principle now stated, which would unquestionably add to the distinctness of vision. With respect to the proportions of the focal lengths of the lenses in this four glass eye-piece, Mr. Coddington states, that if the focal lengths, reckoning from A to D, fig. 79, be as the numbers 3, 4, 4 and 3, and the distances between them on the same scale, 4, 6, and 5, 2, the radii, reckon- ing from the outer surface of A, should be thus:— * / First surface 27 \ ■ i onA A \ Second surface i > nearly plano-conwx. t> / First surface 9 4 • • „„„ B l Second surface 4 } amlmscus - ON THE EYE-PIECES OF TELESCOPES. 351 C HZoJ^le pearly plano-convex. V^IESSL 2 >nble convex. Sir D. Brewster states, that a good achromatic eye-piece may be made of 4 lenses, if their focal lengths, reckoning from that next the object, be as the numbers 14, 21, 27, 32 ; their distances 23, 44, 40; their apertures 5.6; 3.4; 13.5 ; 2.6 ; and the aperture of the diaphragm placed in the interior focus of the fourth eye-glass, 7. Another proportion may be stated: — Suppose the lens next the object A, to be 1^ inch focal length, then B may be 2\ inches, C 2 inches, and D U ; and their distances AB 2J; BC 3| ; and CD 2f. In one of Ramsden's small telescopes, whose object glass was 82 inches in focal length, and its mag- nifying power 15.4, the focal lengths of the eye glasses were A 0.775 of an inch, B 1.025, C 1.01, D 0.79 ;— the distances AB 1.18, BC 1.83, and CD 1.105. In the excellent achromatic telescope of Dollond's construction which belonged to the Due de Chaulnes, the focal lengths of the eye glasses, beginning with that next the object, were 14} lines, 19, 22f , 14 ; their distances 22.48 lines, 46.17, 21.45, and their thickness at the centre, 1.23 lines, 1.25, 1.47. The fourth lens was plano-convex, with the plane side to the eye, and the rest were double convex lenses. This telescope was in focal length 3 feet 5J inches. The magnifying power of this eye-piece, as usually made, differs only in a small degree from what would be produced by using the first or the fourth glass alone, in which case the magnifying power would be somewhat greater, but the vision less distinct, and were the lens next the eye used alone without the field glass, the field of view 352 THE PRACTICAL ASTRONOMER. would be much contracted. Stops should be placed between the lenses A and B, near to B, and a larger one between C and D, to prevent any false light from passing through the lenses to the eye. The more stops that are introduced into a telescope — which should all be blackened — pro- vided they do not hinder the pencils of light pro- ceeding from the object, the better will the in- strument perform. For the information of amateur constructors of telescopes, I shall here state the dimensions of two or three four glassed eye-pieces in my posses- sion, which perform with great distinctness, and present a pretty large field of view. In one of these, adapted to a 44^ inch achromatic, the lens A, next the object, is If inch, focal length, and about 1 inch diameter, with the plane side, next the object. The focal length of the lens B 2^ inches, diameter g inch, with its plane side next A ; distance of these lenses from each other 2^ inches. Distance of the field lens C from the lens B 5|- inches. The small hole or diaphragm between A and B is at the focus of A, and is about I inch diameter, and about f of an inch from the lens B. The field lens C is 2 inches focal length, and 1} inch diameter, with its plane side next the eye. The lens next the eye D is 1 inch focal distance, i inch diameter, and is distant from the field glass If inch, with its plane side next the eye. The magnifying power of this eye-piece is equivalent to that of a single lens whose focal length is half an inch, and with the 44| inch ob- ject glass produces a power of about 90 times. The lens next the eye can be changed for another If inch focal length, which produces a power of 65 ; and the two glasses CD can be changed for another set, of a longer focal distance which pro- ON THE EYE-PIECES OF TELESCOPES. 353 duces a power of 45 times. The whole length of this eye-piece is Hi inches. In another eye-piece, adapted to a pocket achro- matic, whose object glass is 9 inches focal length, the lens A is 1 inch focal length, and \ inch dia- meter; the lens B \\ inch, and \ inch diameter, their distance 1^ inch, the lens C 1^ inch focal length, and f inch diameter ; the eye-lens Df inch focal length, and f inch diameter ; distance be- tween C and D \\ inch. The distance between B and C If inch. The whole length of this eye- piece is 4*i inches, and its power is nearly equal to that of a single lens of \ or iq of an inch focal length, the magnifying power of the telescope be- ing about 16 times. Another eye-piece of much larger dimensions, has the lens A of 2| inches focal length, and f inch diameter : the lens B 2f inches focus and f inch diameter ; and their dis- tance 2f inches ; the lens C 2f inches focus and \\ inch diameter ; the lens D If inch focus and f inch diameter ; distance from each other 2f inches. The distance between the lenses B and C is 4 inches. The magnifying power is equal to that of a single lens \\ inch focal distance. When applied to an achromatic object glass 6 feet 7 inches focal length, it produces a power of about 70 times. This eye-piece has a moveable tube 9 inches in length in which the two lenses next the eye are contained, by pulling out which, and con- sequently increasing the distance between the lenses B and C, the magnifying power may be increased to 100, 120 or 140, according to the distance to which this moveable tube is drawn out. It has also a second and third set of lenses, corresponding to C and D of a shorter focal dis- tance, which produce higher magnifying powers on a principle to be afterwards explained. 354 THE PRACTICAL ASTRONOMER. Description of an eye-piece, tyc. of an old Dutch Achromatic Telescope. About twenty or thirty years ago, I purchased, in an optician's shop in Edinburgh, a small achro- matic telescope, made in Amsterdam, which was supposed, by the optician, to have been constructed prior to the invention of achromatic telescopes by Mr. Dollond. It is mounted wholly of brass, and in all its parts is a piece of beautiful and exqui- site workmanship, and the utmost care seems to have been taken to have all the glasses and dia- phragms accurately adjusted. The object glass is a double achromatic, 6^- inches focal distance and 1 inch diameter, but the clear aperture is only 1 inch diameter. It is perfectly achromatic, and would bear a power of 50 times, if it had a sufficient quantity of light. The following in- scription is engraved on the tube adjacent to the object glass : — " Jan van Deyl en Zoon Invenit et Fecit, Amsterdam, Ao. 1769." Although Dol- lond exhibited the principle of an achromatic telescope, eight or ten years before the date here specified, yet it is not improbable that the artist whose name is here stated, may not have heard of Dollond' s invention ; and that he w r as really, as he assumes, one of the inventors of the achromatic telescope. For, the invention of this telescope by Dollond was not very generally known, except among philosophers and the London opticians, till a number of years after the date above stated. Euler, in his " Letters to a German Princess " — in which telescopes are particularly described, makes no mention of, nor the least allusion to the invention of Dollond, though this was a subject which par- ticularly engaged his attention. Now, these letters ON THE EYE-PIECES OF TELESCOPES. 355 were written in 1762, but were not published till 1770. When alluding to the defects in telescopes arising from the different refrangibility of the rays of light, in Letter 43, and that they might possibly be rectified by means of different transparent substances, he says, ' But neither theory nor practice have hitherto been carried to the degree of perfection necessary to the execution of a structure which should remedy these defects.' Mr. B. Martin, in his 6 Gentleman and Lady's Philo- sophy/ published in 1781, alludes to the achro- matic telescope, but speaks of it as it were but very little, if at all superior to the common re- fracting telescope. And therefore, I think it highly probable that Jan van Deyl, was really an inventor of an achromatic telescope, before he had any notice of what Dollond and others had done in this way some short time before. But my principal object in adverting to this telescope, is to describe the structure of the eye- piece, which is a very fine one, and which is some- what different from the achromatic eye-piece above described. It consists of four glasses, two combined next the eye, and two next the object. Each of these combinations forms an astronomical eye-piece nearly similar to the Huygenian. The lens A, next the object, fig. 80, is f inch focal distance, and £ inch diameter ; the lens B f inch focus, and \ inch diameter, and the distance be- tween them somewhat less than f inch ; the diameter of the aperture e about fg of an inch. This combination forms an excellent astronomical eye-piece, with a large flat field, and its magnify- ing power is equivalent to that of a single lens f or | focal length. The lens C is \ inch focal length, and £ inch diameter ; the lens D \ inch fpcus, and about \ inch diameter ; their distance 356 THE PRACTICAL ASTRONOMER. about i inch, or a small fraction more. The hole at d is about ^ or 2 ~ of an inch diameter, and the distance between the lenses B and C about lg inch. The whole length of the eye-piece is 3} inches — exactly the same size as represented in the engraving. Its magnifying power is equal to that of a single lens J inch focal length ; and consequently the telescope, though only &§• inches long, magnifies 26 times, with great distinctness, though there is a little deficiency of light when viewing land objects, which are not well illuminated. The glasses of this telescope are all plano-convex, with their convex-sides towards the object — except the lens D, which is double convex, but flattest on the side next the eye, and they are all very accurately finished. The two lenses C and I) form an astrono- mical eye-piece nearly similar to that formed by the lenses A and B. The focus of the telescope is adjusted by a screw, the threads of which are formed upon the outside of a tube into which the eye-piece slides. The eye-piece and apparatus connected with it, is screwed into the inside of the main tube, when not in use, when the instrument forms a compact brass cylinder 6 inches long, which is enclosed in a fish-skin case, lined with silk velvet, which opens with hinges. The lenses in the eye-pieces formerly described, though stated to be plano-convexes, are for the ON THE EYE-PIECES OF TELESCOPES. 357 most part crossed glasses, that is ground on tools of a long focus on the one side, and to a short focus on the other. The construction of the eye- piece of the Dutch telescope above described, is one which might be adopted with a good effect in most of our achromatic telescopes ; and I am per- suadedj from the application I have made of it to various telescopes, that it is even superior, in dis- tinctness and accuracy, and in the flatness of field which it produces to the eye-piece in common use. The two astronomical eye-pieces of which it con- sists, when applied to large achromatic telescopes, perform with great accuracy, and are excellently adapted for celestial observations. SECT. 3. — DESCRIPTION OF THE PANCRATIC EYE-TUBE. From what we have stated, when describing the common terrestrial eye-piece now applied to achro- matic instruments, (p. 349, fig. 79.), it appears obvious, that any variety of magnifying powers, within certain limits, may be obtained by removing the set of lenses CD, fig. 79, nearer to or farther from the tube which contains the lenses A and B, on the same principle as the magnifying power of a compound microscope is increased by removing the eye-glasses to a greater distance from the object-lens. If then, the pair of eye-lenses CD be attached to an inner tube that will draw out and increase their distance from the inner pair of lenses, as the tube abed, the magnifying power may be indefinitely increased or diminished, by pushing in or drawing out the sliding tube, and a scale might be placed on this tube, which, if divided into equal intervals, will be a scale of 358 THE PRACTICAL ASTRONOMER. magnifying powers, by which the power of the telescope will be seen at every division, when the lowest power is once determined. Sir David Brewster, in his ' Treatise on New Philosophical instruments/ Book i. chap. vii. page 59, published in 1813, has adverted to this circumstance, in his description of an 6 Eye-piece wire micrometer,' and complains of Mr. Ezekiel Walker, having in the ' Philosophical Magazine ' for August, 1811, described such an instrument as an invention of his own. Dr. Kitchener some years afterwards, described what he called a Pan- cratic or omnipotent eye-piece, and got one made by Dollond, with a few modifications different from that suggested by Brewster and Walker, which were little else than cutting the single tube into several parts, and giving it the appearance of a new invention. In fact, none of these gentlemen had a right to claim it as his peculiar invention, as the principle was known and recognised long be- fore. I had increased the magnifying powers of telescopes, on the same principle, several years before any of these gentlemen communicated their views on the subject, although I never formally constructed a scale of powers. Mr. B. Martin, who died in 1782, proposed many years before, such a moveable interior tube as that alluded to, for varying the magnifying power. In order to give the reader a more specific idea of this contrivance, I shall present him with a figure and description of one of Dr. Kitchener's Pancratic eye-pieces, copied from one lately in my possession. The following are the exact dimen- sions of this instrument, with the focal distances, &c. of the glasses, &c. of which it is composed. ON THE EYE-PIECES OF TELESCOPES. 359 In. Tenths, fig. 81. Length of the whole eye-piece, consisting of four tubes, when fully drawn out, or the dis- TUT tance from A to B. fig. 81. - - 14 4 ^nHT Length of the three tubes on which the H I ill scale is engraved, from the commencement of the divisions at B to their termination at C. - 9 15 IBB Each division into tens is equal to 3-10ths of an inch. When the three inner tubes are shut up to I I C, the length of the eye-piece is exactly 5 5 1 When these tubes are thus shut up, the mag- nifying power for a 31 feet achromatic is 100 times, which is the smallest power. When the QebRT inner tube is drawn out i of an inch, or to WwM the first division, the power is 110, &c. |||||| Focal distance of the lens next the object 1 0 wBM Breadth of Ditto. - - - - - - 0 65 J ; The plane side of this glass is next the object. jf fw g Focal distance of the second glass from the <63P» object 1 5 TpyfT This glass is double and equally convex, Breadth 0 5 MM Distance between these two glasses 1 7 g|i Focal distance of the third or field lens, |P| which is plane on the side next the eye 1 1 Breadth of Ditto. 0 55 MM- Focal distance of the lens next the eye 0 6 Wa Breadth ------- 0 43 M This glass is plane on the side next the eye. Vm Distance between the third and fourth glasses. 1 1 W-JliiW From the figure and description, the reader will be at no loss to perceive how the magnifying power is ascertained by this eye-piece. If the lowest power for a 44 inch telescope be found to be 100 3 when the three sliding tubes are shut into the larger one, then by drawing out the tube next the eye 4 divisions, a power of 140 is produced ; by drawing out the tube next the eye its whole length, and the second tube to the division marked 220, a power of 220 times is produced, and draw- ing out all the tubes to their utmost extent, as represented in the figure, a power of 400 is ob- tained. These powers are by far too high for such a telescope, as the powers between 300 and 400 can seldom or never be used. Were the scale to begin at 50, and terminate at 200, it would be 360 THE PRACTICAL ASTRONOMER. much better adapted to a Si feet telescope. Each alteration of the magnifying power requires a new adjustment of the eye-piece for distinct vision. As the magnifying power is increased, the distance between the eye-glass and the object- glass must be diminished. Dr. Kitchener says, that - the pancratic eye tube gives a better defined image of a fixed star, and shows double stars de- cidedly more distinct and perfectly separated than any other eye tube, and that such tubes will pro- bably enable us to determine the distances of these objects from each other, in a more perfect manner than has been possible heretofore.* These tubes are made by Dollond, London, and are sold for two guineas each. But I do not think they excel, in distinctness, those which are occasionally made by Mr. Tulley and other opticians. MANNER OF USING TELESCOPES. 361 CHAPTER VI. MISCELLANEOUS REMARKS IN RELATION TO TELESCOPES. The following remarks, chiefly in regard to the manner of using telescopes, may perhaps be useful to young observers, who are not much accustomed to the mode of managing these instruments. 1. Adjustments requisite to be attended to in the use of telescopes* When near objects are viewed with a considerable magnifying power, the eye- tube requires to be removed farther from the object-glass than w T hen very distant objects are contemplated. When the telescope is adjusted for an object, 6, 8, or 10 miles distant, a very considerable alteration in the adjustment is requi- site in order to see distinctly an object at the dis- tance of two or three hundred yards, especially if the instrument is furnished with a high magnify- ing power. In this last case, the eye-tube requires to be drawn out to a considerable distance beyond the focus for parallel rays. I have found that, in a telescope which magnifies 70 times, when adjusted for an object at the distance of two miles, the adjustment requires to be altered fully one inch in order to perceive distinctly an object at the distance of two or three hundred yards ; R 362 THE PRACTICAL ASTRONOMER. that is, the tube must be drawn, in this case, an inch farther from the object-glass, and pushed in the same extent, when we wish to view an object at the distance of two or three miles. These adjustments are made, in pocket perspectives, by gently sliding the eye-tube in or out, by giving it a gentle circular or spiral motion till the object appear distinct. In using telescopes which are held in the hand, the best plan is to draw all the tubes out to their full length, and then, looking at the object, with the left hand supporting the main tube near the object-glass, and the right support- ing the eye-tube — gently and gradually push in the eye-piece till distinct vision be obtained. In Gregorian reflecting telescopes this adjustment is made by means of a screw connected with the small speculum; and in large achromatics, by means of a rack and pinion connected with the eye-tube. When the magnifying power of a telescope is comparatively small, the eye- tube requires to be altered only a very little. There is another adjustment requisite to be attended to, in order to adapt the telescope to the eyes of different persons. Those whose eyes are too convex, or who are short-sighted, require the eye-tube to be pushed in, and those whose eyes are somewhat flattened, as old people, require the tube to be drawn out. Indeed there are scarcely two persons whose eyes do not require different adjust- ments in a slight degree. In some cases I have found that the difference of adjustment for two individuals, in order to produce distinct vision in each, amounted to nearly half an inch. Hence the difficulty of exhibiting the sun, moon, and planets through telescopes, and even terrestrial objects, to a company of persons who are unac- quainted with the mode of using or adjusting such MANNER OF USING TELESCOPES. 363 instruments — not one half of whom generally see the object distinctly — for, upon the proper adjust- ment of a telescope to the eye, the accuracy of vision, in all cases, depends ; and no one except the individual actually looking through the instru- ment, can be certain that it is accurately adjusted to his eye, and even the individual himself, from not being accustomed to the view of certain ob- jects, may be uncertain whether or not the adjust- ment be correct. I have found by experience that when the magnifying powers are high, as 150 or 200, the difference of adjustment required for different eyes is very slight ; but when low powers are used, as 20, 30, or 40, the difference of the requisite adjustments is sometimes very considera- ble, amounting to i or ^ of an inch. 2. State of the Atmosphere most proper for observing terrestrial and celestial objects. The atmosphere which is thrown around the globe — while it is essentially requisite to the physical constitution of our world, and the comfort of its inhabitants — is found in many instances a serious obstruction to the accurate performance of teles- copes. Sometimes it is obscured by mists and exhalations, sometimes it is thrown into violent undulations by the heat of the sun and the process of evaporation, and even, in certain cases, where there appears a pure unclouded azure, there is an agitation among its particles and the substances incorporated with them, which prevents the teles- cope from producing distinct vision either of ter- restrial or celestial objects. For viewing distant terrestrial objects, especially with high powers, the best time is early in the morning, a little after sun- rise, and, from that period till about 9 o'clock a.m., in summer ; and, in the evening about two or three hours before sun-set. From about 10 R 2 364 THE PRACTICAL ASTRONOMER. o'clock a.m. till 4 or 5 in the afternoon, in summer, if the sky be clear and the sun shining, there is generally a considerable undulation in the atmos- phere, occasioned by the solar rays and the rapid evaporation, which prevents high powers from being used with distinctness on any telescope, however excellent. The objects at such times, when powers of 50, 70, or 100 are applied, appear to undulate like the waves of the sea, and, not- withstanding every effort to adjust the telescope, they appear confused and indistinct. Even with very moderate magnifying powers this imperfec- tion is perceptible. In such circumstances, I have sometimes used a power of 200 times on distant land objects, with good effect, a little before sunset, when, in the forenoon of the same day, I could not have applied a power of 50 with any degree of distinctness. On days when the air is clear, and the atmosphere covered with clouds, terrestrial objects may be viewed with considerably high powers. When there has been a long-con- tinued drought, the atmosphere is then in a very unfit state for enjoying distinct vision with high magnifying powers, on account of the quantity of vapours with which the atmosphere is then sur- charged, and the undulations they produce. But, after copious showers of rain, especially if accom- panied with high winds, the air is purified, and distant objects appear with greater brilliancy and distinctness than at any other seasons. In using telescopes, the objects at which we look should, if possible, be nearly in a direction opposite to that of the sun. When they are viewed nearly in the direction of the sun, their shadows are turned towards us, and they consequently appear dim and obscure. By not attending to this circumstance, some persons, in trying telescopes, have pro- MANNER OF USING TELESCOPES. 365 nounced a good instrument to be imperfect, which, had it been tried on objects properly illuminated, would have been found to be excellent. In our variable northerly climate the atmosphere is not so clear and serene for telescopic observation as in Italy, the South of France, and in many of the countries which lie within the tropics. The un- dulations of the air, owing to the causes alluded to above, constitute one of the principal reasons why a telescope magnifying above a hundred times can seldom be used with any good effect in viewing terrestrial objects — though I have some- times used a power of nearly 200 with considera- ble distinctness, in the stillness of a summer or autumnal evening, when the rays of the declining sun strongly illuminated distant objects. The atmosphere is likewise frequently a great obstruction to the distinct perception of celestial objects. It is scarcely possible for one who has not been accustomed to astronomical observations, to form a conception of the very great difference there is in the appearance of some of the heavenly bodies in different states of the atmosphere. There are certain conditions of the atmosphere essentially requisite for making accurate observa- tions with powerful telescopes, and it is but seldom, especially in our climate, that all the favourable circumstances concur. The nights must be very clear and serene — the moon absent — no twilight — no haziness — no violent wind — no sudden change of temperature, as from thaw to frost — and no surcharge of the atmosphere with aqueous vapour. I have frequently found that, on the first and second nights after a thaw, when a strong frost had set in, and when the heavens appeared very brilliant, and the stars vivid and sparkling — the planets, when viewed with 366 THE PRACTICAL ASTRONOMER. high powers, appeared remarkably undefined and indistinct ; their margins appeared waving and jagged, and the belts of Jupiter, which at other times were remarkably distinct, were so obscured and ill-defined, that they could with difficulty be traced. This is probably owing to the quantity of aqueous vapour, and perhaps icy particles, then floating in the air, and to the undulations thereby produced. When a hard frost has continued a considerable time, this impediment to distinct observation is in a great measure removed. But I have never enjoyed more accurate and distinct views of the heavenly bodies than in fresh serene evenings, when there was no frost and no wind, and only a few fleecy clouds occasionally hovering around. On such evenings, and on such alone, the highest powers may be applied. I have used magnifying powers on such occasions with good effect, which could not have been applied, so as to ensure distinct vision, more frequently than two or three days in the course of a year. Sir William Herschel has observed, in reference to this point, 6 In beautiful nights, when the out- side of our telescopes is dropping with moisture, discharged from the atmosphere, there are now and then favourable hours in which it is hardly possible to put a limit to the magnifying powers. But such valuable opportunities are extremely scarce, and with large instruments it will always be lost labour to observe at other times. In order therefore, to calculate how long a time it must take to sweep the heavens, as far as they are within the reach of my forty-feet telescope, charged with a magnifying power of 1000, I have had recourse to my journals to find how many favourable hours we may annually hope for in this climate. And, under all favourable cir- MANNER OF USING TELESCOPES. 