Through the Telescope

Appendix Ii.: List of Objects for the Telescope 278

INDEX 285

LIST OF ILLUSTRATIONS

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I. The 40-inch Refractor of the Yerkes Observatory [Frontispiece

II. Six-inch Photo-Visual Refractor, equatorially mounted 30

III. Twenty-inch Reflector, Stanmore Observatory 36

IV. Telescope House and 8-1/2-inch 'With' Reflector 38

V. The Sun, February 3, 1905. Royal Observatory, Greenwich 48

VI. Photograph of Bridged Sunspot (Janssen). Knowledge, February, 1890 50

VII. Solar Surface with Faculæ. Yerkes Observatory 60

VIII. Coronal Streamers: Eclipse of 1898. From Photographs by Mrs. Maunder 70

IX. The Chromosphere and Prominences, April 11, 1894. Photographed by M. H. Deslandres 74

X. Venus. H. MacEwen. Five-inch Refractor 94

XI. The Moon, April 5, 1900. Paris Observatory 102

XII. The Moon, November 13, 1902. Paris Observatory 108

XIII. The Moon, September 12, 1903. Paris Observatory 110

XIV. Region of Maginus: Overlapping Craters. Paris Observatory 112

XV. Clavius, Tycho, and Mare Nubium. Yerkes Observatory 114

XVI. Region of Theophilus and Altai Mountains. Yerkes Observatory 116

XVII. Apennines, Alps, and Caucasus. Paris Observatory 118

XVIII. Chart of the Moon. Nasmyth and Carpenter } } 124 XIX. Key to Chart of Moon. Nasmyth and Carpenter }

XX. Mars: Drawing 1, January 30, 1899--12 hours. Drawing 2, April 22, 1903--10 hours 134

XXI. Chart of Mars. Memoirs of the British Astronomical Association, Vol. XI., Part III., Plate VI. 138

XXII. Jupiter, January 6, 1906--8 hours 20 minutes. Instrument, 9-1/4-inch Reflector 158

XXIII. Jupiter, February 17, 1906. J. Baikie, 18-inch Reflector 166

XXIV. Saturn, July 2, 1894. E. E. Barnard, 36-inch Equatorial 172

XXV. Great Comet. Photographed May 5, 1901, with the 13-inch Astrographic Refractor of the Royal Observatory, Cape of Good Hope 210

XXVI. Photographs of Swift's Comet. By Professor E. E. Barnard 220

XXVII. Region of the Milky Way in Sagittarius, showing a Double Black Aperture. Photographed by Professor E. E. Barnard 232

XXVIII. Irregular Star Clusters. Photographed by E. E. Barnard 256

XXIX. Cluster M. 13 Herculis. Photographed by Mr. W. E. Wilson 258

XXX. Photograph of the Orion Nebula (W. H. Pickering) 262

XXXI. Photographs of Spiral Nebulæ. By Dr. Max Wolf 264

XXXII. Photograph of Whirlpool Nebula (M. 51). Taken by Mr. W. E. Wilson, March 6, 1897 266

LIST OF ILLUSTRATIONS

PRINTED IN THE TEXT

FIG. PAGE

1. Principle of Galilean Telescope 3 2. Principle of Common Refractor 3 3. Dorpat Refractor 7 4. Thirty-inch Refractor, Pulkowa Observatory 9 5. Principle of Newtonian Reflector 11 6. Lord Rosse's Telescope 12 7. Herschel's 4-foot Reflector 13 8. Star--Correct and Incorrect Adjustment 21 9. Small Telescope on Pillar and Claw Stand 26 10. Telescope on Tripod, with Finder and Slow Motions 27 11. Equatorial Mounting for Small Telescope 29 12. Eight-inch Refractor on Equatorial Mounting 32 13. Four-foot Reflector, equatorially mounted 36 14. Drawing of Sunspot 52 15. " " 53 16. " " 56 17. " " 57 18. " " 58 19. Eclipses of the Sun and Moon 69 20. Mercury as a Morning Star. W. F. Denning, 10-inch Reflector 84 21. The Tides 101 22. Lunar Craters 105 23. " " 118 24. Mars 146 25. Jupiter 157 26. Saturn 183

THROUGH THE TELESCOPE

CHAPTER I. The Telescope--Historical

THE TELESCOPE--HISTORICAL

The claim of priority in the invention of this wonderful instrument, which has so enlarged