367 cumstances, it appears, that a year which will afford ninety, or at most, one hundred hours is to be called very productive/ 1 In the equator, with iny twenty feet telescope, I have swept over zones of two degrees with a power of 157, but an allowance of ten minutes in Polar distance must be made for lapping the sweeps over one another where they join. As the breadth of the zones may be increased towards the poles, the northern hemisphere may be swept in about 40 zones ; to these we must add 19 southern zones ; then 59 zones which, on account of the sweeps lapping over one another, about 5 minutes of time in right ascension, we must reckon of 25 hours each, will give 1475 hours. And allowing 100 hours per year, we find that with the 20 feet telescope, the heavens may be swept in about 14 years and three quarters. Now the time of sweeping with different magnifying powers will be as the squares of the powers: and putting^ and t for the power and time in the 20 feet telescope, and P=1000 for the power in the 40 feet instrument, we shall have p 2 : t : : P 2 : ~ =59840. Then making the same allowance for 100 hours per year, it appears that it will require not less than 598 years, to look with the 40 feet reflector, charged with the above-mentioned power, only one single moment into each point of space ; and even then, so much of the southern hemisphere will remain unexplored, as will take up 213 years more to examine. ,# From the above remarks of so eminent an observer, the reader will perceive how difficult it is to explore the heavens with minuteness and accuracy, and with how many disappointments; * Philosophical Transactions for 1800, Vol. XC. p. 80, &c. 368 THE PRACTICAL ASTRONOMER. arising from the state of the atmosphere, the astronomer must lay his account, when employed in planetery or sidereal investigation. Besides the circumstances now stated, it ought to be noticed that a star or a planet is only in a situation for a high magnifying power, about half the time it is above the horizon. The density of the atmos- phere, and the quantity of vapours with which it is charged near the horizon, prevent distinct vision of celestial objects with high powers, till they have risen to at least 15 or 20 degrees in altitude, and the highest magnifiers can scarcely be applied with good effect, unless the object is near the meridian, and at a considerable elevation above the horizon. If the moon be viewed a little after her rising, and afterwards when she comes to her highest elevation in autumn, the difference in her appearance and distinctness will be strikingly per- ceptible. It is impossible to guess whether a night be well adapted for celestial observations, till we actually make the experiment, and instru- ments are frequently condemned, when tried at improper seasons, when the atmosphere only is in fault. A certain observer remarks, — ' I have never seen the face of Saturn more distinctly than in a night when the air has been so hazy, that with my naked eye, I could hardly discern a star of less than the third magnitude.' The degree of the transparency of the air is likewise varying almost in the course of every minute, so that even in the course of the same half hour, planets and stars will appear perfectly defined, and the reverse. The vapours moving and undulating the atmosphere, even when the sky appears clear to the naked eye, will in a few instants destroy the distinctness of vision, and in a few seconds more, MANNER OF USING TELESCOPES. 369 the object will resume its clear and well-defined aspect.* 3. On the magnifying powers requisite for observing the phenomena of the different planets — comets — double stars, &c. There are some objects connected with astro- nomy which cannot be perceived without having recourse to instruments and to powers of great magnitude. But it is a vulgar error to imagine that very large and very expensive telescopes are absolutely necessary for viewing the greater part of the more interesting scenery of the heavens. Most of the phenomena of the planets, comets and double stars and other objects, are visible with instruments of moderate dimensions, so that every one who has a relish for celestial investiga- tions, may, at a comparatively small expense, procure a telescope, for occasional observations, which will show the principal objects and pheno- mena described in books on astronomy. Many persons have been misled by some occasional remarks which Sir W. Herschel made, in reference to certain very high powers which he sometimes put, by way of experiment, on some of his teles- copes, as if these were the powers requisite for viewing the objects to which he refers. For example, it is stated that he once put a power of 6450 times on his 7 feet Newtonian telescope of 6 ,-| inches aperture ; but this was only for the purpose of an experiment, and could be of no * In using telescopes within doors, care should generally be taken, that there be no fires in the apartment where they are placed for ob- servation, and that the air within be nearly of the same temperature as the air of the surrounding atmosphere ; for if the room be filled with heated air, when the windows are opened, there will be a cur- rent of cold air rushing in, and of heated air rushing out, which will produce such an undulation and tremulous motion, as will prevent any celestial object from being distinctly seen. R5 370 THE PRACTICAL ASTRONOMER. use whatever when applied to the moon, the planets and most objects in the heavens. Herschel, through the whole course of his writings, men- tions his only having used it twice, namely on the stars a Lyrae, and y Leonis, which stars can be seen more distinctly and sharply defined with a power of 420. To produce a power of 6450 on such a telescope, would require a lens of only f 7 th of an inch in focal distance, and it is questioned by some whether Herschel had lenses of so small a size in his possession, or whether it is possible to form them with accuracy. Powers requisite for observing the phenomena of the planets.— The planet Mercury requires a con- siderable magnifying power, in order to perceive its phases with distinctness. I have seldom viewed this planet with a less power than 100 and 150, with which powers its half moon, its gibbous, and its crescent phase, may be distinctly perceived. With a power of 40, 50, or even 60 times, these phases can with difficulty be seen, especially as it is generally at a low altitude, when such observations are made. The phases of Venus are much more easily distinguished, especially the crescent phase, which is seen to the greatest advantage about a month before and after the inferior conjunction. With a power not exceeding 25 or 30 times, this phase, at such periods, may be easily perceived. It requires, however, much higher powers to perceive distinctly the variations of the gibbous phase ; and if this planet be not viewed at a considerably high altitude when in a half-moon or gibbous phase, the obscurity and undulations of the atmosphere near the horizon, prevent such phases from being accurately distinguished, even when high powers are applied. Although certain phenomena of the planets may be seen with such low powers as I have now stated, yet, in every instance, the MANNER OF USING TELESCOPES. 371 highest magnifying powers, consistent with dis- tinctness, should be preferred, as the eye is not then strained, and the object appears with a greater degree of magnitude and splendour. The planet Mars requires a considerable degree of magnifying power, even when at its nearest dis- tance from the earth, in order to discern its spots and its gibbous phase. I have never obtained a satisfactory view of the spots which mark the surface, and their relative position, with a less power than 130, 160, or 200 times; and even with such powers, persons not much accustomed to look through telescopes, find a difficulty in dis- tinguishing them. The strongest and most prominent belts of Jupiter, may be seen with a power of about 45 ; which power may be put upon a 20-inch achro- matic, or a 1 foot reflector. But a satisfactory view of all the belts, and the relative positions they occupy, cannot be obtained with much lower powers than 80, 100, or 140. The most common positions of these belts are — one dark and well- defined belt to the south of Jupiter's equator ; another of nearly the same description to the north of it, and one about his north and his south polar circles. These polar belts are much more faint, and consequently not so easily distinguished as the equatorial belts. The moons of this planet, in a very clear night, may sometimes be seen with a pocket 1 foot achromatic glass, magnifying about 15 or 16 times. Some people have pretended that they could see some of these satellites with their naked eye ; but this is very doubtful, and it is probable that such persons mistook certain fixed stars which happened to be near Jupiter for his satellites. But, in order to have a clear and interesting view of these, powers of at least 80 or 100 times should be used. In order to perceive 372 THE PRACTICAL ASTRONOMER. their immersions into the shadow of Jupiter, and the exact moment of their emersions from it, a telescope not less than a 44 inch achromatic, with a power of 150 should be employed. When these satellites are viewed through large telescopes with high magnifying powers, they appear with well defined disks, like small planets. The planet Jupiter has generally been considered as a good test by which to try telescopes for celestial pur- poses. When it is near the meridian and at a high altitude, if its general surface, its belts, and its margin appear distinct and well-defined, it forms a strong presumptive evidence that the instrument is a good one. The planet Saturn forms one of the most interesting objects for telescopic observation. The ring of Saturn may be seen with a power of 45 ; but it can only be contemplated with advantage when powers of 100, 150, and 200 are applied to a 3 or a 5 feet achromatic. The belts of Saturn are not to be seen distinctly with au achromatic of less than 2f inches aperture, or a Gregorian reflector of less than 4 inches aperture, nor with a less magnifying power than 100 times. Sir W. Herschel has drawn this planet with five belts across its disk ; but it is seldom that above one or two of them can be seen by moderate-sized teles- copes and common observers. The division of the double ring, when the planet is in a favora- ble position for observation, and in a high altitude, may sometimes be perceived with a 44-inch achromatic, with an aperture of 2f inches, and with powers of 150 or 180, but higher powers and larger instruments are generally requisite to perceive this phenomenon distinctly ; and even when a portion of it is seen at the extremities of the anscz, the division cannot, in every case, be MANNER OF USING TELESCOPES. 373 traced along the whole of the half-circumference of the ring which is presented to our eye. Mr. Hadley's engraving of Saturn, in the ' Philosophi- cal Transactions' for 1723, though taken with a Newtonian reflector with a power of 228, repre- sents the division of the ring as seen only on the ansae or extremities of the elliptic figure in which the ring appears. The best period for observing this division is when the ring appears at its utmost width. In this position it was seen in 1840, and it will appear nearly in the same position in 1855. When the ring appears like a very narrow ellipse, a short time previous to its disappearance, the division, or dark space between the rings, cannot be seen by ordinary instruments. Sir W. Herschel very properly observes, c There is not perhaps another object in the heavens that presents us with such a variety of extraordinary phenomena as the planet Saturn; a magnificent globe, encompassed by a stupendous double ring ; attended by seven satellites ; ornamented with equatorial belts ; compressed at the poles ; turning upon its axis ; mutually eclipsing its ring and satellites, and eclipsed by them ; the most distant of the rings also turning upon its axis, and the same taking place with the farthest of the satel- lites ) all the parts of the system of Saturn occa- sionally reflecting light on each other ; the rings and moons illuminating the nights of the Saturnian, the globe and satellites enlightening the dark parts of the ring ; and the planet and rings throwing back the sun's beams upon the moons, when they are deprived of them at the time of their conjunc- tions.' This illustrious astronomer states, that with a new 7 feet mirror of extraordinary distinct- ness he examined this planet, and found that the ring reflects more light than the body, and with 374 THE PRACTICAL ASTRONOMER. a power of 570 the colour of the body becomes yellowish, while that of the ring remains more white. On March 11, 1780, he tried the powers of 222, 332, and 440 successively, and found the light of Saturn less intense than that of the ring ; the colour of the body turning, with the high powers, to a kind of yellow white, while that of the ring still remained white. Most of the satellites of Saturn are difficult to be perceived with ordinary telescopes, excepting the 4th, which may be seen with powers of from 60 to 100 times. It was discovered by Huygens in 1655, by means of a common refracting teles- cope 12 feet long, which might magnify about 70 times. The next in brightness to this is the 5th satellite, which Cassini discovered in 1671, by means of a 17 feet refractor, which might carry a power of above 80 times. The 3rd was discovered by the same astronomer in 1672, by a longer telescope ; and the 1st and 2nd, in 1684, by means of two excellent object-glasses of 100 and 136 feet, which might have magnified from 200 to 230 times. They were afterwards seen by two other glasses of 70 and 90 feet, made by Campani, and sent fmm Rome to the Royal Observatory at Paris, by the King's order, after the discovery of the 3rd and 5th satellites. It is asserted, how- ever, that all those 5 satellites were afterwards seen with a telescope of 34 feet, with an aperture of 3f 0 inches, which would magnify about 120 times. These satellites, on the whole, except the 4th and 5th, are not easily detected. Dr. Derham, who frequently viewed Saturn through Huygens' glass of 126 feet focal length, declares, in the preface to his ' Astro-Theology,' that he could never perceive above 3 of the satellites. Sir W. Herschel observes, that the visibility of these MANNER OF USING TELESCOPES. 375 minute and extremely faint objects, depends more on the penetrating than upon the magnifying power of our telescopes ; and that with a 10 feet Newtonian, charged with a magnifying power of only 60, he saw all the 5 old satellites ; but the 6th and 7th, which were discovered and were easily seen with his 40-feet telescope, and were also visible in his 20-feet instrument, were not discernible in the 7 or the 10-feet telescopes, though all that magnifying power can do may be done as well with the 7 -feet as with any larger in- strument. Speaking of the 7th satellite, he says, 1 Even in my 40-feet reflector it appears no bigger than a very small lucid point. I see it, however, very well in the 20-feet reflector ; to which the exquisite figure of the speculum not a little con- tributes.' A late observer asserts, that in 1825, with a 12-feet achromatic, of 7 inches aperture, made by Tulley, with a power of 150, the 7 satellites were easily visible, but not so easily with a power of 200 ; and that the planet appeared as bright as brilliantly burnished silver, and the division in the ring and a belt were very plainly distinguished, with a power of 200. The planet Uranus, being generally invisible to the naked eye, is seldom an object of attention to common observers. A considerable magnifying power is requisite to make it appear in a plane- tary form with a well-defined disk. The best periods for detecting it are, when it is near its opposition to the sun, or when it happens to approximate to any of the other planets, or to a well-known fixed star. When none of these cir- cumstances occur, its position requires to be pointed out by an Equatorial Telescope. On the morning of the 25th January, 1841, this planet happened to be in conjunction with Venus, at 376 THE PRACTICAL ASTRONOMER. which time it was only 4 minutes north of that planet. Several days before this conjunction, I made observations on Uranus. On the evening of the 24th, about 8 hours before the conjunction, the two planets appeared in the same field of the telescope, the one exceedingly splendid, and the other more obscure, but distinct and well-defined. Uranus could not be perceived, either with the naked eye, or with an opera glass ; but could be distinguished as a very small star by means of a pocket achromatic telescope magnifying about 14 times. It is questionable whether, under the most favourable circumstances, this planet can ever be distinguished by the naked eye. With magnifying powers of 30 and 70, it appeared as a moderately lai'ge star with a steady light, but without any sensible disk. With powers of 120, 180, and 250, it presented a round and pretty well-defined disk, but not so luminous and dis- tinct as it would have done in a higher altitude. The Double Stars require a great variety of powers, in order to distinguish the small stars that accompany the larger. Some of them are dis- tinguished with moderate powers, while others re- quire pretty large instruments, furnished with high magnifying eye-pieces. I shall therefore select only a few as a specimen. The star Castor, or . C/J Angles Minutt Distan< in feel Angles Minute Distant in feel Angles Minute Distant in feel Angles Minute Distant in feel 1 3438 31 110.9 1 20626.8 31 665.4 2 1719 32 107.4 2 10313. 32 644.5 3 1146 33 104.2 3 6875.4 33 625. 4 859.4 34 101.1 4 5)56.5 34 606.6 5 687.5 35 98.2 5 4125.2 35 589.3 6 572.9 36 95.5 6 3437.7 36 572.9 7 491.1 37 92 9 7 2946.6 37 557.5 8 429.7 38 90.4 8 2578.2 38 542.8 9 382 39 88.1 9 2291.8 39 528.9 10 343.7 40 85.9 10 2062.6 40 515.6 11 312.5 41 83.8 11 1875.2 41 503.1 12 286.5 42 81.8 12 1718.8 42 491.1 13 264.4 43 79.9 13 1586.7 43 479.7 14 245.5 44 78.1 14 1473.3 44 468.8 15 229.2 45 76.4 15 1375. 45 458.4 16 214.8 46 74.7 16 1298.1 46 448.4 17 202.2 47 73.1 17 1213.3 47 438.9 18 191 48 71.6 18 1145.9 48 429.7 19 181 49 70.1 19 1085.6 49 421. 20 171.8 50 68.7 20 1031.4 50 412.5 21 162.7 51 67.4 21 982.2 51 404.4 22 156.2 52 66.1 22 937.6 52 396.7 23 149.4 53 64.8 23 896.8 53 389.2 24 143.2 54 63.6 24 859.4 54 381.9 25 137.5 55 62.5 25 825. 55 375, 26 132.2 56 61.4 26 793.3 56 368.3 27 127.3 57 60.3 27 763.9 57 361.9 28 122.7 58 59.1 28 736.6 58 355.6 29 118.5 59 58.2 1 29 711.3 59 349.6 30 114.6 60 57.3 ! 30 687.5 60 343.7 452 THE PRACTICAL ASTRONOMER. In this way the distance of a considerably re- mote object, as a town or building at 10 or 12 miles distant, may be very nearly determined ; provided we have the lineal dimensions of a house or other object that stands at right angles to the line of vision. The breadth of a river, of an arm of the sea, or the distance of a light house, whose elevation above the sea or any other point, is known, may likewise in this manner be easily determined. THE EQUATORIAL TELESCOPE. 453 CHAPTER II. ON THE EQUATORIAL TELESCOPE, OR PORTABLE OBSERVATORY. The equatorial instrument is intended to answer a number of useful purposes in Practical Astro- nomy, independently of any particular observatory. Besides answering the general purpose of a Quad- rant, a Transit instrument, a Theodolite, and an Azimuth instrument — it is almost the only instru- ment adapted for viewing the stars and planets in the day time, and for following them in their apparent diurnal motions. It may be made use of in any steady room or place, and performs most of the useful problems in astronomical science. The basis of all equatorial instruments is a revolving axis, placed parallel to the axis of the earth, by which an attached telescope is made to follow a star or other celestial body in the arc of its diurnal revolution, without the trouble of repeated adjustments for changes of elevation, which quad- rants and circles with vertical and horizontal axes require. Such an instrument is not only con- venient for many useful and interesting purposes in celestial observations, but is essentially requi- site in certain cases, particularly in examining and measuring the relative positions of two con- 454 THE PRACTICAL ASTRONOMER. tiguous bodies, or in determining the diameters of the planets, when the spider's-line micrometer is used. Christopher Scheiner is supposed to have been the first astronomer who, in the year 1620, made use of a polar axis, but without any appendage of graduated circles. It was not, however, till the middle of the last century, that any instruments of this description, worthy of the name, were attempted to be constructed. In 1741, Mr. Henry Hindley, a clock-maker in York, added to the polar axis, an equatorial plate, a quadrant of altitude, and declination semicircle ; but when this piece of mechanism was sent to London for sale in 1748, it remained unsold for the space of 13 years. Mr. Short, the optician, published in the Philosophical Transactions, for 1750, a ' de- scription of an equatorial telescope/ which was of the reflecting kind, and was mounted over a com- bination of circles and semicircles, which were strong enough to support a tube, and a speculum of the Gregorian construction 18 inches in focal length. This instrument consisted of a somewhat cumbersome and expensive piece of machinery — a representation of which may be seen in volume III of Martin's ' Philosophia Britannica, or system of the Newtonian philosophy.' Various modifications of this instrument have since been made by Nairne, Dollond, Ramsden, Troughton, and other artists ; but even at the present period, it has never come into very general use, though it is one of the most pleasant and useful instruments connected with astronomical observations. As many of these instruments are somewhat complicated, and very expensive, I shall direct the attention of the reader solely to one which I consider as the most simple — which may be pur- THE EQUATORIAL TELESCOPE. 455 chased at a moderate expence, and is sufficiently accurate for general observations. This instrument consists of the following parts : A horizontal circle EF (fig. 86.) divided into four figure 86. quadrants of 90 degrees each. There is a fixed nonius at N ; and the circle is capable of being turned round on an axis. In the centre of the horizontal circle is fixed a strong upright pillar, which supports the centre of a vertical semicircle 456 THE PRACTICAL ASTRONOMER. AB, divided into two quadrants of 90 degrees each. This is called the semicircle of altitude, and may, at any time, serve the purpose of a quadrant in measuring either altitudes or depres- sions. It has a nonius plate at K. At right angles to the plane of this semicircle, the equa- torial circle MN is firmly fixed. It represents the equator, and is divided into twice 12 hours, every hour being divided into 12 parts of 5 minutes each. Upon the equatorial circle moves another circle, with a chamfered edge, carrying a nonius by which the divisions on the equatorial may be read off to single minutes ; and at right angles to this moveable circle is fixed the semi- circle of declination D, divided into two quadrants of 90 degrees each. The telescope PO, is sur- mounted above this circle, and is fixed to an index moveable on the semicircle of declination, and carries a nonius opposite to (J. The telescope is furnished with 2 or 3 Huygenian eye-pieces, and likewise with a diagonal eye-piece for viewing objects near the zenith. Lastly, there are 2 spirit levels fixed on the horizontal circle, at right angles to each other, by means of w T hich this circle is made perfectly level when observations are to be made. To adjust the equatorial for observation. Set the instrument on a firm support. Then to adjust the levels and the horizontal circle : — Turn the horizontal circle till the beginning O of the divi- sions coincides with the middle stroke of the nonius, or near it. In this situation one of the levels will be found to lie either in a right line joining the 2 foot screws which are nearest the nonius, or else parallel to such a right line. By means of the 2 last screws, cause the bubble in the level to become stationary in the middle of THE EQUATORIAL TELESCOPE. 457 the glass ; then turn the horizontal circle half round, by bringing the other O to the nonius ; and if the bubble remains in the middle, as before, the level is well-adjusted ; if it does not, correct the position of the level, by turning one or both of the screws which pass through its ends, till the bubble has moved half the distance it ought to come to reach the middle, and cause it to move the other half by turning the foot-screws already mentioned. Return the horizontal circle to its first position, and if the adjustments have been well made, the bubble will remain in the middle : if otherwise, the process must be repeated till it bears this proof of its accuracy. Then turn the horizontal circle till 90° stands opposite to the nonius ; and by the foot-screw, immediately oppo- site the other 90°, cause the bubble of the same level to stand in the middle of the glass. Lastly, by its own proper screws set the other level so that its bubble may occupy the middle of its glass. To adjust the line of sight. Set the nonius on the declination semicircle at O, the nonius on the horary circle at VI, and the nonius on the semi- circle of altitude at 90. Look through the teles- cope towards some part of the horizon, where there is a diversity of remote objects. Level the horizontal circle, and then observe what object appears in the centre of the cross-wires, or in the centre of the field of view, if there be no wires. Reverse the semicircle of altitude, so that the other 90° may apply to the nonius ; taking care, at the same time, that the other three noniuses continue at the same parts of their respective graduations as before. If the remote object con- tinues to be seen on the centre of the cross-wires, the line of sight is truly adjusted. To find the correction to be applied to observa- x 458 THE PRACTICAL ASTRONOMER. tions by the semicircle of altitude. Set the nonius on the declination-semicircle to 0, and the nonius on the horary circle to XII ; direct the telescope to any fixed and distant object, by moving the horizontal circle and semicircle of altitude, and nothing else ; note the degree and minute of alti- tude or depression ; reverse the declination-semi- circle, by directing the nonius on the horary circle to the opposite XII ; direct the telescope again to the same object, by means of the horizontal circle and semicircle of altitude, as before. If its altitude or depression be the same as was observed in the other position, no correction will be re- quired ; but, if otherwise, half the difference of the two angles is the correction to be added to all observations made with that quadrant, or half of the semicircle which shows the least angle, or to be subtracted from all the observations made with the other quadrant, or half of the semicircle. When the levels and other adjustments are once truly made, they will be preserved in order for a length of time, if not deranged by violence ; and the correction to be applied to the semicircle of altitude is a constant quantity. Description of the nonius. The nonius— some- times called the vernier — is a name given to a device for subdividing the arcs of quadrants and other astronomical instruments. It depends on the simple circumstance, that if any line be divided into equal parts, the length of each part will be greater, the fewer the divisions ; and con- trariwise, it will be less in proportion as those divisions are more numerous. Thus, in the equatorial now described^ the distance between the two extreme strokes on the nonius is exactly equal to 11 degrees on the limb, but that it is divided into 12 equal parts. Each of these last THE EQUATORIAL TELESCOPE. 459 parts will therefore be shorter than the degree on the limb in the proportion of 11 to 12, that is to say, it will be ^th part, or 5 minutes shorter. Consequently, if the middle stroke be set pre- cisely opposite to any degree, the relative posi- tions of the nonius and the limb must be altered 5 minutes of a degree, before either of the two adjacent strokes next the middle on the nonius, can be brought to coincide with the nearest stroke of a degree ; and so likewise the second stroke on the nonius will require a change of 10 minutes, the third of 15, and so on to 30, when the middle line of the nonius will be seen to be equi-distant between 2 of the strokes on the limb ; after which the lines on the opposite side of the nonius will coincide in succession with the strokes on the limb. It is clear from this, that whenever the middle stroke of the nonius does not stand pre- cisely opposite to any degree, the odd minutes — or distance between it and the degree immediately preceding — may be known by the number of the stroke marked on the nonius, which coincides with any of the strokes on the limb.* In some instruments the nonius-plate has its divisions fewer than the number of parts on the limb to which it is equal ; but when once a clear idea of the principle of any nonius is obtained, it will be easy to transfer it to any other mode in which this instrument is contrived. To find by this equatorial the meridian line, and the time, from one observation of the sun. In order to this it is requisite that the sun's declination, and the latitude of the place be known. The declination of the sun may be found, for every day, in the Nautical Almanack, or any other astronomical Ephemeris; and the * Adams' Introduction to Practical Astronomy. X 2 460 THE PRACTICAL ASTRONOMER, latitude of the place may be found by means of the semicircle of altitude, when the telescope is directed to the sun or a known fixed star. It is likewise requisite to make the observation when the azimuth and altitude of the sun alter quickly ; and this is generally the case, the farther that luminary is from the meridian: — Therefore, at the distance of 3 or 4 hours, either before or after noon, (in summer) adjust the horizontal circle ; set the semicircle of altitude, so that its nonius may stand at the co-latitude of the place ; lay the plane of the last-mentioned semicircle in the meridian, by estimation, its 0 being directed towards the depressed pole ; place the nonius of the declination semicircle to the declination, whether north or south. Then direct the teles- cope towards the sun, partly by moving the declination semicircle on the axis of the equatorial circle, and partly by moving the horizontal circle on its own axis. There is but one position of these which will admit of the sun being seen ex- actly in the middle of the field of view. When this position is obtained, the nonius on the equa- torial circle shows the apparent time, and the circle of altitude is in the plane of the meridian. When this position is ascertained, the meridian may be settled by a land-mark at a distance. With an equatorial instrument, nearly similar to that now described, I formerly made a series of 6 day observations on the celestial bodies/ which were originally published in vol. 36 of * Nicholson's Journal of Natural Philosophy,' and which occupy twenty pages of that journal. Some of these observations I shall lay before the reader, after having explained the manner in which they are made. The instrument was made by Messrs. W. and THE EQUATORIAL TELESCOPE. 461 S. Jones, opticians, Holborn, London. The telescope which originally accompanied the instru- ment was an achromatic refractor, its object-glass being 8^ inches focal distance, and one inch diameter. This telescope, not admitting suffi- ciently high magnifying powers for the observa- tions intended, was afterwards thrown aside for another telescope, having an object-glass 20 inches focal length, and If inch diameter, which was attached to the equatorial machinery in place of the small telescope. It was furnished with mag- nifying powers of 15, 30, 45, 60, and 100 times. The instrument was placed on a firm pedestal about three feet high. The feet of this pedestal had short iron pikes, which slipped into corre- sponding holes in the floor of the apartment adja- cent to a south window, so that when the direc- tion of the meridian was found, and the circles properly adjusted, the instrument was in no danger of being shifted from this position. Though this instrument generally stood fronting the southern part of the heavens, yet the equatorial part, along with the telescope, could occasionally be removed to another position fronting the north and north-west, for observing the stars in those quarters. Manner of observing stars and planets in the day -time by the equatorial. Before such observa- tions can be made, the semicircle of altitude must be placed in the meridian, and the degree and minute pointed out by the nonius on the horizon- tal circle, when in this position, noted down in a book, so that it may be placed again in the same position, should any derangement afterwards happen. The semicircle of altitude must be set to the co-latitude of the place ; that is, to what the latitude wants of 90°. Suppose the latitude 462 THE PRACTICAL ASTRONOMER. of the place of observation be 52° 30' north, this latitude subtracted from 90°, leaves 37° 30' for the co-latitude ; and therefore, the semicircle of altitude — on which the equatorial circle is fixed — must be elevated to 37° 30', and then the equa- torial circle on the instrument coincides with the equator in the heavens. Lastly, the telescope must be adjusted on the declination semicircle, so as exactly to correspond with the declination of the heavenly body to be viewed. If the body is in the equator, the telescope is set by the index at 0 on the semicircle of declination, or at the middle point between the two quadrants, and then when the telescope, along with the semi- circle of declination, is moved from right to left, or the contrary, it describes an arc of the equator. If the declination of the body be north, the teles- cope is elevated to the northern division of the semicircle ; if south, to the southern part of it. These adjustments being made, take the dif- ference between the Right Ascension of the sun and the body to be observed ; and if the Right Ascension of the body be greater than that of the sun, subtract the difference from the time of ob- servation ; if not, add to the time of observation.* The remainder in one case, or the sum in the other, will be the hour and minute to which the nonius on the equatorial circle is to be set ; which being done, the telescope will point to the star or planet to whose declination the instrument is ad- justed. When the heavenly body is thus found, it may be followed, in its diurnal course, for hours, * Or find the sun's right ascension for the given day ; substract this from the star or planet's right ascension, and the remainder is the ap- proximate time of the star's coming to the meridian. The difference between this time and the time of observation, will then determine the point to which the telescope is to be directed. THE EQUATORIAL TELESCOPE. 463 or as long as it remains above the horizon. For as the diurnal motion of a star is parallel to the equator, the motion of the telescope on the equa- torial circle, will always be in the star's diurnal arc ; and should it have left the field of the telescope for any considerable time, it may be again re- covered, by moving the telescope onward accord- ing to the time which elapsed since it was visible in the field of view. We may illustrate what has been now stated by an example or two. Suppose on the 30th April, 1841, at 1 o'clock, p.m. we wished to see the star Aldebaran. The Right Ascension of this star is 4 h 27 m ; and the sun's Right Ascension for that day at noon, as found in ' White's Ephemeris/ or the ' Nautical Alma- nack/ is 2 h 30 m . Subtract this last number from 4 h 27 m , and the remainder l h 57 ra , shows that the star comes to the meridian on that day at 57 minutes past 1 o'clock, p.m. And as the time of observation is 1 p.m., the nonius which moves on the equatorial circle must be set to 3 minutes past XI, as the star is at that hour 57 minutes from the meridian. The declination of Aldebaran is 16° IT north, to which point on the semicircle of de- clination, the telescope must be adjusted, and then the star will be visible in the field of view. Again, suppose we wished to observe the planet Venus on the 1st January, 1842, at 12 o'clock noon. The sun's Right Ascension on that day is 18 h 46 m , and that of Venus 17 h 41 m , from which the sun's Right Ascension being subtracted, the remainder is 22 h 55 m , or 55 minutes past 10, a.m. Here, as the Right Ascension of Venus is too small to have the sun's Right Ascension taken from it, we borrow 24 hours, and reckon the remainder from XII at noon. As the planet at 12 noon, is 1 hour 5 minutes past the meridian, 484 THE PRACTICAL ASTRONOMER. the nonius on the equatorial circle must be set to that point, and the telescope adjusted to 23° 6' of south declination, which is the declination of Venus for that day, when this planet will appear in the field of view. Observations on the fixed stars and planets, made in the day-time by the Equatorial. For the purpose of illustrating the descriptions now given, and for affording some information respecting celestial day observations, I shall select a few of the observations above alluded to, which I formerly published in Nicholson's Journal, along with a few others which have been since made. These observations were made with a view to determine the following particulars : — 1. What stars and planets may be conveniently seen in the day-time, when the sun is above the horizon ? 2. What degrees of magnifying power are requi- site for distinguishing them ? 3. How near their conjunction with the sun they may be seen ? and 4. Whether the diminution of the aperture of the object-glass of the telescope, or the increase of magnifying power, conduces most to render a star or a planet visible in day-light. Having never seen such observations recorded in books of astro- nomy or in scientific journals, I was induced to continue them, almost every clear day for nearly a year, in order to determine the points now specified. Some of the results are stated in the following pages. Observations on fixed stars of the first magni- tude. April 23, 1813, at 10 h 15 m , a.m., the sun being 5^ hours above the horizon. Saw the star Vega, or » Lyrae, very distinctly with a power of 30 times. Having contracted the aperture of the object-glass to r 0 of an inch, saw it on a darker ground, but not more plainly than before. OBSERVATIONS ON THE FIXED STARS. 465 Having contracted the aperture still farther, to half an inch, I perceived the star, but not so distinctly as before, The sky being very clear, and the star in a quarter of the heavens nearly opposite to the sun, I diminished the magnifying power to 15, and could still perceive the star, but indistinctly ; it was just perceptible. August 23, at 0 h 12 m , p.m., saw the star Capella, or a Auriga, with a power of 60, and immediately afterwards with a power of 30 ; the aperture un- diminished. With this last power it appeared extremely distinct, but not so brilliant and splen- did as with the former power. Having diminished the aperture to f 0 of an inch, it appeared on a darker ground, though in the former case, it was equally perceptible. A few minutes afterwards, could distinguish it with a power of 15, the aper- ture being contracted to half an inch. It appeared very small ; it was with difficulty the eye could fix upon it in the field of the telescope ; but when it was once perceived, its motion across the field of view could be readily followed. It could not be perceived, when the diminished aperture was remov- ed. The sun was then shining in meridian splendour. August 10th, 9 h 30 m , a.m. Saw the star Sirius with a power of 60, the aperture contracted to £ inch. Saw it likewise when the aperture was diminished to half an inch, but not so distinctly as through the aperture of ^ inch. Having put on a power of 30, could distinguish it distinctly enough through each of the former apertures, and likewise when they were removed ; but somewhat more distinctly with the apertures of nine-tenths and half an inch than without them. At this time the star was 2 h 42 m in time of Right Ascen- sion west of the sun, having an elevation above the horizon of about 17° 10'; the sun shining X 5 466 THE PRACTICAL ASTRONOMER. bright, and the sky very much enlightened in that quarter of the heavens where the star appeared. There was also a considerable undulation of the air, which is generally the case in the hot morn- ings of summer — which renders a star more dif- ficult to be perceived than in the afternoon, especially when it is viewed at a low altitude. June 4th, l h 30 m , p.m., saw Sirius with a power of 30 with great distinctness, the aperture not contracted. The star was then within l h 50 m , in time of Right Ascension east from the sun. August 24th, 9 h 5 m , a.m., saw the star Procyon, or a Canis- Minor is distinctly with a power of 60, the aperture not contracted. When diminished to ft inch, it appeared rather more distinct, as the ground on which it was seen was darker. With a power of 30, and the aperture contracted to ^ inch, could perceive it, but somewhat indistinctly. When the equatorial motion was performed, in order to keep it in the field of view, it was some- time before the eye could again fix upon it. When the aperture was diminished to half an inch, it could not be perceived. Saw it when both the apertures were removed, but rather more dis- tinctly with the aperture of f 0 inch. The difference in the result of this observation, from that of Capella, above stated, was owing to the star's proximity to the sun, and the consequent illumi- nation of the sky in that quarter where it ap- peared. Its difference in Right Ascension from that of the sun was then about 2 h 5m of time, and its difference of declination about 4° 50'.* This * The right ascensions, declinations, longitudes, &c, stated in these memoranda — which were noted at the time of observation — are only approximations to the truth ; perfect accuracy in these respects being of no importance in such observations. They are, however, in gene- ral, within a minute or two of the truth. The times of the observa- tions, too, are noted in reference — not to the astronomical, but to the OBSERVATIONS ON THE FIXED STARS. 487 star may be considered as one of those which rank between the first and second magnitudes. Similar observations to the above were made and frequently repeated on the stars Rigel, Aldebaran, Betelguese Cor-Leonis and other stars of the first magnitude, which gave nearly the same results. The stars Altares and Fomal- haut are not so easily distinguished, on account of their great southern declination, and conse- quent low elevation above the horizon. The following observation on Arcturus may be added. June 3rd, observed Arcturus very distinctly, a little before 7 in the evening, the sun being about l h 40 m above the horizon, and shining bright — with a power of 15 ; the aperture not contracted. It appeared very small but distinct. This star is easily distinguishable at any time of the day with a power of 30. Observations on stars of the second magnitude. May 5, 1813, at 6 h , p.m. ; the sun being an hour and three quarters above the horizon. Saw Alphard, or a Hydrae, a star of the second mag- nitude, with a power of 60 ; the aperture diminished to $ inch. A few minutes afterwards could perceive it, but indistinctly, with a power of 30, the aperture contracted as above. It could not be seen very distinctly with this power, till about half an hour before sun-set. It was then seen rather more distinctly when the aperture was contracted than without the contraction. May 7th. Saw the star Deneb, or jS Leonis, distinctly with a power of 60, about an hour and a half be- fore sun-set. August 20th. Saw Ras Alkague, or a Ophiuchi, at 4 h 40 m , p.m., with a power of civil day. The astronomical day commences at 12 noon, and the hours are reckoned, without interruption, to the following noon. The civil day commences at 12 midnight. 468 THE PRACTICAL ASTRONOMER. 100, the sun being nearly 3 hours above the hori- zon, and shining bright. Perceived it about an hour afterwards, with a power of 60 — with the aperture contracted to § inch, and also when this contraction w r as removed. The star was seen nearly as distinctly in the last case as in the first. August 27, 5 h , p.m., the same star appeared quite distinct with a power of 60, the aperture not contracted. It did not appear more distinct when the aperture was contracted to £ inch. The sun was then more than 2 hours above the horizon. August 28th. Saw the star Pollux, or |3 Gemini, 2 hours after sun-rise with a power of 60, aperture un- dimished. November 12th, l h 30', p.m. Saw the star Altair, or « Aquiloe, with an 8^- inch teles- cope, 1 inch aperture, carrying a power of 45, the aperture not contracted. Having contracted the aperture a little, it appeared somewhat less dis- tinct. This star is reckoned by some to belong to the class of stars of the first magnitude ; but in White's 'Ephemeris' and other Almanacks, it is generally marked as being of the second magni- tude. It forms a kind of medium between stars of the 1st and of the 2nd magnitude. Similar observations, giving the same results, were made on the stars Ballatrix, Orion's Girdle, a Andromedae, a Pegasi, Alioth, Benetnach, North Crown, or a Coronas Borealis, and various other stars of the same magnitude. From the above and several hundreds of similar observations, the following conclusions are deduced. ]. That a magnifying power of 30 times is sufficient for distinguishing a fixed star of the first magnitude, even at noon-day, at any season of the year ; provided it have a moderate degree of elevation above the horizon, and be not within 30° or 40° of the sun's body. Also, that, by a OBSERVATIONS ON THE PLANETS. 469 magnifying power of 15, a star of this class may be distinguished, when the sun is not above an hour and a half above the horizon. But, in every case, higher powers are to be preferred. Powers of 45 or 60, particularly the last, were found to answer best in most cases, as with such powers the eye could fix on the star with ease, as soon as it entered the field of the telescope. 2. That most of the stars of the 2nd magnitude may be seen with a power of 60, when the sun is not much more than 2 hours above the horizon ; and, at any time of the day, the brightest stars of this class may be seen with a power of 100, when the sky is serene, and the star not too near the quarter in which the sun appears. 3. That, in every instance, an increase of mag- nifying power has the principal effect in rendering a star easily perceptible. That diminution of aperture, in most cases, produces a very slight effect; in some cases, none at all; and, when the aperture is contracted beyond a certain limit, it produces a hurtful effect. The cases in which a moderate contraction is useful, are the two follow- ing : — 1. When the star appears in a bright part of the sky, not far from that quarter in which the sun appears. 2. When an object-glass of a large aperture, and a small degree of magnifying power, is used. In almost every instance the contrac- tion of the object-glass of the 8|-inch telescope with a power of 45, had a hurtful effect. But when the 20-inch telescope carried a power of only 15, the contraction served to render the object more perceptible. Observations on the Planets made in the day-time. Some of the planets are not so easily distin- guished in the day-time as the fixed stars of the 470 THE PRACTICAL ASTRONOMER. first magnitude. The one which is most easily distinguished at all times, is the planet Venus. 1. Observations on Venus, My observations on this planet commenced about the end of August, 1812, about three or four weeks after its inferior conjunction. About that period, between ten and eleven in the forenoon, with a power of 45, it appeared as a beautiful crescent, quite distinct and well-defined, with a lustre similar to that of the moon about sun-set, but of a whiter colour. The view of its surface and phase was fully more distinct and satisfactory than what is obtained in the evening after sun-set; for, being at a high elevation, the undulation near the horizon did not affect the distinctness of vision. The planet was then very distinctly seen with a power of 7 times, when it appeared like a star of the first or second magnitude. I traced the variation of its phases, almost every clear day, till the month of May, 1813. As at that time, it was not far from its superior conjunction with the sun, I wished to ascertain how near its conjunction with that luminary it might be seen ; and particularly whether it might not be possible, in certain cases, to see it at the moment of its conjunction. The expressions of all astronomical writers pre- vious to this period, when describing the phases of Venus, either directly assert, or, at least imply, that it is impossible to see that planet, in any instance, at the time of its superior conjunction. This is the language of Dr. Long, Dr. Gregory, Dr. Brewster, Ferguson, Adams, B. Martin, and most other writers on the science of astronomy. How far such language is correct will appear from the following observations and remarks. April 24, 1813, 10* 50' a.m. Observed Venus with a power of 30, the aperture not contracted. She was then about 31 minutes, in time, of right OBSERVATIONS ON THE PLANETS. 471 ascension, distant from the sun. Their difference of declination 3° 59'. She appeared distinct and well-defined. With a power of 100, could dis- tinguish her gibbous phase. May 1st, 10 h 20 m , a.m. Viewed this planet with a power of 60 ; the aperture not contracted. It appeared dis- tinct. Saw it about the same time with a power of 15, the aperture being contracted to & inch. Having contracted the aperture to \ inch, saw it more distinctly. When the contracted apertures were removed, the planet could with difficulty be distinguished, on account of the direct rays of the sun striking on the inside of the tube of the tele- scope. The sun was shining bright, and the planet about 25' of time in R.A. west of his cen- tre, their difference of declination being 8° 7'. May 7th, 10 h , a.m. Saw Venus distinctly with a power of 60, the sun shining bright. It was then about 19' in time of R.A. and 4° 27' in longitude west of the sun ; their difference of declination being 2° 18'. I found a diminution of aperture particularly useful when viewing the planet at this time, even when the higher powers were applied. This was the last observation I had an opportu- nity of making prior to the conjunction of Venus with the sun, which happened on May 25 th, at 9 h 30 m , a.m. Its geocentric latitude at that time being about 16' south, the planet must have passed almost close by the sun's southern limb. Cloudy weather for nearly a month after the last observa- tion, prevented any further views of the planet, when it was in that part of the heavens which was within the range of the instrument. The first day that proved favourable after it had passed the superior conjunction, was June 5th. The following is the memorandum of the observation then taken. June 5th, 9 b , a.m. Adjusted the Equatorial 472 THE PRACTICAL ASTRONOMER. Telescope for viewing the planet Venus, but it could not be perceived, on account of the direct rays of the sun entering the tube of the tele- scope. I contrived an apparatus for screening his rays, but could not get it conveniently to move along with the telescope; and therefore deter- mined to wait till past eleven, when the top of the window of the place of observation would inter- cept the solar rays. At ll h 20 m , a.m., just as the sun had passed the line of sight from the eye to the top of the window, and his body was eclipsed by it, I was gratified with a tolerably dis- tinct view of the planet, with a power of 60. The aperture being contracted to f 0 inch. The distinctness increased as the sun retired, till, in two or three minutes, the planet appeared per- fectly well-defined. Saw it immediately after- wards, with a power of 30, the aperture contracted as before. Saw it also quite distinctly with a power of 15 ; but it could not be distinguished with this power, when the contracted aperture was removed. At this time Venus was just 3° in longitude, or about 13' in time of R. A. east of the sun's centre, and of course only about 2f degrees from his eastern limb ; the difference of their declination being 27', and the planet's latitude ll 7 north. Several years afterwards, I obtained views of this planet, when considerably nearer the sun's margin than as stated in the above observation, particularly on the 16th October, 1819, when Venus was seen when only 6 days and 19 hours past the time of the superior conjunction. At that time its distance from the sun's eastern limb was only 1° 28' 42". A subsequent observation proved that Venus can be seen when only 1° 27', from the sun's margin — which I consider as ap- OBSERVATIONS ON THE PLANETS. 473 proximating to the nearest distance from the sun at which this planet is distinctly visible. — I shall only state farther the two or three following observations. June 7th, 1813, 10 h , a.m. Saw Venus with a power of 60, the aperture being contracted to re inch — the direct rays of the sun not being inter- cepted by the top of the window. The aperture having being further contracted to \ inch, could perceive her, but not quite so distinctly. When the contractions were removed, she could scarcely be seen. She was then 3° 33' in longitude, and nearly 15 minutes in time of R.A. distant from the sun's centre. Some fleeces of clouds having moved across the field of view, she was seen re- markably distinct in the interstices — the sun at the same time, being partly obscured by them. — August 19th, l h 10', p.m. Viewed Venus with a magnifying power of 100. Could perceive her surface and gibbous phase almost as distinctly as when the sun is below the horizon. She appeared bright, steady in her light, and well defined, with- out that glare and tremulous appearance she exhi- bits in the evening when near the horizon. She was then nearly on the meridian. On the whole, such a view of this planet is as satisfactory, if not preferable, to those views we obtain with an ordinary telescope in the evening, when it is visi- ble to the naked eye. All the particulars above stated have been con- firmed by many subsequent observations continued throughout a series of years. I shall state only two recent observations which show that Venus may be seen somewhat nearer the sun than what is deduced from the preceding observations, and at the point of its superior conjunction. March 10th, 1842, observed the planet Venus, then very near 474 THE PRACTICAL ASTRONOMER. the sun, at 19 minutes past 11, a.m. It had passed the point of its superior conjunction with the sun, on the 5th March, at l h 19 m , p.m. The difference of right ascension between the sun and the planet was then about 6^ minutes of time, or about 1° 37^', and it was only about 1° 21' distant from the sun's eastern limb. It appeared quite distinct and well-defined, and might perhaps have been seen on the preceding day, had the observa- tion been then made. — The following observation shows that Venus may be seen still nearer the sun than in the preceding observations, and even at the moment of its superior conjunction. On the 2nd of October, 1843, this planet passed the point of its superior conjunction with the sun, at 4 h 15 m , p.m. At two o'clock, p.m. — only two hours before the conjunction, I perceived the planet dis- tinctly, and kept it in view for nearly ten minutes, till some dense clouds intercepted the view. It appeared tolerably distinct and well-defined, though not brilliant, and with a round full face, and its apparent path was distinctly traced several times across the field of view of the telescope. I per- ceived it afterwards, about half past four, p.m., only a few minutes after it had passed the point of conjunction, on which occasion it appeared less distinct than in the preceding observation, owing to the low altitude of the planet, being then only a few degrees above the horizon. The observa- tions, in this instance, were made not with an equa- torial instrument, which I generally use in such observations, but with a good achromatic telescope 44^ inches focal distance, mounted on a common tripod, with a terrestrial power of 95 times. A conical tube about ten inches long was fixed on the object-end of the telescope, at the extremity of which an aperture, \\ inch diameter was placed, OBSERVATIONS ON THE PLANETS. 475 so as to intercept, as much as possible, the direct ingress of the solar rays. The top of the upper sash of the window of the place of observation was likewise so adjusted as to intercept the greater part of the sun's rays from entering the tube of the telescope. The sun's declination at that time was 3° 26' south, and that of Venus 2° 12' south; con- sequently, the difference of declination was 1° 14' = the distance of Venus from the sun's cen- tre; and as the sun's diameter was about 16', Venus was then only 58' from the sun's northern limb, or 6' less than two diameters of the sun. This is the nearest approximation to the sun at which I have ever beheld this planet, and it de- monstrates that Venus may be seen even when within a degree of the sun's margin ; and it is perhaps the nearest position to that luminary in which this planet can be distinctly perceived. It shows that the light reflected from the surface of Venus is far more brilliant than that reflected from the surface of our moon ; for no trace of this nocturnal luminary can be perceived, even when at a much greater distance from the sun, nor is there any other celestial body that can be seen within the limit now stated. This is the first ob- servation, so far as my information extends, of Venus having been seen at the time of her superior conjunction.* The practical conclusion from this observation is, that, at the superior conjunction of this planet, when its distance from the sun's margin is not less than 58', its polar and equatorial diameter may be measured by a micrometer, when it will be determined whether or not Venus be of a sphe- roidal figure. The Earth, Mars, Jupiter and * This observation is inserted in the ' Edinburgh Philosophical Journar for January, 1844. 476 THE PRACTICAL ASTRONOMER. Saturn are found to be not spheres but spheroids, having their polar shorter than their equatorial diameters. But the true figure of Venus has never yet been ascertained, because it is only at the superior conjunction that she presents a full enlightened hemisphere, and when both diameters can be measured, except at the time when she transits the sun's disk, which happens only twice in the course of 120 years.* * The late Mr. Benjamin Martin, when describing the nature of the solar telescope, in his ' Philosophia Britannica^ Vol. iii. p. 85, gives the following relation : — ' 1 cannot here omit to mention a very unusual pJienomenon that I observed about ten years ago in my darkened room. The window looked towards the west, and the spire of Chichester Cathedral was before it at the distance of 50 or 60 yards. I used very often to divert myself by observing the pleasant manner in which the sun passed behind the spire, and was eclipsed by it for sometime ; for the image of the sun and of the spire were very large, being made by a lens of 12 feet focal distance. And once as I observed the oc^ cultation of the sun behind the spire, just as the disk disappeared, I saw several small, bright, round bodies or balls running toward the sun from the dark part of the room, even to the distance of 20 inches. I observed their motion was a little irregular, but rectilinear, and seemed accelerated as they approached the sun. These luminous globules appeared also on the other side of the spire, and preceded the sun, running out into the dark room, sometimes more, sometimes less, together in the same manner as they followed the sun at its oc- cultation. They appeared to be in general one-twentieth of an inch in diameter, and therefore, must be very large luminous globes in some part of the heavens, whose light was extinguished by that of the sun, so that they appeared not in open day light ; but whether of the meteor kind, or what sort of bodies they might be, I could not con- jecture.' Professor Hansteen mentions, that when employed in mea- suring the zenith distances of the pole star, he observed a somewhat similar phenomenon, which he described as 4 a luminous body which passed over the field of the universal telescope — that its motion was neither perfectly equal nor rectilinear, but resembled very much the unequal and somewhat serpentine motion of an ascending rocket ; ' and he concluded that it must have been 4 a meteor ' or' shooting star 1 descending from the higher regions of the atmosphere. 1 In my frequent observations on Venus, to determine the nearest positions to the sun in which that planet could be seen, I had several times an opportunity of witnessing similar phenomena. I was not a little surprised, when searching for the planet, frequently to perceive a body pass across the field of the telescope, apparently of the same 1 See Edinburgh Philosophical Journal, for April, 1825. No. XXIV, OBSERVATIONS ON THE PLANETS. 477 The following conclusions are deduced from the observations made on Venus. 1. That this planet may be seen distinctly, with a moderate degree of magnifying power, at the moment of its superior conjunction ivith the sun 9 size as Venus, though sometimes larger and sometimes smaller, so that 1 frequently mistook that body for the planet, till its rapid motion undeceived me. In several instances four or five of these bodies ap- peared to cross the field of view, sometimes in a perpendicular, and, at other times in a horizontal direction. They appeared to be lumi- nous bodies, somewhat resembling the appearance of a planet when viewed in the day-time with a moderate magnifying power. Their motion was nearly rectilinear, but sometimes inclined to a waving or serpentine form, and they appeared to move with considerable rapidity — the telescope being furnished with a power of about 70 times. I was for a considerable time at a loss what opinion to form of the nature of these bodies ; but having occasion to continue these observations almost every clear day for nearly a twelvemonth, I had frequent op- portunities of viewing this phenomenon in different aspects ; and was at length enabled to form an opinion as to the cause of at least some of the appearances which presented themselves. In several instances, the bodies alluded to appeared much larger than usual, and to move with a more rapid velocity ; in which case I could plainly perceive that they were nothing else than birds of different sizes, and appa- rently at different distances, the convex surfaces of whose bodies, in certain positions, strongly reflected the solar rays. In other instances, when they appeared smaller, their true shape was undistinguishable by reason of their motion and their distance. Having inserted a few remarks on this subject, in No. XXV. of the Edinburgh Philosophical Journal for July, 1825, particularly in re- ference to Professor Hansteen's opinion, that article came under the review of M. Serres, Sub-Prefect of Embrun, in a paper inserted in the Annates de Chemie^ for October, 1825, entitled, ' Notices regarding fiery meteors seen during the day.' 1 In the discussion of this sub- ject, M. Serres admits that the light reflected \ery obliquely from the feathers of a bird is capable of producing an effect similar to that which I have now described ; but that 4 the explanation ought not to be generalized.'' He remarks, that, while observing the sun at the re- peating circle, he frequently perceived, even through the coloured glass adapted to the eye-piece, large luminous points which traversed the field of the telescope, and which appeared too well defined not to admit them to be distant, and subtended too large angles to imagine them birds. In illustration of this subject he states the following facts. On the 7th September, 1 820, after having observed for some time the eclipse of the sun which happened on that day, he intended to take a walk in the fields, and on crossing the town, he saw a nume- rous group of individuals of every age and sex, who had their eyes See Edinburgh Philosophical Journal, for July, 1826, p. 114. 478 THE PRACTICAL ASTRONOMER. when its geocentric latitude, either north or south, at the time of conjunction, is not less than 1° 14', or, when the planet is about 58' from the sun's limb. This conclusion is deduced from the obser- vation of Oct. 2, 1843, # stated above. 2. Another conclusion is — that during the space fixed in the direction of the sun. Further on, he perceived another group having their eyes in like manner turned towards the sun. He questioned an intelligent artist who was among them to learn the ob- ject that fixed his attention. He replied, 6 We are looking at the stars which are detaching themselves from the sun.' 6 You may look yourself ; that will be the shortest way to learn the fact.' He looked, and saw, in fact, not stars, but balls of fire of a diameter equal to the largest stars, which were projected in various directions from the upper hemisphere of the sun, with an incalculable velocity, and although this velocity of projection appeared the same in all, yet they did not all attain the same distance. These globes were projected at unequal and pretty short intervals. Several were often projected at once, but always diverging from one another. Some of them described a right line, and were extinguished in the distance ; some described a para- bolic line, and were in like manner extinguished ; others again, after having removed to a certain distance in a right line, retrograded upon the same line, and seemed to enter, still luminous, into the sun's disk. The ground of this magnificent picture was a sky blue, somewhat tinged with brown. Such was his astonishment at the sight of so majestic a spectacle, that it was impossible for him to keep his eyes off it till it ceased, which happened gradually as the eclipse wore off and the solar rays resumed their ordinary lustre. It was remarked by one of the crowd that ' the sun projected most stars at the time when it was palest ; 1 and that the circumstance which first excited attention to this phenomenon was that of a woman who cried out ' Come here ! — come and see the flames that are issuing from the sun ! ' I have stated the above facts because they may afterwards tend to throw light upon certain objects or phenomena with which we are at present unacquainted. The phenomenon of 6 falling stars ' has of late years excited considerable attention, and it seems now to be admitted, that, at least, certain species of these bodies descend from regions far beyond the limits of our atmosphere. This may be pronounced as certain with regard to the 6 November Meteors.' May not some of the phenomena described above, be connected with the fall of meteoric stones — the showers of falling stars seen on the 12th and 13th of November, or other meteoric phenomena whose causes we have hitherto been unable to explain ? Or, may we conceive that certain celestial bodies, with whose nature and destination we are as yet unacquainted, may be revolving in different courses in the regions around us — some of them opaque and others luminous, and whose light is undistin- guishable by reason of the solar effulgence ? * For an explanation of the manner of viewing Venus at her superior conjunction, see 1 Celestial Scenery,' 5th thousand, p. 102. OBSERVATIONS ON THE PLANETS. 479 of 583 days, or about 19 months — the time this planet takes in moving from one conjunction with the sun to a like conjunction again — when its latitude at the time of its superior conjunction exceeds 1° 14 N , it may be seen with an equatorial telescope every clear day without interruption, except about the period of its inferior conjunc- tion, when its dark hemisphere is turned towards the earth, and a short time before and after it. When its geocentric latitude is less than 1 14 v , it will be hid only about four days before, and the same time after its superior conjunction. During the same period it will be invisible to the naked eye, and consequently no observations can be made upon it with a common telescope, for nearly six months, and sometimes more, according as its declination is north or south, namely about two or three months before, and the same time after its superior conjunction, except where there is a very free and unconfined horizon. In regard to the time in which this planet can be hid about the period of its inferior conjunction, I have ascertained from observation, that it can never be hid longer than during a space of 2 days 22 hours; having seen Venus, about noon, like a fine slender crescent, only 35 hours after she had passed the point of her inferior conjunction ; and in a late instance she was seen when little more than a day from the period of conjunction. The longest time, therefore, that this planet can be hid from view during a period of 5SS days, is only about 10 days ; and when its latitude at the time of the superior conjunction, equals or exceeds 1° 14', it can be hid little more than two days. This is a circumstance which cannot be affirmed of any other celestial body, the sun only excepted. 3. That every variation of the phases of this planet — from a slender crescent to a full enlight- 480 THE PRACTICAL ASTRONOMER. ened hemisphere — may, on every clear day, be conveniently exhibited by means of the equatorial telescope. This circumstance renders this instru- ment peculiarly useful in the instruction of the young in the principles of astronomy. For, if the phase which Venus should exhibit at any particular time be known, the equatorial telescope may be directed to the planet, and its actual phase in the heavens be immediately exhibited to the astrono- mical pupil. 4. Since it is only at the period of the superior conjunction that this planet presents a full enlightened hemisphere, and since it is only when this phase is presented that both its diameters can be measured — it is of some importance that observations be made on it at the moment of conjunction, by means of powerful telescopes fur- nished with micrometers, so as to determine the difference (if any) between its polar and equatorial diameters. 5. Another conclusion from the observations on Venus, is, that a moderate diminution of the aperture of the object-glass of the telescope is useful, and even necessary in viewing this planet when near the sun. Its effect is owing in part to the direct solar rays being thereby more effec- tually excluded ; for when these rays enter directly into the tube of the telescope, it is very difficult, and almost impossible to perceive this planet, or any other celestial body when in the vicinity of the sun. Observations on Jupiter and other planets. This planet is very easily distinguished in the day-time with a very moderate magnifying power, when it is not within 30° or 35° of the sun. The following extract from my memorandums may OBSERVATIONS ON THE PLANETS. 481 serve as a specimen. May 12, 1813, l h 40 m , p.m. Saw Jupiter with a power of 15 times, the aper- ture not contracted. The planet appeared so distinct with this power, that I have reason to be- lieve, it would have been perceived with a power of 6 or 7 times. When the aperture was contrac- ted ^ inch, and afterwards to half an inch, there was little perceptible difference in its appearance. It was then about 58° in longitude, east of the sun. Though Jupiter when at a considerable distance from the sun, and near his opposition, appears to the naked eye with a brilliancy nearly equal to that of Venus, yet there is a very striking differ- ence between them, in respect of lustre, when viewed in day-light. Jupiter, when viewed with a high magnifying power, in the day-time, always exhibits a very dull cloudy appearance ; whereas Venus appears with a moderate degree of splen- dour . About the end of June 1813, between 5 and 6 in the evening, having viewed the planet Venus, then within 20° of the sun, and which appeared with a moderate degree of lustre, I directed the telescope to Jupiter, at that time more than 32° from the sun, when the contrast between the two planets was very striking, Jupiter appearing so faint as to be just discernible, though his apparent magnitude was nearly double that of Venus. In this observation a power of 65 was used. In his approach towards the sun, about the end of July, I could not perceive him when he was within 16° or 17° of his conjunction with that luminary. — These circumstances furnish a sensible and popular proof, independently of astronomical calculations, that the planet Jupiter is placed at a much greater distance from the sun than Venus ; since its light is so faint as to be scarcely percepti- Y 482 THE PRACTICAL ASTRONOMER. ble when more than 20 degrees from the sun, while that of Venus is distinctly seen amidst the full splendour of the solar rays, when only about a degree from the margin of that luminary. With a power of 65 I have been enabled to dis- tinguish the belts of Jupiter before sunset, but could never perceive any of his satellites till the sun was below the horizon. There are no obser- vations which so sensibly and strikingly indicate the different degrees of light emitted by the dif- ferent planets as those which are made in the day- time. To a common observer, during night, Jupiter and Venus appear, in a clear sky, nearly with equal brilliancy, and even Mars, when about the point of his opposition to the sun, appears with a lustre somewhat similar, though tinged with a ruddy hue; but when seen in day-light their aspect is very dissimilar. This circumstance evidently indicates, 1. that these planets are placed at different distances from the sun, and conse- quently are furnished with different degrees of light proportional to the square of their distances from that luminary ; — and 2. that there are cer- tain circumstances connected with the surfaces and atmospheres of the planetary bodies, which render the light they emit more or less intense, indepen- dently of their different distances from the central luminary. For Mars, though much nearer to the sun than Jupiter, is not so easily distinguished in the day-time, and, even in the night-time, appears with a less degree of lustre. My observations on Saturn in day-light, have not been so frequent as those on Jupiter. I have been enabled to distinguish his ring several times before sun-set, with a power of 65 ; but his great southern declination, and consequent low altitude, at the periods when these observations were made, OBSERVATIONS ON THE PLANETS, 483 were unfavourable for determining the degree of his visibility in day-light ; for a planet or a star is always more distinctly perceptible in a high than in a low altitude, on account of the superior purity of the atmosphere through which a celestial object is seen when at a high elevation above the horizon. This planet, however, is not nearly so distinctly visible in day-light as Jupiter, and I have chiefly seen it, when the sun was not more than an hour or two above the horizon, but never at noon-day ; although it is probable that with powerful instruments it may be seen even at that period of the day. The planet Mars is seldom distinctly visible in the day-time, except when at no great distance from its opposition to the sun. The following is a memorandum of an observation on Mars, when in a favourable position. October 24, 1836. Saw the planet Mars distinctly with a power of about 60, at 40 minutes past 9 a.m., the sun having been above the horizon nearly three hours. It appeared tolerably distinct, but scarcely so brilliant as a fixed star of the first magnitude, but with apparently as much light as Jupiter generally exhibits when viewed in day-light. It could not be traced longer at the time, so as to ascertain if it could be seen at mid-day ; on account of the interposition of the western side of the window of the place of observation. The ruddy aspect of this planet — doubtless caused by a dense atmosphere with which it is environed — is one of the causes which prevents its appearing with brilliancy in the day-time. With respect to the planet Mercury, I have had opportunities of observing it several times after sun-rise, and before sunset, about 10 or 12 days before and after its greatest elongation from the sun, with a power of 45. I have several times searched for Y 2 484 THE PRACTICAL ASTRONOMER. this planet about noon, but could not perceive it. The air, however, at the times alluded to, was not very clear, and I was not certain that it was within the field of the telescope ; and therefore, I am not convinced but that, with a moderately high power, it may be seen even at noon-day. Such are some specimens of the observations I have made on the heavenly bodies in the day-time, and the conclusions which may be deduced from them. I have been induced to communicate them, from the consideration, that the most minute facts, in relation to any science, are worthy of being known, and may possibly be useful. They may at least gratify the astronomical tyro with some information which he will not find in the common treatises on astronomy, and may perhaps excite him to prosecute a train of similar observa- tions for confirming or correcting those which have been noted above. Besides the deductions already stated, the fol- lowing general conclusions may be noted. — 1. That a celestial body may be as easily distinguished at noon-day, as at any time between the hours of nine in the morning and three in the afternoon, except during the short days in winter. 2. They are more easily distinguished at a high than at a low altitude — in the afternoon than in the morn- ing, especially if their altitudes be low — and in the northern region of the heavens than in the southern. The difficulty of perceiving them at a low altitude is obviously owing to the thick vapours near the horizon. Their being less easily distinguished in the morning than in the afternoon is owing to the undulations of the atmosphere, which are generally greater in the morning than in the afternoon. This may be evidently per- ceived by looking at distant land-objects at those UTILITY OF DAY OBSERVATIONS. 485 times, in a hot day, through a telescope which magnifies about 40 or 50 times, when they will be found to appear tremulous and distorted in conse- quence of these undulations, especially if the sun be shining bright. In consequence of this cir- cumstance, we can seldom use a high terrestrial power with effect on land objects, except early in the morning, and a short time before sun-set. Their being more easily distinguished in the northern region of the heavens is owing to that part of the sky being of a deeper azure, on account of its being less enlightened than the southern with the splendour of the solar rays. Utility of Celestial Day Observations. The observations on the heavenly bodies in the day-time, to which I have now directed the atten- tion of the reader, are not to be considered as merely gratifications of a rational curiosity, but may be rendered subservient to the promotion of astronomical science. As to the planet Venus — when I consider the degree of brilliancy it exhi- bits, even in day-light, I am convinced that useful observations might frequently be made on its sur- face in the day-time, to determine some of its physical peculiarities and phenomena. Such ob- servations might set at rest any disputes which may still exist respecting the period of rotation of this planet. Cassini, from observations on a bright spot, which advanced 20° in 24 h 34 m determined the time of its rotation to be 23 hours, 20 minutes. On the other hand, Bianchini, from similar obser- vations, concluded that its diurnal period was 24 days and 8 hours. The difficulty of deciding between these two opinions, arises from the short time in which observations can be made on this planet, either before sun-rise, or after sun-set, 486 THE PRACTICAL ASTRONOMER. which prevents us from tracing, with accuracy, the progressive motion of its spots for a sufficient length of time. And, although an observer should mark the motion of the spots at the same hour, on two succeeding evenings, and find they had moved forward about 15° in 24 hours, he would still be at a loss to determine, whether they had moved only 15°, in all, since the preceding obser- vation, or had finished a revolution and 15° more. If, therefore, any spots could be perceived on the surface of Venus in the day-time, their motion might be traced, when she is in north declination, for 12 hours or more, which would completely settle the period of rotation. That it is not im- probable that spots, fitted for this purpose, may be discovered on her disk in the day-time, appears from some of the observations of Cassini, who saw one of her spots when the sun was more than eight degrees above the horizon.* The most dis- tinct and satisfactory views I have ever had of this planet were those which I obtained in the day- time, in summer, when it was viewed at a high altitude, with a 44^ inch achromatic telescope, carrying a power of 150. I have at such times distinctly perceived the distinction between the shade and colour of its margin, and the superior lustre of its central parts, and some spots have occasionally been seen, though not so distinctly marked as to determine its rotation. Such dis- tinct views are seldom to be obtained in the even- ing after sun-set, on account of the undulations of the atmosphere, and the dense mass of vapours through which the celestial bodies are viewed when near the horizon. Nor do I consider it altogether improbable that * See Long's Astronomy, vol. 2, p. 487, — and Encyclopedia Bri- tannica, vol. li. p. 436, 3rd edition. UTILITY OF DAY OBSERVATIONS. 487 its satellite (if it have one, as some have supposed) may be detected in the day time, when this planet is in a favourable position for such an observation ; particularly when a pretty large portion of its enlightened surface is turned towards the earth, and when its satellite, of course, must present a similar phase. About the period of its greatest elongation from the sun, and soon after it assumes a crescent phase, in its approach to the inferior conjunction, may be considered as the most eligi- ble times for prosecuting such observations. If this supposed satellite be about one third or one fourth of the diameter of its Primary, as Cassini, Short, Baudouin, Montbarron, Montaigne, and other astronomers supposed, it must be nearly as large as Mercury, which has been frequently seen in day-light. If such a satellite have a real exist- ence, and yet undistinguishable in day-light, its surface must be of a very different quality for reflecting the rays of light from that of' its pri- mary ; for it is obvious to every one who has seen Venus with a high power, in the day-time, that a body of equal brilliancy — though four times less in diameter — would be quite perceptible, and exhibit a visible disk. Such observations, how- ever, would be made, with a much greater effect in Italy and other Southern countries, and parti- cularly in Tropical climates, such as the southern parts of Asia and America, and in the West India Islands, where the sky is more clear and serene, and where the planet may be viewed at higher altitudes, and for a greater length of time, without the interruption of clouds, than in our island. Again, the apparent magnitudes of the fixed stars — the quantity of light they respectively emit — and the precise class of magnitude which should be assigned to them — might be more accu- 488 THE PRACTICAL ASTRONOMER. rately determined by day observations, than by their appearance in the nocturnal sky. All the stars which are reckoned to belong to the first magnitude are not equally distinguishable in day- light. For example, the stars Aldebaran and Procyon are not so easily distinguished, nor do they appear with the same degree of lustre by day, as the stars a Lyrce and Capella. In like manner the stars Altair, Alphard, Deneb Has Alkague, considered as belonging to the second magnitude, are not equally distinguishable by the same aperture and magnifying power — which seems to indicate, that a different quantity of light is emitted by these stars, arising from a difference either in their magnitude, their distance, or the quality of the light with which they are irradiated. The following are likewise practical purposes to which celestial day observations may be applied. In accurately adjusting Circular and Transit in- struments, it is useful, and even necessary, for determining the exact position of the meridian, to take observations of certain stars, which differ greatly in zenith distance, and which transit the meridian nearly at the same time. But as the stars best situated for this purpose, cannot, at every season, be seen in the evenings, we must, in certain cases, wait for several months till such ob- servations can be made, unless we make them in the day-time, which can very easily be done, if the instrument have a telescope adapted to it, fur- nished with such powers as those above stated, or higher powers if required. I have likewise made use of observations on the stars in the day time for adjusting a clock or watch to meantime, when the sun was in a situation beyond the range of the instrument, or obscured by clouds, and when I did not choose to wait till the evening. This may, at UTILITY OF DAY OBSERVATIONS. 489 first view, appear to some as paradoxical ; since the finding of a star in daylight depends on our knowing its right Ascension from the sun, and this last circumstance depends, in some measure, on our knowing the true time. But if a watch or clock is known not to have varied above seven or eight minutes from the time, a star of the first magnitude may easily be found, by moving the telescope a little backwards or forwards, till the star appear ; and when it is once found, the exact variation of the movement is then ascertained, by comparing the calculations which were previously necessary, with the time pointed out by the nonius on the Equatorial circle — or, in other words, by ascertaining the difference between the time as- sumed, and the time indicated by the instrument, when the star appears in the centre of the field of view. All this may be accomplished in five or six minutes. Besides the practical purposes now stated, the Equatorial telescope is perhaps the best instru- ment for instructing a learner in the various opera- tions of practical astronomy, and particularly for enabling him to distinguish the names and posi- tions of the principal stars. For, when the right Ascension and Declination of any star is known, from astronomical tables, the telescope may be immediately adjusted to point to it, which will infallibly prevent his mistaking one star for another. In this way, likewise, the precise position of the planet Mercury, Uranus, Vesta, Juno, Ceres, Pallas — a small comet, a nebula, a double star, or any other celestial body not easily distinguish- able by the naked eye, may be readily pointed out, when its right Ascension and Declination are known to a near approximation. In conclusion, I cannot but express my sur- Y 5 490 THE PRACTICAL ASTRONOMER. prise, that the Equatorial telescope is so little known, even by many of the lovers of astrono- mical science. In several respectable academies in this part of Britain, and, if I am not misin- formed, in most of our universities, this instru- ment is entirely unknown. This is the more un- accountable, as a small equatorial may be pur- chased for a moderate sum ; and as there is no single instrument so well adapted for illustrating all the operations of Practical Astronomy. Where very great accuracy is not required, it may occa- sionally be made to serve the general purposes of a transit instrument for observing the passages of the sun and stars across the meridian. It may likewise be made to serve as a theodolite for sur- veying land and taking horizontal angles— as a Quadrant for taking angles of altitude — as a level — as an equal altitude instrument — an azimuth instrument for ascertaining the sun's distance from the north or south points of the horizon — and as an accurate Universal Sun Dial, for finding the exact mean or true time, on any occasion when the sun is visible. The manner of applying it to these different purposes will be obvious to every one who is in the least acquainted with the nature and construction of this instrument. The price of a small Equatorial instrument, such as that described p. 454, is about 16 guineas, exclusive of some of the eye-pieces, which were afterwards added for the purpose of making parti- cular observations. Instruments of a larger size, and with more complicated machinery, sell from 50 to 100 guineas and upwards. Messrs. W. and S. Jones, Holborn, London, construct such instru- ments. ON THE QUADRANT. 491 ON THE QUADRANT. Every circle being supposed to be divided into 360 equal parts, or degrees, — it is evident, that 90 degrees, or the fourth part of a circle, will be sufficient to measure all angles, between the hori- zon of any place and the line perpendicular to it which goes up to the zenith. Thus, in fig. 87, the line CB represents the plane of the horizon. ACBH, the quadrant, AC the perpendicular to the horizon, and A the zenith point. If the lines BC and CA represent a pair of compasses with the legs standing perpendicular to each other, and the curved lines AB, DE and FG, the quarter of as many circles of different sizes — it is evident that although each of these differs from the others in size, yet that each contains the same portion of a circle, namely a quadrant or fourth part ; and thus it would be from the smallest to the largest quadrant that could be formed, — they figure 87. C L a E B 492 THE PRACTICAL ASTRONOMER. would all contain exactly 90 degrees each. By the application of this principle the comparative measure of angles may be extended to an indefi- nite distance. By means of an instrument con- structed in the form of a quadrant of a circle, with its curved edge divided into 90 equal parts, the altitude of any object in the heavens can at any time be determined. There are various constructions of this instru- ment, some of them extremely simple, and others considerably complex and expensive, according to the degree of accuracy which the observations re- quire. The following is a description of the Pillar Quadrant, as it was made by Mr. Bird, for the observatory of Greenwich, and several con- tinental observatories. This instrument consists of a quadrant EE H GL (fig. 88.) mounted on a pillar B, which is sup- ported by a tripod AA, resting on three foot screws. The quadrant, the pillar, and the hori- zontal circle all revolve round a vertical axis. A telescope H is placed on the horizontal radius, and is directed to a meridian mark previously made on some distant object for placing the plane of the instrument in the meridian, and also for setting the zero, or beginning of the scale truly horizontal. This is sometimes done by a level instead of a telescope, and sometimes by a plumb- line G, suspended from near the centre, and brought to bisect a fine dot made on the limb, where a microscope is placed to examine the bisec- tion. The weight or plummet at the end of the plumb-line is suspended in the cistern of water 6, which keeps it from being agitated by the air. A similar dot is made for the upper end of the plumb-line upon a piece of brass, adjustable by a screw d, in order that the line may be exactly ON THE QUADRANT. 493 at right angles to the telescope, when it is placed at o. The quadrant is screwed by the centre of its frame, against a piece of brass e with three screws, and this piece is screwed to the top of the pillar B with other three screws. By means of the first three screws, the plane of the quadrant can be placed exactly parallel to the vertical axis, and by the other screws the telescope H can be placed exactly perpendicular to it. The nut of the delicate screw L is attached to the end of the telescope F, by a universal joint. The collar for the other end is jointed in the same manner to a 494 THE PRACTICAL ASTRONOMER. clamp which can be fastened to any part of the limb. A similar clamp-screw and slow motion is seen at n for the lower circle, which is intended to hold the circle fast, and adjust its motion. The divisions of the lower, or horizontal circle, are read by verniers, or noniuses, fixed to the arms of the tripod at I and m, and, in some cases three are used to obtain greater accuracy. In using this quadrant, the axis of the tele- scope H is adjusted to a horizontal line, and the plane of the quadrant to a vertical line, by the means already stated. The screw of the champ L is then loosened, and the telescope directed to the star, or other object, whose altitude is re- quired. The clamp screw being fixed, the observer looks through the telescope, and with the nut of the screw L he brings the telescope into a position where the star is bisected by the intersection of the wires in the field of the telescope. The divi- sions are then to be read off upon the vernier, and the altitude of the star will be obtained. By means of the horizontal circle D, all angles in the plane of the horizon may be accurately measured — such as the amplitudes and azimuths of the celes- tial bodies. Quadrants of a more simple construction than the above, may be occasionally used, such as Gunter's, Cole's, Sutton's and others ; but none of these are furnished with telescopes, or telescopic sights, and therefore an altitude cannot be ob- tained by them with the same degree of accu- racy as with that which has been now described. By means of the Quadrant, not only the alti- tudes of the heavenly bodies may be determined, but also the distances of objects on the earth by observations made at two stations — the altitude of fireballs and other meteors in the atmosphere — - THE ASTRONOMICAL CIRCLE. 495 the height of a cloud, by observation on its altitude and velocity — and numerous other pro- blems, the solution of which depends upon angular measurements. A Mural Quadrant is the name given to this instrument when it is fixed upon a wall of stone, and in the plane of the meridian, such as the quadrant which was erected by Flam- stead in the Observatory at Greenwich. Although the quadrant was formerly much used in astrono- mical observations, yet it may be proper to state, that its use has now been almost completely super- seded by the recent introduction of Astronomical Circles, of which we shall now give the reader a very short description, chiefly taken from Trough- ton's account of the instrument he constructed, as found in Sir D. Brewster's Supplement to Ferguson's Astronomy. THE ASTRONOMICAL CIRCLE. An astronomical circle is a complete circle sub- stituted in place of the quadrant, and differs from it only in the superior accuracy with which it enables the astronomer to make his observations. The large vertical or declination circle CC (fig. 89.) is composed of two complete circles strength- ened by an edge bar on their inside, and firmly united at their extreme borders by a number of short braces or bars which stand perpendicular between them, and which keep them at such a distance as to admit the achromatic telescope TT. This double circle is supported by 16 conical bars, firmly united along with the telescope, to a hori- zontal axis. The exterior limb of each circle is divided into degrees and parts of a degree, and these divisions are divided into seconds by means 496 THE PRACTICAL ASTRONOMER. figure 89. of the micrometer microscopes mm, which read oft the angle on opposite sides of each circle. The cross wires in each microscope may be moved over the limb till they coincide with the nearest divi- sion of the limb, by means of the micrometer screws cc 9 and the space moved through is ascer- tained by the divisions on the graduated head above THE ASTRONOMICAL CIRCLE. 497 c, assisted by a scale within the microscope. The microscopes are supported by two arms proceed- ing from a small circle concentric with the hori- zontal axis, and fixed to the vertical columns. This circle is the centre upon which they can turn round nearly a quadrant for the purpose of employ- ing a new portion of the divisions of the circle, when it is reckoned prudent to repeat any delicate observations upon any part of the limb. At h is represented a level for placing the axis in a true horizontal line, and at k is fixed another level parallel to the telescope, for bringing the zero of the divisions to a horizontal position. The horizontal axis to which the vertical circle and the telescope are fixed, is equal in length to the distance between the vertical pillars, and its pivots are supported by semi-circular bearings, placed at the top of each pillar. These two vertical pillars are firmly united at their bases to a cross bar /. To this cross bar is also fixed a vertical axis about three feet long, the lower end of which terminat- ing in an obtuse point, rests in a brass conical socket firmly fastened at the bottom of the hollow in the stone pedestal D, which receives the verti- cal axis. This socket supports the whole weight of the moveable part of the instrument. The upper part of the vertical axis is supported by two pieces of brass, one of which is seen at e 3 screwed to the ring i, and containing a right angle, or Y. At each side of the ring, opposite to the points of contact, is placed a tube contain- ing a heliacal spring, which, by a constant pressure on the axis, keeps it against its bearings, and permits it to turn, in these four points of contact, with an easy and steady motion. The two bear- ings are fixed upon two rings capable of a lateral adjustment; the lower one by the screw d to in- 498 THE PRACTICAL ASTRONOMER. cline the axis to the east or west, while the screw b gives the upper one i a motion in the plane of the meridian. By this means the axis may be adjusted to a perpendicular position as exactly as by the usual method of the tripod with feet screws. These rings are attached to the centre piece s, which is firmly connected with the upper surface of the stone by six conical Tubes A,A,A, &c, and brass standards at every angle of the pedestal. Below this frame lies the azimuth circle EE consisting of a circular limb, strength- ened by ten hollow cones firmly united with the vertical axis, and consequently turning freely along with it. The azimuth circle EE is divided and read off in the same manner as the vertical circle. The arms of the microscopes BB project from the ring i 9 and the microscopes themselves are adjustable by screws, to bring them to zero and to the diameter of the circle. A little above the ring i is fixed an arm L which embraces and holds fast the vertical axis with the aid of a champ screw. The arm L is connected at the extre- mity with one of the arms A, by means of the screw a 9 so that by turning this screw, a slow motion is communicated to the vertical axis and the azimuth circle. In order to place the instrument in a true ver- tical position, a plumb-line, made of fine silver wire, is suspended from a small hook at the top of the vertical tube n, connected by braces with one of the large pillars. The plumb-line passes through an angle in which it rests, and by means of a screw may be brought into the axis of the tube. The plummet at the lower end of the line is immersed in a cistern of water t 9 in order to check its oscillations, and is supported on a shelf proceeding from one of the pillars. At the lower THE ASTRONOMICAL CIRCLE. 499 end of the tube n are fixed two microscopes o and p 9 at right angles to one another, and opposite to each is placed a small tube containing a lucid point. The plumb-line is then brought into such a position by the screws d, b, and by altering the suspension of the plumb-line itself, that the image of the luminous point, like the disk of a planet, is formed on the plumb-line, and accurately bisected by it. The vertical axis is then turned round, and the plumb-line examined in some other position. If it still bisects the luminous point, the instrument is truly vertical ; but if it does not, one half of the deviation must be corrected by the screws d b, and the other half by altering the suspension of the line till the bisection of the circular image is perfect in every position of the instrument. It is not many years since Circular Repeating instruments came into general use. The principle on which the construction of a repeating circle is founded appears to have been first suggested by Professor Mayer of Gottingen, in 1758; but the first person who applied this principle to measure round the limb of a divided instrument, was Borda, who about the year 1789, caused a repeat- ing circle to be constructed that would measure with equal facility horizontal and vertical angles. Afterwards, Mr. Troughton greatly improved the construction of Borda's instrument by the intro- duction of several contrivances which ensure, at the same time, its superior accuracy and conveni- ence in use ; and his instruments have been intro- duced into numerous observatories. Circular in- struments, on a large scale, have been placed in the Royal Observatory of Greenwich, and in most of the principal observatories on the continent of Europe. Although it is agreed on all hands that 500 THE PRACTICAL ASTRONOMER. greater accuracy may be obtained by a repeating circle, than by any other having the same radius, yet there are some objections to its use which do not apply to the altitude and azimuth circle. The following are the principal objections, as stated in Vol. I., of the 6 Memoirs of the Astronomical Society of London.' 1. The origin of the repeat- ing circle is due to bad dividing, which ought not to be tolerated in any instrument in the present state of the art. 2. There are three sources of fixed error which cannot be exterminated, as they depend more on the materials than on the work- manship ; first, the zero of the level changes with variations of temperature ; secondly, the resis- tance of the centre work to the action of the tangent screws ; and thirdly, the imperfection of the screws in producing motion, and in securing permanent positions. 3. The instrument is ap- plied with most advantage to slowly moving or circumpolar stars ; but in low altitudes these stars are seen near the horizon, where refraction inter- feres. 4. Much time and labour are expended, first in making the observations, and again in reducing them. 5. When any one step in a series of ob- servations is bad, the whole time and labour are absolutely lost. 6. When the instrument has a telescope of small power, the observations are charged with errors of vision, which the repeating circle will not cure. 7. This instrument cannot be used as a transit instrument, nor for finding the exact meridian of a place. A great variety of directions is necessary in order to enable the student of practical astronomy thoroughly to understand and to apply this instru- ment to practice, which the limited nature of the present work prevents us from detailing. — As this instrument consists of a variety of complicated THE TRANSIT INSTRUMENT, 501 pieces of machinery, it is necessarily somewhat expensive. A six inch brass astronomical circle for altitudes, zenith or polar distances, azimuths, with achromatic telescope, &c, is marked in Messrs. W. and S. Jones' catalogue of astronomi- cal instruments, at £27 6s. A circle 12 inches diameter, from £36 15s. to £68 5s. An 18 inch ditto, of the best construction, £105. The larger astronomical circles for public observatories, from 100 to a 1000 guineas and upwards, accord- ing to their size, and the peculiarity of their con- struction. THE TRANSIT INSTRUMENT. A Transit instrument is intended for observing celestial objects as they pass across the meridian. It consists of a telescope fixed at right angles to a horizontal axis — which axis must be so supported that what is called the line of collimation, or the line of sight of the telescope, may move in the plane of the meridian. This instrument was first invented by Romer in the year 1689, but has since received great improvements by Troughton, Jones and other modern artists. Transit instruments may be divided into two classes, Portable, and Fixed. The portable instrument, when placed truly in the meridian, and well adjusted, may be advantageously used as a stationary instrument in an observatory, if its dimensions be such as to admit of a telescope of 3i feet focal length ; but when the main tube is only from 20 to 30 inches long, with a proportional aperture, it is more suited for a travelling instrument to give the exact time ; and, when carried on board a ship in a voyage of discovery, may be taken on shore at 502 THE PRACTICAL ASTRONOMER. any convenient place, for determining the solar time of that place, and for correcting the daily rate of the Chronometer giving the time at the first meridian, so that the longitude of the place of observation may be obtained from the difference of the observed and indicated times, after the proper corrections have been made. The following is a brief description of one of Mr. Troughton's Portable Transit Instruments. In fig. 90. PP is an achromatic telescope firmly fixed, by the middle to a double conical and hori- zontal axis HH, the pivots of which rest on figure 90. THE TRANSIT INSTRUMENT. 503 angular bearings called Ys, at the top of the standards B,B, rendered steady by oblique braces DD, fastened to the central part of the circle, AA. In large fixed instruments, the pivots and angular bearings are supported on two massive stone pil- lars, sunk several feet into the ground, and are sometimes supported by mason-work, to secure perfect stability, The axis HH has two adjust- ments, one for making it exactly level, and the other for placing the telescope in the meridian. A graduated circle L is fixed to the extremity of the pivot which extends beyond one of the Ys, and the two radii that carry the verniers aa, are fitted to the extremities of the pivot in such a way as to turn round independent of the axis. The double verniers have a small level attached to them, and a third arm b, which is connected with the standard B by means of a screw s. If the verniers are placed by means of the level, in a true horizontal position, when the axis of the telescope is hori- zontal, and the arm b screwed by the screw s to the standard B, the verniers will always read off the inclination of the telescope, and will enable the observer to point it to any star, by means of its meridian altitude. The whole instrument rests on three foot screws entered into the circle AA. In the field of view of the telescope, there are several parallel vertical wires, crossed at right angles with a horizontal one, and the telescope is sometimes furnished with a diagonal eye-piece, for observing stars near the zenith. A level like- wise generally accompanies the instrument, in order to place it horizontal, by being applied to the pivots of the axis. In order to fix the transit instrument exactly in the meridian, a good clock regulated to sidereal time is necessary. This regulation may be effected 504 THE PRACTICAL ASTRONOMER. by taking equal altitudes of the sun or a star before and after they pass the meridian, which may be done by small quadrants, or by a good sextant. The axis H of the instrument is then to be placed horizontal by a spirit level, which accompanies the transit, and the greatest care must be taken that the axis of vision describes in the heavens a great circle of the sphere. To as- certain whether the telescope be in the plane of the meridian, observe by the clock when a circum- polar star seen through the telescope transits both above and below the pole ; and if the times of describing the eastern and western parts of its circuit be equal, the telescope is then in the plane of the meridian ; otherwise, certain adjustments must be made. When the telescope is at length perfectly adjusted, a land-mark must be fixed upon, at a considerable distance — the greater the better. This mark must be in the horizontal direction of the intersection of the cross wires, and in a place where it can be illuminated, if possible, in the night time, by a lantern hanging near it ; which mark being on a fixed object, will serve at all times afterwards for examining the position of the telescope. Various observations and adjustments are requi- site in order to fixing a transit instrument exactly in the plane of the meridian. There is the ad- justment of the level — the horizontal adjustment of the axis of the telescope — the placing of the parallel lines in the focus of the eye-glass, so as to be truly vertical, and to determine the equatorial value of their intervals — the collimation in azimuth, so that a line passing from the middle vertical line to the optical centre of the object-glass, is at right angles with the axis of the telescope's motion — the collimation in altitude, so that the horizontal line THE TRANSIT INSTRUMENT. 505 should cross the parallel vertical lines, not only at right angles, but also in the optical centre of the field of view — with various other particulars ; but of which our limited space will not permit us to enter into details. Those who wish to enter into all the minute details in reference to the construc- tion and practical application of this and the other instruments above described, as well as all the other instruments used by the Practical Astro- nomer, will find ample satisfaction in perusing the Rev. Dr. Pearson's Introduction to Practical Astronomy, 4to., Vol. II. A portable Transit instrument, with a cast-iron stand, the axis 12 inches in length, and the achro- matic telescope about 20 inches, packed in a case, sells at about 16 guineas: with a brass-framed stand and other additions, at about 20 guineas. Transit instruments of larger dimensions are higher in proportion to their size, &c. z 506 THE PRACTICAL ASTRONOMER. CHAPTER III. ON OBSERVATORIES. In order to make observations, with conveni- ence and effect, on the heavenly bodies, it is ex- pedient that an observatory, or place for making the requisite observations, be erected in a proper situation. The following are some of the leading features of a spot adapted for making celestial observations: 1. It should command an extensive visible horizon all around, particularly towards the south and the north. 2. It should be a little elevated above surrounding objects. 3. It should be, if possible, at a considerable distance from manufactories, and other objects which emit much smoke or vapour, and even from chimney-tops where no sensible smoke is emitted, as the heated air from the top of funnels causes undulations in the atmosphere. 4. It should be at a distance from swampy ground or valleys that are liable to be covered with fogs and exhalations. 5. It should not, if possible, be too near public roads, particularly if paved with stones, and frequented by heavy carriages, as in such situations, undu- lations and tremulous motions may be produced, injurious to the making of accurate observations with graduated instruments. 6. It is expedient that the astronomical observer should have access ON OBSERVATORIES. 507 to some distant field within a mile of the observa- tory, on which a meridian mark may be fixed, after his graduated instruments are properly ad- justed. The distance at which a meridian mark should be erected will depend in part on the focal length of the telescope generally used for making observations on the Right Ascensions and declina- tions of the stars. It should be fixed at such a distance that the mark may be distinctly seen without altering the focus of the telescope when adjusted to the sun or stars, which, in most cases, will require to be at least half a mile from the place of observation, and more if it can be obtained. Observatories may be distinguished into public and private. A private observatory may be com- prehended in a comparatively small building, or in the wing of a building of ordinary dimensions for a family, provided the situation is adapted to it. Most of our densely-peopled towns and cities, which abound in narrow streets and lanes, are generally unfit for good observatories, unless at an elevated position at their extremities. Public observatories, where a great variety of instruments is used, and where different observers are em- ployed, require buildings of larger dimensions, divided into a considerable number of apartments, The observatory of Greenwich is composed prin- cipally of two separate buildings — one of which is the observatory properly so called, where the assistant lives and makes all his observations ; the other is the dwelling-house in which the astrono- mer-royal resides. The former consists of three rooms on the ground-floor, the middle of which is the assistant's sitting and calculating room, furnished with a small library of such books only as are necessary for his computations, and an z 2 508 THE PRACTICAL ASTRONOMER. accurate clock made by the celebrated Graham, which once served Dr. Halley as a transit-clock. Immediately over this is the assistant's bed-room, with an alarum to awake him to make his obser- vations at the proper time. The room on the eastern side of this is called the transit-room, in which is an 8 feet transit instrument, with an axis of 3 feetj resting on 2 pieces of stone, made by Mr. Bird, but successively improved by Messrs. Dollond, Troughton and others. Here is also a chair to observe with, the back of which lets down to any degree of elevation that convenience may require. On the western side is the quad- rant room, with a stone pier in the middle running north and south, having on its eastern face a mural quadrant of 8 feet radius, by which obser- vations are made on the southern quarter of the meridian, through an opening in the roof, of 3 feet wide, produced by means of two sliding shutters. On the western face is another mural quadrant of 8 feet radius, the frame of which is of iron, and the arch of brass, which is occasion- ally applied to the north quarter of the meridian. In the same room is the famous zenith sector, 12 feet long, with which Dr. Bradley made the ob- servations which led to the discovery of the nuta- tion of the earth's axis and the aberration of the light of the fixed stars. Here are also Dr. Hooke's reflecting quadrant and three time- keepers by Harrison. On the south side of this room a small wooden building is erected for the purpose of observing the eclipses of Jupiter's satellites, occultations of stars by the moon, and other phenomena which require merely the use of a telescope, and the true or mean time. It is furnished with sliding shutters on the roof and sides to view any part of the hemisphere from the ON OBSERVATORIES. 509 Prime Vertical down to the southern horizon. It contains a 40-inch achromatic, with a triple ob- ject-glass ; and also a 5 feet achromatic by Messrs. John and Peter Dollond — a 2 feet reflecting teles- cope by Edwards, and a 6 feet reflector by Hers- chel. Above the dwelling-house is a large octa- gonal room, which is made the repository for certain old instruments, and for those which are too large to be used in the other apartments. Among many other instruments, it contains an excellent 10 feet achromatic by Dollond, and a 6 feet reflector by Short. Upon a platform, in an open space, is erected the great reflecting teles- cope constructed by Mr. Ramage of Aberdeen, on the Herschelian principle, which has a specu- lum of 15 inches diameter, and 25 feet focal length, remarkable for the great accuracy and brilliancy with which it exhibits celestial objects. Various other instruments of a large size, and of modern construction, have of late years been introduced into this observatory, such as the large and splen- did transit instrument constructed by Troughton, in 1816 — the two large mural circles by Troughton and Jones — the transit clock, by Mr. Hardy, and several other instruments and apparatus which it would be too tedious to enumerate and describe. Every observatory, whether public or private, should be furnished with the following instru- ments. 1. A transit instrument for observing the meridian passage of the sun, planets and stars. 2. A good clock whose accuracy may be depended upon. 3. An achromatic telescope, at least 44 inches focal distance, w 7 ith powers of from 45 to 180 for viewing planetary and other phenomena — or, a good reflecting telescope at least 3 feet long, and the speculum 5 inches diameter. 4. An equatorial instrument, for viewing the stars and 510 THE PRACTICAL ASTRONOMER. planets in the day-time, and for finding the Right Ascension and declination of a comet, or any other celestial phenomenon. Where this instrument is possessed, and in cases where no great degree of accuracy is required, the equa- torial may be made to serve the general purposes of a transit instrument. A private observatory might be constructed in any house which has a commanding view of the heavens, provided there is an apartment in it, in which windows may be placed, or openings cut out fronting the north, the south, the east and the west. The author of this work has a small observatory erected on the top of his house, which commands a view of 20 miles towards the east, 30 miles towards the west, and north-west, and about 20 miles towards the south, at an elevation of above 200 feet above the level of the sea, and the banks of the Tay, which are about half a mile distant. The apartment is 12i feet long by 8^- wide, and 8£ feet between the floor and the roof. It has an opening on the north by which observations can be made on the pole-star ; a win- dow on the south by which the meridian-passages of the heavenly bodies may be observed ; another opening towards the east, and a fourth opening, consisting of a door, towards the west. There is a pavement of lead on the outside, all around the observatory-room, enclosed by a stone parapet S\ feet high, the upper part of which is coped with broad flat stones, in certain parts of which groves or indentations are made for receiving the feet of the pedestal of an achromatic telescope, which form a steady support for the telescope in the open air, when the weather is calm and serene, and when observations are intended to be made on any region of the heavens. By placing an ON OBSERVATORIES. 511 instrument on this parapet, it may be directed to any point of the celestial canopy, except a small portion near the northern horizon, which is partly intercepted by a small hill. In the follow- ing ground-plan, fig. 91. AAA, is the parapet surrounding the observatory-room ; BBB, a walk around it nearly 3 feet broad, covered with lead. O is the apartment for the observatory, having an opening C to the north, another opening D to the east, E is a window which fronts the south, and P is a door fronting the west, by which an access figure 91. North South 512 THE PRACTICAL ASTRONOMER. is obtained to the open area on the outside. GHI is an area on the outside towards the south, covered with lead, 15 feet long from Gr to H, and 6^ feet from E to I, from which a commanding view of the southern, eastern and western por- tions of the heavens may be obtained : eeee are positions on the top of the parapet where a teles- cope may be conveniently placed, when observa- tions are intended to be made in the open air. The top of this parapet is elevated about 30 feet from the level of the ground. On the roof of the observatory, about 12 feet above its floor, on the outside is a platform of lead, surrounded by a railing, 6 feet by 5, with a seat, on which obser- vations either on celestial or terrestrial objects may occasionally be made. K is a door or hatch- way, which forms an entrance into the observa- tory from the apartments below, which folds down, and forms a portion of the floor. In the perspective view of the building fronting the title-page, the position and general aspect of the observatory -part of the building may be more distinctly perceived. In public observatories, where zenith or polar distances require to be measured, it is necessary that there should be a dome, with an opening across the roof and down the north and south walls. Should an altitude or azimuth circle, or an equatorial instrument be used, they will re- quire a revolving roof with openings and doors on two opposite sides, to enable an observer to follow a heavenly body across all the cardinal points. The openings may be about 15 inches wide, and the roof needs not be larger than what is requisite for giving room to the observer and the instru- ment, lest its bulk and weight should impede its easy motion. There have been various plans ON OBSERVATORIES. 513 adopted for revolving domes. Fig. 92 represents a section of the rotatory dome constructed at East Sheen by the Rev. Dr. Pearson. This dome turns round on three detached spheres of lignum vitae, in a circular bed, formed partly by the dome, and partly by the cylindrical frame-work, which surrounds the circular room of 9 feet diameter. A section of this bed forms a square which the sphere just fills, so as to have a small play to allow for shrinking ; and, when the dome is carried round, the spheres, having exactly equal diameters of 4£ inches each, when placed figure 92. 514 THE PRACTICAL ASTRONOMER. at equal distances from one another, keep their relative places, and move together in a beautifully smooth manner. These spheres act as friction rollers in two directions at the four points of con- tact, in case any obstacle is opposed to their pro- gressive motion by the admission of dirt, or by any change of figure of the wood that composes the rings of the dome, and of the gang-way. No groove is here made, but what the weight of the roof resting on the hard sphere occasions. The dome itself moves twice round for the balls once, and has, in this way, its friction diminished. The wood of this dome is covered by Wyatt's patent copper, one square foot of which weighs upwards of a pound ; and the copper is so turned over the nails that fix it at the parts of junction, that not a single nail is seen in the whole dome. This covering is intended to render the dome more permanent than if it had been made of wood alone. At the observatory at Cambridge the dome is made chiefly of iron. In the figure a, a represents one of the two oblong doors that meet at the apex of the cone, and a piece of sheet-copper bent over the upper end of the door which shuts last, keeps the rain from entering at the place of junction. The two halves of the dome are united by brass rods passing through the door-cheeks of wainscot at a and a by means of nuts that screw upon their ends, which union allows the dome to be separated into two parts when there may be occasion to dis- place it. The wooden plate 66, which appears in a straight line, is a circular broad ring to which the covering wainscot boards are made fast above the eaves, and cc is a similar ring forming the wall-plate or gang-way on which the dome rests and revolves. ON OBSERVATORIES. 515 figure 92*. Fig. 92* shows a small door that lies over the summit of the dome, and may be separately opened for zenith observations ; the rod of metal with a ring at the lower end passing through it, serves to open and shut this door, and at the same time carries upon its upper end a large ball that falls back on the roof when the door is open, and keeps the door in a situation to be acted upon by the hook of a handle that is used for this purpose. The doors aa being curved, are made to open in two halves, the upper one being opened first, on account of its covering the end of the other ; and the observer may open one or two doors as may best suit his purpose. The weight of this dome is such that a couple of wedges, inserted by a gentle blow between the rings bb and cc 9 will keep it in its situation under the influence of the strongest wind. It may not be improper to remark, that in all observatories, and in every apartment where celes- tial observations are made, there should, if possi- ble, be a uniform temperature ; and consequently a fire should never be kept in such places, par- ticularly when observations are intended to be made, as it would cause currents of air through the doors and other openings, which would be injurious to the accuracy of observations. When a window is opened in an ordinary apartment where a fire is kept, there is a current of heated air which rushes out at the top, and a current of 516 THE PRACTICAL ASTRONOMER. cold air which rushes in from below, producing agitations and undulations, which prevent even a good telescope from showing celestial objects dis- tinct and well denned ; and, I have no doubt, that many young observers have been disappointed in their views of celestial phenomena, from this cir- cumstance, when viewing the heavenly bodies from heated rooms in cold winter evenings ; as the aerial undulations before the telescope pre- vent distinct vision of such objects as the belts of Jupiter, the spots of Mars, and the rings of Saturn. ON ORRERIES. 517 CHAPTER IV. ON ORRERIES OR PLANETARIUMS. An orrery is a machine for representing the order, the motions, the phases, and other pheno- mena of the planets. Although orreries and planet- ariums are not so much in use as they were half a century ago, yet as they tend to assist the con- ceptions of the astronomical tyro in regard to the motions, order, and positions of the bodies which compose the solar system, it may not be inexpe- dient shortly to describe the principles and con- struction of some of these machines. The reason why the name Orrery was at first given to such machines, is said to have been owing to the following circumstance. Mr. Rowley, a mathematical-instrument-maker, having got one from Mr. George Graham, the original inventor, to be sent abroad with some of his own instru- ments, he copied it and made the first for the Earl of Orrery. Sir R. Steele, who knew nothing of Mr. Graham's machine — thinking to do justice to the first encourager, as well as to the inventor of such a curious instrument, called it an Orrery, and gave Mr. Rowley the praise due to Mr. Graham. The construction of such machines is not a modern invention. The hollow sphere of Archimedes was apiece of mechanism of this kind, 518 THE PRACTICAL ASTRONOMER. having been intended to exhibit the motions of the sun, the moon, and the five planets, according to the Ptolemaic system. The next orrery of which we have any account was that of Posidonius, who lived about 80 years before the Christian era, of which Cicero says, c If any man should carry the sphere of Posidonius into Scythia or Britain, in every revolution of which the motions of the sun, moon and five planets, were the same as in the heavens, each day and night, who in those barbarous countries could doubt of its being finished — not to say actuated — by perfect reason V The next machine of this kind, which history records, was constructed by the celebrated Boethius, the Christian Philosopher, about the year of Christ 510 — of which it was said * that it was a machine pregnant with the universe — a portable heaven — a compendium of all things/ After this period, we find no instances of such mechanism of any note till the 16th century, when science began to revive, and the arts to flourish. About this time the curious clock in Hampton Court Palace was constructed, which shows not only the hours of the day, but the motions of the sun and moon through all the signs of the zodiac, and other celes- tial phenomena. Another piece of mechanism of a similar kind is the clock in the cathedral of Strasburg, in which besides the clock part, is a celestial globe or sphere with the motions of the sun, moon, planets and the firmament of the fixed stars, which was finished in 1574. Among the largest and most useful pieces of machinery of this kind, is the great sphere erected by Dr. Long in Pembroke Hall in Cambridge. This machine, which he called the Uranium, con- sists of a planetarium which exhibits the motion of the earth and the primary planets, the sun, and ON ORRERIES. 519 the motion of the moon round the earth, all en- closed within a sphere. Upon the sphere, besides the principal circles of the celestial globe, the Zodiac is placed, of a breadth sufficient to contain the apparent path of the moon, with all the stars over which the moon can pass, also the ecliptic, and the heliocentric orbits of all the planets. The Earth in the planetarium has a moveable horizon, to which a large moveable brass circle within the sphere may be set coincident, representing the plane of the horizon continued to the starry hea- vens. The horizons being turned round sink below the stars on the east side, and make them appear to rise, and rise above the stars on the west side, and make them appear to set. On the other hand, the earth and the horizon being at rest, the sphere may be turned round to represent the apparent diurnal motion of the heavens. In order to complete his idea on a large scale, the Doctor erected a sphere of 18 feet diameter, in which above 30 persons might sit conveniently, the entrance to which is over the South Pole, by six steps. The frame of the sphere consists of a number of iron meridians, the northern ends of which are screwed to a large round plate of brass with a hole in the centre of it ; through this hole, from a beam in the ceiling, comes the north pole, a round iron rod about three inches long, and which supports the upper part of the sphere, to its proper elevation for the latitude of Cambridge, so much of it as is invisible in England being cutoff, and the lower or southern ends of the meridians terminate on, and are screwed down to a strong circle of oak 13 feet diameter, which, when the sphere is put in motion, runs upon large rollers of lignum vitae, in the manner that the tops of some wind-mills turn round. Upon the iron meridians 520 THE PRACTICAL ASTRONOMER* is fixed a zodiac of tin painted blue, on which the ecliptic and heliocentric orbits of the planets are drawn and the stars and constellations traced. The whole is turned round with a small winch, with as little labour as it takes to wind up a Jack, although the weight of the iron, tin, and the wooden circle is above a thousand pounds. This machine, though now somewhat neglected, may still be seen in Pembroke Hall, Cambridge, where 1 had an op- portunity of inspecting it in November, 1839. The essential parts of the machine still remain nearly in the same state as when originally con- structed in 1758. The machine which I shall now describe is of a much smaller and less complex description than that which has been noticed above, and may be made for a comparatively small expense, while it exhibits, with sufficient accuracy, the motions, phases, and positions of all the primary planets, with the exception of the new planets, which cannot be accurately represented on account of their orbits crossing each other. In order to the construction of the Planetarium to which I allude, w r e must compare the proportion which the annual revolutions of the primary planets bear to that of the Earth. This proportion is expressed in the following table, in which the first column is the time of the Earth's period in days ; the second, that of the planets ; and the third and fourth are numbers very nearly in the same proportion to each other. 365J : 88 : : 83 : 20 for Mercury. 365J : 224§ : : 52 : 32 for Venus. 365J : 687 : : 40 : 75 for Mars. 3654 : 4332i : : 7 : 83 for Jupiter. 365i : 10759J : : 5 : 148 for Saturn. 365| : 30686' : : 3 : 253 for Uranus. On account of the number of teeth required ON ORRERIES. 521 for the wheel which moves Uranus, it is frequently omitted in Planetariums, or the planet is placed upon the arbor which supports Saturn. If w r e now suppose a spindle or arbor with six wheels fixed upon it in an horizontal position, having the number of teeth in each corresponding to the numbers in the third column, namely the wheel figure 93. AM (fig. 93.) of 83 teeth, BL of 52, CK of 50, for the earth, DI of 40, EH of 7, and FG of 5 ; and another set of wheels moving freely about an arbor having the number of teeth in the fourth column, namely AN of 20, BO of 32, CP of 50 —for the earth ; DQ of 75, ER of 83, and FS of 148. Then, if these two arbors of fixed and moveable wheels be made of the size, and fixed at the distance here represented, the teeth of the former will take hold of those of the latter, and turn them freely when the machine is in motion. These arbors, with their wheels, are to be placed in a box of a proper size, in a perpendicular posi- tion ; the arbor of fixed wheels to move in pivots at the top and bottom of the box, and the arbor 522 THE PRACTICAL ASTRONOMER. of the moveable wheels to go through the top of the box, and having on the top a wire fixed, and bent at a proper distance into a right angle upwards, bearing on the top a small round ball, representing its proper planet. If then, on the lower part of the arbor of fixed wheels, be placed a pinion of screw-teeth, a winch turning a spindle with an endless screw, playing in the teeth of the arbor, will turn it with all its wheels, and these wheels will turn the others about with their planets, in their proper and res- pective periods of time. For, while the fixed wheel CK moves its equal CP once round, the wheel AM will move AN a little more than four times round, and will consequently exhibit the motion of Mercury ; the wheel EH will turn the wheel ER about } - 2 round, representing the proportional motion of Jupiter ; and the wheel EG will turn the wheel FS, about round, and represent the motion of Saturn, and so of all the rest. The following figure (fig. 94.) represents the figure 94. ON ORRERIES. 523 appearance of the instrument when completed. Upon the upper part of the circular box is pasted a Zodiac circle divided into 12 signs, and each sign into 30 degrees, with the corresponding days of the month. The wheel-work is understood to be within the box, which may either be supported by a tripod, or with four feet, as here represented. The moon, and the satellites of Jupiter, Saturn and Uranus, are moveable only by the hand. When the winch W is turned, then all the primary planets are made to move in their respective velo- cities. The ball in the centre represents the Sun, which is either made of brass or of wood gilded with gold. By this Planetarium, simple as its construction may appear, a variety of interesting exhibitions may be made and problems performed, which may be conducive to the instruction of young students of astronomy. I shall mention only a few of those as specimens. 1. When the planets are placed in their respec- tive positions by means of an Ephemeris or the Nautical Almanack, the relative positions of those bodies in respect to each other, the quarters of the heavens where they may be observed, and whether they are to be seen in the morning before sun-rise or in the evening after sun-set, may be at once determined. For example, on the 19th of December, 1844, the heliocentric fleeces of the planets are as follows : — Uranus 2° Aries ; Saturn 8° 27 of Aquarius ; Jupiter 7° 4' Aries ; Mars 12° 45' Libra; the Earth 27° 46' Gemini; Venus 29° 48' Virgo ; Mercury 7° 53' Pisces. When the planets are placed on the planetarium in these positions, and the eye placed in a line with the balls representing the Earth and the Sun, all those situated to the left of the sun are to the east of 524 THE PRACTICAL ASTRONOMER. him, and are to be seen in the evening, and those on the right, in the morning. In the present case, Uranus, Saturn, Jupiter, and Mercury are evening stars, and Mars and Venus can only be seen in the morning. Jupiter is in an aspect nearly quartile, or 3 signs distant from the sun, and Uranus is nearly in the same aspect. Saturn is much nearer the sun, and Mercury is not far from the period of its greatest eastern elongation. Mars is not far from being in a quartile aspect, west of the sun, and Venus is near the same point of the heavens, approaching to the period of its greatest western elongation, and con- sequently will be seen before sun-rise as a beauti- ful morning star. Jupiter and Uranus, to the east of the sun, appear nearly directly opposite to Venus and Mars, which are to the west of the sun. The phase,* of Venus is nearly that of a half- moon, and Mercury is somewhat gibbous, ap- proaching to a half-moon phase. If, now, we turn the machine by the winch till the Index of the earth point at the 8th of August, 1845, we shall find the planets in the following positions : — Mars and Saturn are nearly in opposition to the sun ; Venus and Mercury are evening stars at no great distance from each other, and Jupiter is a morning star. In like manner if we turn the machine till the Index point to any future months, or even succeeding years, the various aspects and positions of the planets may be plainly perceived. When the planets are moved by the winch, in this machine, we see them all at once in motion around the sun, * The balls which represent the different planets, on this machine, have their hemispheres painted black, with the white side turned directly to the sun, so that if the eye be placed in a line with the earth, and the planet, particularly Mercury and Venus, its phase in the heavens, at that time, as viewed with a telescope, may be dis- tinctly perceived. ON ORRERIES. 525 with the same respective velocities and periods of revolution which they have in the heavens. As the planets are represented in the preceding positions, Mercury, Jupiter and Mars, are evening stars, and Venus, Saturn, and Uranus, morning stars, if we suppose the earth placed in a line with our eye and the sun. 2. By this instrument, the truth of the Coper- nican or Solar system is clearly represented. When the planets are in motion, we perceive the planets Venus and Mercury to pass both before and behind the sun, and to have two conjunctions. We observe Mercury to be never more than a certain angular distance from the sun, as viewed from the earth, namely 27° ; and Venus 47°. We perceive that the superior planets, particularly Mars, will be sometimes much nearer to the earth than at others, and therefore must appear larger at one time than at another, as they actually appear in the heavens. We see that the planets cannot appear from the earth to move with uniform velocity ; for when nearest they appear to move faster, and slower when most remote. We like- wise observe that the planets appear from the figure 95. 526 THE PRACTICAL ASTRONOMER. earth to move sometimes direct, or from west to east, then become retrograde, or from east to west, and between both to be stationary. All which particulars exactly correspond with celestial ob- servations. For illustrating these particulars there is a simple apparatus represented by fig. 95, which consists of a hollow wire with a slit at top which is placed over the arm of Mercury or Venus at E. The arm DG represents a ray of light coming from the planet at D to the earth at F. The planets being then in motion, the planet D, as seen in the heavens from the earth at F, will undergo the several changes of position, which we have described above, sometimes appearing to go backwards and at other times forwards. The wire prop, now supposed to be placed over Mercury at E, may likewise be placed over any of the other planets, particularly Mars, and similar phenomena will be exhibited. This machine may likewise be used to exhibit the falsity of the Ptolemaic system, which places the Earth in the centre, and supposes the sun and all the planets to revolve around it. For this pur- pose, the ball representing the Sun is removed, and placed on the wire or pillar which supports the Earth, and the ball representing the Earth is placed in the centre. It will then be observed, that the planets Mercury and Venus, being both within the orbit of the sun, cannot at any time be seen to go behind it, whereas, in the heavens we as often see them go behind as before the sun. Again, it shows that as the planets move in circular orbits about the central earth, they ought at all times to appear of the same magnitude ; while, on the contrary, we observe their apparent magni- tudes in the heavens to be very variable ; Mars, for example, appearing sometimes nearly as large ON ORRERIES. 527 as Jupiter, and at other times only like a small fixed star. Again, it is here shown that the planets may be seen at all distances from the sun ; for example, when the sun is setting, Mercury and Venus, according to this arrangement, might be seen, not only in the south but even in the eastern quarter of the heavens — a phenomenon which was never yet observed in any age ; Mercury never appearing beyond 27° of the Sun, nor Venus beyond 48°. In short, according to the system thus represented, it is seen, that the motions of the planets should all be regular, and uniformly the same in every part of their orbits, and that they should all move the same way, namely from west to east ; whereas, in the heavens, they are seen to move with variable velocities, sometimes appearing stationary, and sometimes moving from east to west, and from west to east. All which circumstances plainly prove that the Ptolemaic cannot be the true system of the universe. A Planetarium, such as that now described, might be constructed with brass wheel-work, for about 5 guineas. The brass wheel-work of one which I long since constructed cost about 3 guineas, and the other parts of the apparatus about 2 guineas more. The following are the prices of some instruments of this kind as made by Messrs. Jones, 30, Lower Holborn, London. 6 An Orrery, showing the motions of the Earth, Moon, and inferior planets, Mercury and Venus, by wheel-work, the board on which the instrument moves being 13 inches diameter, £4 : 14s. 6d.' 6 A Planetarium showing the motions of all the primary planets by wheel- work with 1 j inch or 3 inch papered globes, — according to the wheel-work and the neatness of the stands, from £7: 17s; 6d. to £10: 10s.' 6 Ditto, with wheel- work to show the parallelism of the Earth's axis, the motions of the Moon, her phases, &c, £18 : 18s. 1 4 Ditto, with wheel-work, to show the earth's diurnal motion, on a brass stand in mahogany case, £22 : Is.' 6 A small Tellurian, showing the motion of the Earth and Moon, &c, £1 : 8s. 528 THE PRACTICAL ASTRONOMER. Henderson's planetarium. The following is a description of the most com- plete and accurate planetarium I have yet seen. The calculations occupied more than eight months. For this article I am indebted to my learned and ingenious friend Dr. Henderson, F.R.A.S., who is known to many of my readers by his excellent astronomical writings. figure 96. Section of the wheel-work of a Planetarium for shewing with the utmost degree of accuracy the mean tropical revolutions of the planets round the sun, calculated by E. Henderson, LL.D. &c. In the above section the dark horizontal lines represent the wheel-work of the Planetarium, and the annexed numerals, the numbers of teeth in the given wheel. The machine has three axes or arbors, indicated by the letters A, B, C. — Axis ' C/ the ' Yearly axis/ is assumed to make one revolution in 365,242,236 days, or, in 365 days 5 h 48 m 49. 19 s , and is furnished with wheels 17, Henderson's planetarium. 529 44, 54, 36, 140, 96, 127, 86, which wheels are all firmly riveted to said axis, and consequently they turn round with it in the same time. Axle * B ' is a fixture ; it consists of a steel rod, on which a system of pairs of wheels revolve ; thus wheels 40 and 77 are made fast together by being riveted on the same collet represented by the thick dark space between them, as also of the rest: the several wheels on this axis may be written down thus ; f 79 ^, gg, 8 -, 30, & , 96, g, %. On axis A a system of wheels, furnished with tubes revolve, and these tubes carry hori- zontal arms, supporting perpendicular stems with the planets. The wheels on this axis are 173, 1, 111, 119, 1, g?, 83, 239,96, 128,72. From the following short description the nature of their several actions will, it is presumed, be readily understood — viz., MERCURY'S On the axis 6 C ' at the bottom is PERIOD. wheel 86, which turns round in 365 days 5 h 48 m 49. 19 s , this wheel im- pels a small wheel of 22 teeth, to which is made fast to wheel 67, both revolving together at the foot of axis B ; wheel 67 drives a wheel of 72 once round in the period of 87 days, 23 h 14 m 36. I s : this last men- tioned wheel has a long tube, which turns on the steel axis A, and car- ries a horizontal arm with the planet Mercury round the sun in the time above noted. VENUS'S On axis ' C ' is wheel 127, which PERIOD. drives wheel 47, to which is riveted a wheel of 77 teeth, which impels a wheel of 128 teeth on axis A, and 2 A 530 THE PRACTICAL ASTRONOMER. causes it to make a revolution in 224 days, 16 h 41 m 31. I s , and is furnished with a tube, which revolves over that of Mercury and ascends through the cover of the machine, and bears an arm on which is placed a small ball representing this planet in the time stated. THE EARTH'S The motion of the earth round PERIOD. fae sun is simply effected as follows — the assumed value of axis ' C ; ' the ' Yearly axis * is 365 days 5 h 48 ra 49. 19 s ; hence a system of wheels having the same numbers of teeth, or at all events, the first mover, and last wheel impelled must be equal in their numbers of teeth ; in this machine three wheels are employed, thus ; a wheel having 96 teeth is made fast to the Yearly axis C and of course moves round with it in a mean solar year, as above noted, this wheel impels another wheel of 96 teeth, on axis B, and this in its turns drives a third wheel of 96 teeth on axis A, and is fur- nished with a long tube which re- volves over that of Venus, and ascends above the cover-plate of the machine, and bears a horizontal arm which supports a small terrestrial globe, which revolves by virtue of said wheels once round the sun in 365 days 5 h 48 m 49. 19 s . MARS' PERIOD. The revolution of this planet is effected as follows — ^a wheel of 140 teeth is made fast to the yearly axis Henderson's planetarium. 531 THE ASTE- RIODS. VESTA'S PERIOD. JUNO'S PERIOD. CERES' PERIOD. C, and drives on axis B a wheel of 65 teeth, to which is fixed a whee of 59 teeth, which impels a large wheel of 239 teeth on axis A once round the sun in 686 days 22 h 18 m 33, 6 s , this last-mentioned wheel is also furnished with a tube which revolves over that of the earth, and carries a horizontal arm bearing the ball representing Mars, and causes it to complete a revolution round the sun in the period named. The period of Vesta is accom- plished thus, viz. On the Yearly axis C, is made fast a wheel of 36 teeth, which drives a wheel of 65 teeth on axis B, to which is fixed a wheel of 41 teeth, which impels a wheel of 83 teeth on axis A, once round in 1336 days 0 h 2l m 19. 8 s : The tube of which last wheel ascends on that of Mars, and like the rest bears an arm supporting a ball representing this planet. For the revolution of Juno, the yearly axis C is furnished with a wheel of 54 teeth, which impels a wheel of 50 teeth on axis B, to which is made fast a wheel of 27 teeth which turns a wheel of 127 teeth on axis A, once round in 1590 days 17 h 35 m 2. 7 s , and the tube of which ascends on that of Vesta, and supports a horizontal arm which carries a small ball representing this planet in the period named. The revolution of Ceres is derived 2 A 2 532 THE PRACTICAL ASTRONOMER. from the period of Juno, because wheel-work taken from the unit of a solar year was not sufficiently accurate for the purpose, therefore on Juno's wheel of 127 teeth is fixed a wheel of 123 teeth, which drives a thick little bevel sort of wheel of 30 teeth on axis B : the reason of this small wheel being bevelled is to allow its teeth to suit both wheels jf ; wheel 30 drives wheel 130, on axis A once round In 1681 days, 6 h 17 m 22.4 s and the tube of wheel 130 turns on the tube of Juno, and as- cends in a similar manner with the rest and carries an horizontal arm supporting a small ball representing this planet, and is caused to revolve round the Sun in the above men- tioned period (the period of Ceres to that of Juno is as 130 is to 123 ; hence the wheels used.) PALLAS'S The Period of Pallas could not be PERIOD. derived from the solar year with sufficient accuracy, and recourse was had to an engrafted fraction on the period of Ceres, thus. On wheel 130 of Ceres is made fast a wheel of 122 teeth, which drives a wheel of 81 teeth on axis B, to which is fixed a wheel 79 which impels a wheel of 119 teeth on axis A, and is fur- nished with a tube which ascends, and turns on that of Ceres, and supports a horizontal arm, which bears a small ball representing this planet, which by virtue of the above Henderson's planetarium. 533 JUPITER'S PERIOD. SATURN'S PERIOD. URANUS'S PERIOD. train of wheels is caused to com- plete a revolution round the Sun in 1681 d 10 h 28 m 25.1 a . The motion of this planet is de- rived from the period of a solar year ; from the ' yearly axis ' thus, on this axis is made fast a wheel of 44 teeth which turns a wheel of 94 teeth on axis B, to which is riveted a small wheel of 20 teeth, which impels a wheel on axis A having 111 teeth, which is furnished with an ascending tube which revolves over that of Pallas, and bears an horizon- tal arm which supports a ball repre- senting this planet, which by the said train of wheels is caused to revolve round the Sun in 4330 d 14 h 39 ra 35.7 s . The periodic revolution of Saturn is also taken from the solar year — viz., a small wheel of 17 teeth is fixed to the ' yearly axis 9 near its top, and drives a wheel of 129 teeth on axis B, to which is made fast a wheel of 49 teeth, which turns a wheel of 190 teeth on axis A, whose tube ascends and revolves on that of Jupiter's tube, and supports an arm, having a ball representing Saturn and its rings, and which by the train of wheels is caused to per- form a revolution round the sun in the period of 10746 d 19 h 16 m 50.9 s . The revolution of this planet could not be attained with sufficient accuracy from the period of a solar 534 THE PRACTICAL ASTRONOMER. year — the period is engrafted on that of Saturn's, thus, a wheel of 117 teeth is made fast to wheel 190 of Saturn, and consequently revolves in Saturn's period. This wheel of 117 teeth drives a wheel on axis B, having 77 teeth, to which is fixed a wheel of 40 teeth, which turns on axis A, a large wheel of 173 teeth, whose tube ascends and revolves over that of Saturn, and carries a hori- zontal arm which supports a ball representing this planet, which is caused to complete its revolution by such a train of wheels in the period of 30589 d 8 h 26 m 58.4 s . Such is a brief description of the motions of this comprehensive and very accurate machine. The axis A, on which the planetary tubular wheels revolve, performs a rotation in 25 days 10 hours, by virtue of the following train of wheels, § + | of 24 hours, that is, a pinion of 14 is assumed to revolve in 24 hours, and to drive a wheel of 61 teeth, to which is fixed a pinion of 12 , which turns the wheel 70 in the period noted ; to this wheel-axis, it is made fast, and by revolving with it, exhibits the Sun's rotation. The machine is turned by a handle or winch, which is assumed to turn round in 24 hours, and from this rotation of 24 hours a train of wheel- work is required to cause the 6 yearly axis ■ C, to turn once round in 365 d 5 h 48 m 49.19 s , which is effected in the followingmanner— viz, the train found DIURNAL HAND. Henderson's planetarium. 535 by the process of the reduction of con- tinuous fractions is rl + ts + ir that is, in the train for turning the sun, the same pinion 14 turns the same wheel 61, and turns a pinion of 18 leaves, to which is fixed a wheel of 144 teeth, having a pinion of 23 leaves, which impels a large wheel of 241 teeth once round in 365.242236 d or 365 d 5 h 48 m 49.19 s , this last-men- tioned wheel of 241 teeth is made fast to the under part of the * yearly axis ' C at D, the handle having a pinion of 14 leaves therefore, and transmitting its motion through the above train, causes the yearly axis to revolve in the same period. J^^S^S^J" The planetarium is also furnished with a system ot wheels tor regis- tering dates for either 10,000 years past or to come, the arrangement is not shewn in the engraving (to pre- vent confusion) but it might be shortly described thus: — Near the top of the yearly axis is a hooked piece e, which causes the tooth of a wheel of 100 teeth to start forward yearly, consequently 100 starts of said wheel will cause it to revolve in 100 solar years, and it has a hand which points on a dial on the cover of the machine the years; thus for the pre- sent year this hand will be over the number 45. This last-named wheel of 100 teeth has a pin which causes a tooth of another wheel of 100 teeth to start once in 100 years, 536 THE PRACTICAL ASTRONOMER. hence this last wheel will complete one revolution in 10,000 years, and it is for this purpose the former index or hand moves over a number yearly. The second index will pass over a number every 100 years — for the present year the second hand or index will be over the number 1 8, and will continue over it until the first index moves forward to 99, then both indexes will move at one time, viz., the first index to OO on the first concentric circle of the dial, and the second index to 19, denoting the year 1900, and so of the rest. By the ecliptic being di- vided in a series of four spirals, the machine makes a distinction between common and leap years, and indi- cates the common year as containing 365 days, and the leap-year 366 days, by taking in a day in February every fourth year ; thus for any given period for 10,000 years past or to come, the various situations and aspects of the planets may be ascer- tained by operating with this machine, and this for thousands of years without producing a sensible error either in space or time. This planetarium wheel-work is enclosed in an elegant mahogany box of twelve sides — is about 5 feet in diameter by 10 inches in depth ; at each of the twelve angles, or sides, small brass pillars rise and support a large Ecliptic circle on which are engraven Henderson's planetarium. 537 the signs, degrees and minutes of the Ecliptic — the days of the month, &c. This mahogany box with the wheel- work is supported by a tripod stand three feet in hight, and motion is communicated to the several balls representing the planets by turning the handle as before described. A Planetarium of this complicated sort, costs sixty guineas. The following is a tabular view of the wheel- work, periods, &c. Planets' Names. Wheel-work. r ropical periods pro- r luced by the wheel- ""rue mean Tro- pical Periods of the Planets. Mercury % + !° faYear da. ho. m. s. 87. 23.14. 36.1 da. ho. m. s. 87. 23. 14. 36 Venus 47 , 128 127 77 224.16. 41.31.1 224. 16. 41. 36 The Earth Prime mover 96+96 + 96 „ 365. 5.48.49.19 365. 5.48.49 Mars 65 239 140 59 686. 22. 18. 33.6 686. 22. 18. 34 Vesta 65 83 36 41 1335. 0.21.19.8 1335. 0.21.20 Juno 50 127 54 27" 1590.17.35. 2.7 1590.17.35. 1 Ceres ^ + 30 of Juno 1681. 6.17.22.4 1681. 6.17.29 Pallas JS + ll 9 of Ceres 122 79 1681. 10. 28. 25.1 1681.10. 28,42 Jupiter 94 + li 1 of a Yeai 44 20 4330. 14. 39 . 35.7 4330. 14. 39. 32 Saturn 129 190 17 49 10746.19. 16.50.9 10746. 19. 16. 52 Uranus XL + of Saturr 117 40 l 30589. 8.26.58.4 30589: 8.26.59 The Sun's Rotation 6 2 + L° of 24 ho. 14 12 \ 61 4. 144 4. 241 J 14 + 18* 23 " 25.10. 0. 0 25.10. 0. 1 The tropical period of the Earth round the Sun. 365. 5.48.49.11 ) 365. 5.48.49 2 A 5 538 THE PRACTICAL ASTRONOMER, In the month of October last year, Dr. Hen- derson made a series of calculations for a new Planetarium for the use of schools. It shows with considerable accuracy for 700 days, the mean tropical revolutions of the Planets round the sun — the machine consists of a system of brass wheels peculiarly arranged, and is enclosed in a circular case three feet in diameter, the top of which has the signs and degrees of the ecliptic laid down on it, as also the days of the months, &c. This Planetarium costs only 45s. or on a tripod stand, table-high, 55s. ; the machine is put in motion by a handle on the outside. To the teachers and others connected with education this Pla- netarium must be of great importance, for without a proper elucidation of the principles of astronomy, that of Geography must be but confusedly un- derstood, This Planetarium is at present made by Mr. Dollond, 9, White Conduit Grove, Isling- ton, London. The Tellurianis a small instrument which should be used in connection with the Planetarium formerly described. This instrument is intended to show the annual motion of the earth, and the revolution of the moon around it. It also illustrates the moon's phases, and the motion of her nodes, the inclination of the Earth's axis, the causes of eclipses, the variety of seams, and other pheno- mena. It consists of about eight wheels, pinions and circles. A small instrument of this descrip- tion may be purchased for about one pound eight shillings, as stated in the note, page 527. on saturn's ring. 539 ON THE VARIOUS OPINIONS WHICH WERE ORIGI- NALLY FORMED OF SATURN'S RING. The striking and singular phenomenon con- nected with the planet Saturn — though now ascertained beyond dispute to be a Ring, or Rings, surrounding its body at a certain distance — was a subject of great mystery, and gave rise to numer- ous conjectures and controversies, for a consider- able time after the invention of the telescope by which it was discovered. Though it was first discovered in the year 1610, it was nearly 50 years afterwards, before its true form and nature were determined. Galileo was the first who dis- figure 97- 540 THE PRACTICAL ASTRONOMER. covered anything uncommon connected with Saturn : through his telescope he thought he saw that planet appear like two smaller globes on each side of a larger one ; and after viewing the planet in this form for two years, he was surprised to see it becoming quite round, without its adjoining globes, and some time afterwards to appear in the triple form. This appearance is represented in fig. 1 of the above engraving. In the year 1614, Scheiner, a German astronomer, published a representation of Saturn, in which this planet is exhibited as a large central globe, with two smaller bodies, one on each side, partly of a conical form, attached to the planet and forming a part of it, as shown fig. 2. In the year 1640 and 1643, Ric- ciolus, an Italian mathematician and astronomer, imagined he saw Saturn as represented in fig. 3. consisting of a central globe, and two conical shaped bodies completely detached from it, and published an account of it corresponding to this view. Hevelius, the celebrated astronomer of Dantzig, author of the Selenographia and other works, made many observations on this planet about the years 1643, 1649 and 1650, in which he appears to have obtained different views of the planet and its appendages, gradually approximat- ing to the truth, but still incorrect. These views are represented in figures 4, 5, 6, and 7. Fig. 4 nearly resembles two hemispheres, one on each side of the globe of Saturn. The other figures very nearly resemble the extreme parts of the ring as seen through a good telescope, but he still seems to have considered them as detached from each other as well as from Saturn. Figures 8 and 9 are views given by Ricciolus at a period posterior to that in which he supposed Saturn and his appendages in the form delineated on saturn's ring. 541 in fig. 3. In these l^st delineations the planet was supposed to be enclosed in an elliptical ring, but this ring was supposed to be fixed to its two opposite sides. Fig. 10, is a representation by Eustachius Divini, a celebrated Italian optician at Bologna. The shades represented on Saturn and the elliptical curve are incorrect, as this planet presents no such shadowy form. The general appearance here pre- sented is not much unlike that which the ring of Saturn exhibits, excepting that at the upper side the ring should appear covering a portion of the orb of Saturn. But Divini seems to have conceived that the curve on each side was attached to the body of Saturn. For when Huygens published his discovery of the ring of Saturn in 1659, Divini contested its truth, because he could not perceive the ring through his own telescopes; and he wrote a treatise on the subject in opposition to Huygens, in 1660, entitled 'Brevis Annotatio in Systema Saturninum.' Huygens immediately replied to him, and Divini wrote a rejoinder in 1661. —Fig. 11 is the representation given by Francis Fontana, a Neapolitan astronomer. This figure represents Saturn as having two crescents, one on each side, attached to its body, with intervals between the planet and the crescents. Fig. 12 is a view delineated by Gassendus, a celebrated French phi- losopher. It represents the planet as a large ellip- soid, having a large circular opening near each end, and, if this representation were the true one, each opening would be at least 30,000 miles in diameter. Fig. 13, which is perhaps the most singular of the whole, is said to be one of the views of this planet given by Ricciolus. It repre- sents two globes — each of which, in the proportion they here bear to Saturn, must be more than 542 THE PRACTICAL ASTRONOMER, thirty thousand miles in diameter. These globes, were conceived as being attached to the body of Saturn by curves or bands, each of which, in the proportion represented, must have been at least 7000 miles in breadth, and nearly 40,000 miles long. This would have exhibited the planet Saturn as a still more singular body than what we have found it to be; but no such construction of a planet has yet been found in the universe, nor is it probable that such a form of a planetary body exists. It is remarkable that only two general opinions should have been formed respecting the construc- tion of Saturn— as appears from these representa- tions — either that this planet was composed of three distinct parts, separate from each other, — or that the appendage on each side was fixed to the body of the planet. The idea of a ring surround- ing the body of the planet, at a certain distance from every part of it, seems never to have been thought of till the celebrated Huygens, in 1655, 1656 and 1657, by numerous observations made on this planet, completely demonstrated that it is surrounded by a solid and permanent ring, which never changes its situation, and, without touching the body of the planet, accompanies it in its revo- lution around the sun. As the cause of all the erroneous opinions above stated was owing to the imperfection of the telescopes which were then in use, and their deficiency in magnifying power, — this ingenious astronomer set himself to work in order to improve telescopes for celestial observa- tions. He improved the art of grinding and polishing object-glasses, which he finished with his own hands, and produced lenses of a more correct figure, and of a longer focal distance than what had previously been accomplished. He first con- on Saturn's ring. 543 structed a telescope 12 feet long, and afterwards one 23 feet long, which magnified about 95 times; whereas Galileo's best telescope magnified only about 33 times. He afterwards constructed one 123 feet long, which magnified about 220 times. It was used without a tube, the object-glass being placed upon the top of a pole and connected by a cord with the eye-piece. With such telescopes this ingenious artist and mathematician discovered the fourth satellite of Saturn, and demonstrated that the phenomenon, which had been so egre- giously misrepresented by preceding astronomers, consisted of an immense ring surrounding the body, and completely detached from it. His numerous observations and reasonings on this subject were published in Latin, in 1659, in a quarto volume of nearly 100 pages, entitled 6 Systema Saturnium, sive de causis mirandorum Saturni Phenomenon, et Comite ejus Planet a Nova,' from which work the figures and some of the facts stated above have been extracted. ON THE SUPPOSED DIVISIONS OF THE EXTERIOR RING OF SATURN. From the period in which Huygens lived till the time when Herschel applied his large telescopes to the heavens, few discoveries were made in relation to Saturn. Cassini, in 1671, discovered the fifth satellite of this planet; in 1672, the third; and the first and second in March, 1684. In 1675, Cassini saw the broad side of its ring bisected quite round by a dark elliptical line, of which the inner part appeared brighter than the outer. In 1722, Mr. Hadley, with his 5 feet Newtonian Reflector observed the same phenomenon, and 544 THE PRACTICAL ASTRONOMER. perceived that the dark line was stronger next the body, and fainter towards the upper edge of the ring. Within the ring he also discovered two belts across the disk of Saturn. But it does not appear that they had any idea that this dark line was empty space separating the ring into two parts. This discovery was reserved for the late Sir W. Herschel, who made numerous observa- tions on this planet, and likewise ascertained that the ring performs a revolution round the planet in ten hours and thirty minutes. Of late years, some observers have supposed that the exterior ring of Saturn is divided into several parts, or, in other words, that it consists of two or more concentric rings. The following are some of the observations on which this opinion is founded. They are chiefly extracted from Captain Kater's Paper on this subject, which was read before the Astronomical Society of London. The observations, we are told, were made in the years 1825 and 1826, and remained unpublished, from a wish on the part of the observer to witness the appearances again. The planet Saturn has been much observed by Captain Kater, for the purpose of trying the light, &c. 3 for which the ring and satellites are good tests. The instruments which were employed in the present investigations were two Newtonian Reflectors — one by Watson, of 40 inches focus and 6^ aperture ; and another by Dollond, of 68 inches focus, and 6f aperture. The first, under favourable circumstances, gave a most excellent image, the latter is a very good instrument. The following are extracts from the author's journal. Nov. 25, 1825. — The double ring beautifully defined, perfectly distinct all around, and the prin- on saturn's ring. 545 cipal belts well seen. I tried many concave glasses, and found that the image was much sharper than with convex eye-glasses, and the light apparently much greater. Dollond, 259, the best power, 480, a single lens, very distinct. Nov. 30, the night very favourable, but not equal to the 25 th. The exterior ring of Saturn is not so bright as the in- terior, and the interior is less bright close to the edge next the planet. The inner edge appears more yellow than the rest of the ring, and nearer in colour to the body of the planet. Dec. 17. — The evening extremely fine. With Dollond, I perceived the outer ring of Saturn to be darker than the inner, and the division of the ring all around with perfect distinctness ; but with Watson I fancied that I saw the outer ring separated by numerous dark divisions extremely close, one stron- ger than the rest, dividing the ring about equally. This was seen with my most perfect single eye- glass power. A careful examination of some hours confirmed this opinion. — Jan. 16 and 17, 1826. — Captain Kater believed that he saw the divisions with the Dollond, but was not positive. Concave eye-glasses found to be superior to convex. Feb. 26, 1826. — The division of the outer ring not seen with Dollond. On the 17th Dec, when the divi- sions were most distinctly seen, Captain Kater made a drawing of the appearance of Saturn and his rings. The phenomena were witnessed by two other persons on the same evening, one of whom saw several divisions in the outer ring, while the other saw one middle division only ; but the latter person was short-sighted, and unaccustomed to telescopic observations. It may be remarked, how- ever, that these divisions were not seen on other evenings, which -yet were considered very favour- able for distinct vision. 546 THE PRACTICAL ASTRONOMER. It is said that the same appearances were seen by Mr. Short, but the original record of his obser- vations cannot be found. In Lalande's Astronomy (3rd edition, article 3351,) it is said, € Cassini re- marked that the breadth of the ring was divided into two equal parts by a dark line having the same curvature as the ring, and the exterior por- tion was the less bright. Short told me that he observed still more singular phenomena with his large telescope of 12 feet. The breadth of the ansae, or extremities of the ring; was, according to him, divided into two parts, — an inner portion without any break in the illumination, and an outer divided by several lines concentric with the circumference ; which would lead to a belief, that there are several rings in the same plane' De Lambre and Birt severally state that Short saw the outer ring divided, probably on the authority of Lalande. In Brewster's Ferguson's Astronomy, vol. ii, p. 125, 2nd edition, there is the following note on this subject. 6 Mr. Short assures us, that with an excellent telescope, he observed the sur- face of the ring divided by several dark concentric lines, which seem to indicate a number of rings proportional to the number of dark lines which he perceived.' In Dec. 1813, at Paris, Professor Quetelet saw the outer ring divided with the achromatic tele- scope of 10 inches aperture, which was exhibited at the exposition. He mentioned this the follow- ing day to M. de la Place, who observed, that € those or even more divisions, were conformable to the system of the world/ On the other hand the division of the outer ring was not seen by Sir W. Herschel in 1792, nor by Sir J. Herschel in 1826, nor by Struve in the same year ; and on several occasions when the atmospheric conditions were on saturn's ring. 547 most favourable, it has not been seen by Captain Kater. It has been remarked by Sir W. Herschel, Struve and others, that the exterior ring is much less brilliant than the interior. And it is asked, May not this want of light in the outer ring arise from its having a very dense atmosphere? and may not this atmosphere in certain states admit of the divisions of the exterior ring being seen, though, under other circumstances, they remain invisible ? The above observations are said to have been con- firmed by some recent observations by Decuppis at Rome, who announced, some years ago, that Sa- turn's outer ring is divided into two or three con- centric rings. Some of the observations stated above, were they perfectly correct, would lead to the conclusion that Saturn is encompassed with a number of rings, concentric with and parallel to each other. But while such phenomena as described above are so seldom seen, even by the most powerful telescopes and the most accurate observers, a cer- tain degree of doubt must still hang over the subject; and we must suspend our opinion on this point, till future observations shall either confirm or render doubtful those to which we have referred. Should the Earl of Rosse's great telescope, when finished for observation, be found to perform ac- cording to the expectations now entertained, and in proportion to its size and quantity of light, we shall expect that our doubts will be resolved in regard to the supposed divisions of the ring of Saturn. APPENDIX, BRIEF DESCRIPTION OF THE EARL OF ROSSe's TELESCOPE. This telescope, the largest and most magnificent that ever was attempted, reflects the greatest honour on the genius, the inventive powers, and the scientific acquirements of its noble contriver, as well as on the elevated station in which he is placed. With rank and fortune, and every cir- cumstance that usually unfit men for scientific pursuit, he has set a bright example to his com- peers of the dignity and utility of philosophical studies and investigations, and of the aids they might render to the progress of science, were their wealth and pursuits directed in a proper channel. Previously to his Lordship's attempting the construction of his largest — or 6 Monster Teles- cope,' he had constructed one with a speculum of 3 feet in diameter, which was considered one of the most accurate and powerful instruments that had ever been made, not excepting even Sir W. EARL OF ROSSE'S TELESCOPE. 549 HerschePs forty-feet Reflector. In the account of this telescope, published in the Philosophical Transactions for 1840, his Lordship speaks of the possibility of a speculum of six feet in diameter being cast. At that time, it was considered by some as little short of a chimera to attempt the construction of such a monstrous instrument. But the idea no sooner occurred to this ingenious and persevering nobleman than he determined to put it to the test, and the result has been attended with complete success. The materials of which this speculum is composed are copper and tin, united very nearly in their atomic proportions, namely, copper 126.4 parts, to tin 58.9 parts. This compound has a specific gravity of 8.8, and it is found to preserve its lustre with more splen- dor, and to be more free from pores than any other. A foundry was constructed expressly for the purpose of casting the speculum. Its chimney built from the ground was 18 feet high, and 16^ square at the base, tapering to four at the top. At each of its sides, communicating with it by flue, was sunk a furnace 8 feet deep, and square, with a circular opening 4 feet in diameter. About seven feet from the chimney was erected a large crane, with the necessary tackle for elevating and carrying the crucibles from the furnace to the mould, which was placed in a line with the chim- ney and crane, and had three iron baskets sup- ported on pivots hung round it ; and four feet far- ther on was the annealing oven. The crucibles which contained the metal were each 2 feet in diameter, %\ deep, and together weighed one ton and a half ; they were of cast iron and made to fit the baskets at the side of the mould. These baskets were hung on wooden uprights or pivots, to one of these on each side was attached a lever, 550 APPENDIX. by depressing which it might be turned over, and the contents of the crucible poured into the mould. The bottom of the mould was made by binding together tightly layers of hoop-iron, and turning the required shape on them edgewise. This mould conducted the heat away through the bottom, and cooled the metal towards the top in infinitely small layers, while the interstices, though close enough to prevent the metal from escaping, were sufficiently open to allow the air to penetrate. This bottom was six feet in diameter and 5\ inches thick, and was made perfectly horizontal by means of spirit levels, and was surrounded by a wooden frame ; a wooden pattern, the exact size of the speculum, being placed on the iron ; sand was well packed between it and the frame, and the pattern was removed. Each of the crucibles containing the melted metal was then placed in its basket, and every thing being ready for discharging their contents, they were at the same instant turned over, and the mould being filled, the metal in a short time safely set into the required figure. Whilst it was red hot, and scarcely solid, the frame-work was removed, and an iron ring con- nected with a bar which passed through the oven, being placed round it, it was drawn in by means of a capstan at the other side, on a railroad, when charcoal being lighted in the oven, and turf fires underneath it, all the openings were built up, and it was left for sixteen weeks to anneal. It was cast on the 13th of April, 1842, at 9 o' clock in the evening. The crucibles were ten hours heat- ing in the furnaces before the metal was intro- duced, which in about ten hours more was suffi- ciently fluid to be poured. When the oven was opened the speculum was found as perfect as when it entered it. It was then removed to the grinding EARL OF ROSSE'S TELESCOPE. 551 machine, where it underwent that process, and afterwards was polished, without any accident having occurred. This speculum weighed three tons, and lost about one eighth of an inch in grinding. Lord Rosse has since cast another speculum of the same diameter four tons in weight. He can now, with perfect confidence, undertake any casting, so great an improvement has the form of mould which he has invented proved. The speculum was placed on an equilibrium bed, composed of nine pieces resting on points at their centres of gravity; the pieces were lined with pitch and felt, before the speculum was placed on them. The speculum box is also lined with felt and pitched ; this prevents any sudden change of tem- perature affecting the speculum by means of the bad conducting power of the substances employed. A vessel of lime is kept in connection with the speculum-box to absorb the moisture, which otherwise might injure the mirror. The process of grinding was conducted under water, and the moving power employed was a steam-engine of three-horse power. The Polisher is connected with the machinery by means of a large ring of iron, which loosely encircles it ; and instead of either the speculum or the polisher being sta- tionary, both move with a regulated speed ; the ring of the polisher, and therefore the polisher itself, has a transverse and a longitudinal motion ; it makes 80 strokes in the minute, and 24^ strokes backward and forward for every revolution of the mirror, and at the same time 1^ 0 strokes in the transverse direction. The extent of the latter is % of the diameter of the speculum. The sub- stance made use of to wear down the surface was emery and water, a constant supply of these was 552 APPENDIX. kept between the grinder and the speculum. The Grinder is made of cast iron, with grooves cut lengthways, across and circularly on its face. The polisher and speculum have a mutual action upon each other ; in a few hours, by the help of the emery and water, they are both ground truly cir- cular, whatever may have been their previous defects. The grinding is continued till the re- quired form of surface is produced ; and this is ascertained in the following manner. There is a high tower over the house in which the speculum is ground, on the top of which is fixed a pole, to which is attached the dial of a watch ; there are trap doors which open, and by means of a tem- porary eye-piece, allow the figure of the dial to be seen in the speculum brought to a slight polish. If the dots on the dial are not sufficiently well- defined, the grinding is continued ; but if they appear satisfactorily, the polishing is commenced. It required six weeks to grind it to a fair surface. The polisher was cut into grooves, to prevent the abraded matter from accumulating in some places more than in others — a thin layer of pitch was spread over it, it was smeared over with rouge and water, and a supply of it kept up till the machi- nery brought it to a fine black polish. The length of time employed for polishing the 3 feet specu- lum was six hours.* This large telescope is now completed, or nearly so. The tube is 56 feet long, including the spe- culum box, and is made of deal, one inch thick, hooped with iron. On the inside, at intervals of * The above description has been selected and abridged from a small volume entitled 8 The Monster Telescope, erected by the Earl of Rosse, Parsontown,' — and also from the 1 Illustrated London News' of September 9th, 1843. In the volume alluded to a more particular description will be found, accompanied with engravings. EARL OF ROSSE'S TELESCOPE. 553 8 feet, there are rings of iron 3 inches in depth and 1 inch broad, for the purpose of strengthening the sides. The diameter of the tube is 7 feet. It is fixed to mason-work, in the ground, to a large universal hinge which allows it to turn in all di- rections. At 12 feet distance, on each side, a wall is built, 72 feet long, 48 high on the outer side, and 56 on the inner — the walls being 24 feet dis- tant from each other, and lying exactly in the meridional line. When directed to the south, the tube may be lowered till it become almost hori- zontal ; but when pointed to the north, it only falls till it is parallel with the earth's axis, point- ing then to the pole of the heavens. Its lateral movements take place only from wall to wall, and this commands a view for half an hour on each side of the meridian — that is, the whole of its motion from east to west is limited to 15 degrees. At present it is fitted up in a temporary way to be used as a Transit instrument ; but it is ulti- mately intended to connect with the tube-end galleries, machinery which shall give an automaton movement, so that the telescope shall be used as an Equatorial Instrument. All the works con- nected with this instrument are of the strongest and safest kind ; all the iron-work was cast in his Lordship's laboratory by men instructed by him- self, and every part of the machinery was made under his own eye, by the artizans in his own neighbourhood, and not a single accident worth mentioning happened during the whole proceed- ing- The expence incurred by his Lordship in the erection of this noble instrument was not less than twelve thousand pounds ! besides the money ex- pended in the construction of the telescope of three feet diameter. Sufficienttimehasnot yet been 2 B 554 APPENDIX. afforded for making particular observations with this telescope ; but from slight trials which have been made, even under unfavourable circumstances, it promises important results. Its great superio- rity over every telescope previously constructed consists in the great quantity of light it reflects, and the brilliancy with which it exhibits objects even when high powers are applied. It has a re- flecting surface of 4,071 square inches, while that of Herschel's 40-feet telescope had only 1811 square inches on its polished surface, so that the quantity of light reflected from the speculum is considerably more than double that of Herschel's largest reflector. This instrument has already exceeded his Lordship's expectations. Many ap- pearances before invisible in the Moon, have been perceived, and there is every reason to expect that new discoveries will be made by it in the Nebulce, double and triple stars, and other celestial objects. The following is an extract of a com- munication from Sir James South, on this subject, addressed to the Editor of the ( Times.' ' The leviathan telescope on which the Earl of Rosse has been toiling upwards of two years, although not absolutely finished, was on Wednesday last directed to the Sidereal Heavens. The letter which I have this morning received from its noble maker, in his usual unassuming stile, merely states, that the metal only just polished, was of a pretty good figure, and that with a power of 500, the nebula known as No. 2. 9 of Messier's cata- logue, was even more magnificent than the nebula, No. IS of Messier, when seen with his Lordship's telescope of 3 feet diameter, and 27 feet focus. Cloudy weather prevented him from turning the leviathan on any other nebulous object. Thus, then, we have all danger of the metal breaking EARL OF ROSSE'S TELESCOPE. 555 before it could be polished, overcome. Little more, however, will be done with it for some time, as the Earl is on the eve of quitting Ireland for England to resign his post at York as President of the British Association. I look forward with intense anxiety to witness its first severe trial, when all its various appointments shall be com- pleted, in the confidence that those who may then be present, will see with it what man has never seen before. The diameter of the large metal is 6-feet, and its focus 54 feet ; yet the immense mass is manageable by one man. Compared with it, the working telescopes of Sir William Hersche], which in his hands conferred on astronomy such inestimable service, and on himself astronomical immortality, were but playthings/ The following is a more recent account of ob- servations made by this telescope, chiefly extracted from Sir James South's description of this tele- scope, inserted in the Times of April 16th, 1845, and the 6 Illustrated London News ' of April 19. * The night of the 5th of March, 1845, was the finest I ever saw in Ireland. Many nebulas were observed by Lord Rosse, Dr. Robinson and myself. Most of them were for the first time since their creation, seen by us as groups or clusters of stars ; while some, at least to my eyes, showed no such resolution. Never, however, in my life did I see such glorious sidereal pictures as this instrument afforded us. Most of the nebulae we saw I certainly have observed with my own large achromatic ; but although that instrument, as far as relates to magnifying power, is probably inferior to no one in existence, yet to compare these nebulas, as seen with it and the G-feet telescope, is like comparing, as seen with the naked eye, the dinginess of the planet Saturn to the brilliancy 2 B 2 556 APPENDIX. of Venus. The most popularly-known nebulae observed this night were the ring nebulae in the Canes Venatici, or the 51st of Messier's cata- logue, which was resolved into stars with a mag- nifying power of 548, and the 94th of Messier, which is in the same constellation, and which was resolved into a large globular cluster of stars, not much unlike the well-known cluster in Hercules, called also 13th Messier.' Perfection of figure, however, of a telescope, must be tested, not by nebulae, but by its performance on a star of the first magnitude. If it will, under high power, show the star round and free from optical appen- dages, we may safely take it for granted it will not only show nebulae well, but any other celestial object as it ought. To determine this point, the telescope was directed to Regulus, with the entire aperture, and a power of 800, and 6 I saw 3 says Sir James, 6 with inexpressible delight, the star free from wings, tails or optical appendages ; not indeed like a planetary disk, as in my large achro- matic, but as a round image resembling voltaic light between charcoal points ; and so little aber- ration had this brilliant image, that I could have measured its distance from, and position with any of the stars in the field with a spider's line micro- meter, and a power of 1,000, without the slightest difficulty ; for, not only was the large star round, but the telescope, although in the open air, and the wind blowing rather fresh, was as steady as a rock.' e On subsequent nights, observations of other nebulaa, amounting to some 30 or more, removed most of them from the list of nebulae, where they had long figured, to that of clusters ; while some of these latter, more especially 5 Messier, exhi- bited a sidereal picture in the telescope such as EARL OF ROSSE's TELESCOPE. 557 man before had never seen, and which for its mag- nificence baffles all description. Several double stars were seen with various apertures of the tele- scope, and with powers between 860 and 800 ; and as the Earl had told us before we should, — before the speculum was inserted in the tube, in consequence of his having been obliged to quit the superintendence of the polishing at the most critical part of the process, — we found that a ring of about 6 inches broad, reckoning from the cir- cumference of the speculum, was not perfectly polished, and to that the little irradiation seen about Regulus was unquestionably referable. The only double stars of the 1st class which the weather permitted us to examine with it were Xi Ursae Majoris, and Gamma Virginis, which I could have measured with the greatest confidence. D' Arrest's comet we observed on the 12th of March, with a power of 400, but nothing worthy of notice was detected. Of the Moon, a few words must suffice. Its appearance in my large achromatic of 12 inches aperture is known to hun- dreds of readers ; let them then imagine that with it they look at the moon, whilst with Lord Rosse's 6 feet they look into it> and they will not form a very erroneous opinion of the performance of the Leviathan. On the loth of March, when the moon was 7 days old, I never saw her unillu- minated disk so beautifully, nor her mountains so temptingly measurable. On my first looking into the telescope, a star of about the 7th magnitude was some minutes of a degree from the moon's dark limb, and its occultation by the moon ap- peared inevitable. The star, however, instead of disappearing the moment the moon's edge came in contact with it, apparently glided on the moon's dark face, as if it had been seen through a trans- 558 APPENDIX. parent moon, or as if the star were between me and the moon. It remained on the moon's disk nearly two seconds of time, and then disappeared. I have seen this apparent projection of a star on the moon's face several times, but from the great brilliancy of the star, this was the most beautiful I ever saw. The cause of this phenomenon is involved in impenetrable mystery.' The following is a representation of the Great Rosse Telescope, along with part of the buildings with which it is connected. In the interior face of the eastern wall a very strong iron arc of about 43 feet radius is firmly fixed, provided with ad- justments, whereby its surface facing the telescope may be set very accurately in the plane of the meridian. On this bar, lines are drawn, the inter- val between any adjoining two of which, corres- ponds to one minute of time on the Equator. The tube and speculum, including the bed on which the speculum rests, weigh about 15 tons. The telescope rests on an universal joint placed on masonry about 6 feet below the ground, and is elevated or depressed by a chain and windlass ; and although it weighs about 15 tons, the instru- ment is raised by two men with great facility. Of course, it is counterpoised in every direction. The observer when at work, stands in one of four gal- leries, the three highest of which are drawn out from the western wall, while the fourth or lowest has for its base an elevating platform, along the horizontal surface of which a gallery slides from wall to wall by a machinery within the observer's reach, but which a child may work. When the telescope is about half an hour east of the meri- dian, the galleries, hanging over the gap between the walls, present to a spectator below an appear- ance somewhat dangerous 5 yet the observer, with EARL OF ROSSE's TELESCOPE. 559 common prudence, is as safe as on the ground, and each of the galleries can be drawn from the wall to the telescope's side so readily, that the observer needs no one else to move it for him. figure 98. The above figure represents only the upper part of the tube of the telescope, at which the observer stands when making his observations. The telescope is at present of the Newtonian con- struction, and consequently, the observer looks into the side of the tube at the upper end of the telescope, but it is proposed to throw aside the plane speculum, and to adapt it to the Front vieiv, on the plan already described (see pp. 806, 313, &c.) so that the observer will sit or stand with his back towards the object, and his face looking down upon the speculum ; and, in this position, he will sometimes be elevated between 50 and 60 feet above the ground. As yet, the telescope has 560 APPENDIX. no equatorial motion, but it very shortly will ; and at no very distant day, clock-work will be connected with it, when the observer will, while observing, be almost as comfortable, as if he were reading at a desk by his fire-side. The following figure shews a section of the machinery connected with this telescope. It exhibits a view of the inside of the eastern wall, with all the machinery as seen in section, A is the mason-work on the ground, B the universal figure 99. joint, which allows the tube to turn in all direc- tions ; C the speculum in its tube ; D the box ; E the eye-piece ; F the moveable pulley ; G the fixed one ; H the chain from the side of the tube ; I the chain from the beam ; K the counterpoise ; L the lever ; M the chain connecting it with the tube ; Z the chain which passes from the tube to the windlass over a pulley on a truss-beam which runs from W to the same situation on the opposite wall — the pulley is not seen. X is a railroad on EARL OF ROSSE'S TELESCOPE. 561 which the speculum is drawn either to or from its box ; part is cut away to show the counterpoise. The dotted line a represents the course of the weight R as the tube rises or falls ; it is a segment of a circle of which the chain I is the radius. The tube is moved from wall to wall by the ratchet and wheel at R ; the wheel is turned by the handle O, and the ratchet is fixed to the circle on the wall. The ladders in front, as shown in the preceding sketch, enable the observer to follow the tube in its ascent to where the galleries on the side wall commence ; these side galleries are three in number, and each can be moved from wall to wall by the observer, after the tube, the motion of which he also accomplishes by means of the handle O. I shall conclude the description of this wonder- ful instrument in the words of Sir James South. c What will be the power of this telescope when it has its Le Mairean form ' [that is, when it is fitted up with the front view] ' it is not easy to divine ; — what nebulae will it resolve into stars ; in what nebulae wiil it not find stars; — how many satellites of Saturn will it show us ; — how many will it indicate as appertaining to Uranus; — how many nebulae never yet seen by mortal eye, will it present to us ; — what spots will it show us on the various planets ; will it tell us what causes the variable brightness of many of the fixed stars ; — will it give us any information as to the constitu- tion of the planetary nebulae ;— will it exhibit to us any satellites encircling them ; will it tell us why the satellites of Jupiter, which generally pass over Jupiter's face as disks nearly of w r hite light, sometimes traverse it as black patches ; — will it add to our knowledge of the physical construction of nebulous stars; — of that mysterious class of 562 APPENDIX. bodies which surround some stars, called, for want of a better name, ? photospheres ;' — will it show the annular nebulae of Lyra, merely as a brilliant luminous ring, or will it exhibit it as thousands of stars arranged in all the symmetry of an ellipse ; will it enable us to comprehend the hitherto in- comprehensible nature and origin of the light of the great nebulae of Orion ; — will it give us, in easily appreciable quantity, the parallax of some of the fixed stars, or will it make sensible to us the parallax of the nebulae themselves; — - finally, having presented to us original portraits of the moon and of the sidereal heavens, such as man has never dared even to anticipate — will it, by Daguerreotype aid, administer to us copies founded upon truth, and enable astronomers of future ages to compare the moon and heavens as they then may be, with the moon and heavens as they were ? Some of these questions will be an- swered affirmatively, others negatively, and that, too, very shortly ; for the noble maker of the noblest instrument ever formed by man, "has cast his bread upon the waters, and will, with God's blessing, find it before many days." HINTS TO AMATEURS. 563 HINTS TO AMATEURS IN ASTRONOMY RESPECTING THE CONSTRUCTION OF TELESCOPES. As there are many among the lower ranks of the community who have a desire to be possessed of a telescope, which will show them some of the prominent features of celestial scenery, but who are unable to purchase a finished instrument at the prices usually charged by Opticians, the fol- lowing hints may perhaps be acceptable to those who are possessed of a mechanical genius. The lenses of an Achromatic telescope may be purchased separately from glass-grinders or Opti- cians, and tubes of a cheap material may be pre- pared by the individual himself for receiving the glasses. The following are the prices at which achromatic object-glasses for astronomical tele- scopes are generally sold. Focal length 30 inches, diameter 2^ inches, from 2 to 3^ guineas. Focal length 42 inches, diameter 2f inches, from 5 to 8 guineas. Focal length 42 inches, diameter 3^ inches, from 12 to 20 guineas. Focal length 42 inches, diameter 3f inches, from 25 to 30 guineas. Eye-pieces, from 10s c 6d. to 18 shillings. The smallest of these lenses, namely that of 2i inches diameter, if truly achromatic, may be made to bear a power of from 80 to 100 times, in clear weather, for celestial objects, which will show Jupiter's moons and belts, Saturn's ring and other celestial phenomena. The tubes may be made either of tin plates, papier mache, or wood. Wood, however, is rather a clumsy article, and it is sometimes liable to warp, yet excellent tubes have sometimes been made of it. Perhaps the cheapest and most convenient of all tubes when 564 APPENDIX. properly made, are those formed of paper. In forming these a wooden roller of the proper dia- meter should be procured, and paper of a proper size, along with book-binder's paste. About three or four layers only of the paper should be pasted at one time, and, when sufficiently dry, it should be smoothed by rubbing it with a smooth stick or ruler ; after which another series of layers should be pasted on, and allowed to dry as before, and so on till the tube has acquired a sufficient degree of strength and firmness. In this way, I have, by means of a few old Newspapers, and similar materials, formed tubes as strong as if they had been made of wood. If several tubes be in- tended to slide into each other, the smallest tube should be made first, and it will serve as a roller for forming the tube into which it is to slide. An achromatic object glass of a shorter focal distance, and a smaller diameter than any of those stated above, may be fitted up as a useful astro- nomical telescope, when a better instrument cannot be procured. In the Pawn-broker's shops in London, and other places, an old achromatic tele- scope, with an object-glass 20 inches focal dis- tance and about 1^ inch diameter, may be pur- chased at a price varying from 15 to 20 shillings. By applying an astronomical eye-piece to such a lens, if a good one, it may bear a power, for celes- tial objects, of 50 or 60 times. If two piano* convex glasses, f inch focal distance, be placed with their convex sides near to each other, they will form an eye-piece which will produce a power on such an object-glass, of above 50 times, which will show Jupiter's belts and satellites, Saturn's ring, the solar spots, and the mountains and cavities of the moon. I have an object-glass of this description which belonged to an old teles- HINTS TO AMATEURS. 565 cope, which cost me only 12 shillings, and with which I formerly made some useful astronomical observations. It was afterwards used as the teles- cope of a small Equatorial instrument, and, with it, I was enabled to perceive stars of the first and second magnitude, and the planets Venus, Jupiter, and Mars, in the day-time. But, should such a glass be still beyond the reach of the astronomical amateur, let him not altogether despair. He may purchase a single lens, 3 feet focal distance, for about a couple of shillings, and by applying an eye-glass of 1 inch focus, which may be procured for a shilling, he will obtain a power of 36 times, which is a higher power than Galileo was able to apply to his best telescope ; and consequently, with such an instru- ment, he will be enabled to perceive all the celes- tial objects which that celebrated astronomer first described^ and which excited so much wonder, at that period, in the learned world. But, whatever kind of telescope may be used, it is essentially requisite that it be placed on a firm stand in all celestial observations : and any common mechanic can easily form such a stand at a trifling expence. There is a certain optical illusion to which most persons are subject, in the first use of telescopes, especially when applied to the celestial bodies, on which it may not be improper to make a remark. The illusion to which 1 allude is this — that they are apt to imagine, the telescope does not magnify nearly so much as it really does. They are apt to complain of the small appearance which Jupiter and Saturn, for example, present when magnified 160 or 200 times. With such powers they are apt to imagine, that these bodies do not appear so large as the moon to the naked eye. Yet it can be proved that Jupiter, when nearest the earth, 2 C 566 APPENDIX. viewed with such a power, appears about 5 times the diameter of the full moon, and 25 times larger in surface. This appears from the following cal- culation. Jupiter, when in opposition, or nearest, the Earth, presents a diameter of 47" : the mean apparent diameter of the moon is about 31'. Multiply the diameter of Jupiter by the magnify- ing power, 200, the product is 9400'' or 156' or 2° 36', which, divided by 31', the moon's diameter, produces a quotient of 5, showing that this planet with such a power appears five times larger in diameter than the full moon to the naked eye, and consequently 25 times larger in surface. Were a power of only 50 times applied to Jupiter, when nearest the earth, that planet would appear some- what larger than the full moon. For 47 y/ multi- plied by 50 gives 2350" or 89', which is 8' more than the diameter of the moon. Yet with such a power most persons would imagine that the planet does not appear one third of the size of the full moon. The principal mode by which a person may be experimentally convinced of the fallacy to which I allude is the following : — At a time when Jupiter happens to be within a few degrees of the moon, let the planet be viewed through the telescope with the one eye, and the magnified image of the planet be brought into contact with the moon as seen with the other eye — the one eye looking at the moon, and the other viewing the magnified image of Jupiter through the telescope when brought into apparent contact with the moon — then it will be perceived, that with a magnifying power of 50 the image of Jupiter will completely cover the moon as seen by the naked eye ; — and with a power of 200 — when the moon is made to appear in the centre of the magnified image of the HINTS TO AMATEURS. 567 planet—it will be seen that Jupiter forms a large and broad circle around the moon, appearing at least 5 times greater than the diameter of the moon. This experiment may be varied as follows : Suppose a person to view the moon through a small telescope or opera-glass, magnifying three times, he will be apt to imagine, at first sight, that she is not in the least magnified, but rather some- what diminished. But let him bring the image as seen in the telescope in contact with the moon as seen with the naked eye, and he will plainly per- ceive the magnifying power, by the size of the image. It may be difficult in the first instance to look, at the same time, at the magnified image and the real object, but a few trials will render it easy. THE END. L. SEELEY PRINTER, THAMES DITTO*. ERRATA. Page 72 line 4 for EI, read FI. — depend, read depends. 103 30 135 10 — refacting, read refracting. 136 10 — colour, read colours. 146 27 — G, read C. 146 32 — prisms, read prism. 153 35 — 28o 3', read 28<> 10' — some, read since, dele that. 165 32 165 33 166 5 for these, read their. 166 21 — those, read their. 178 32 — variety, read vanity. 187 7 — in, read an. 187 11 — (p. 103.), read (p. 72.) 189 30 — lens, read lenses. 199 31 — punice, read pumice. 216 10 — nine, read ten. 236 — 12, 13 — ■ " more distant from," read 337 27 — 1, read 1|.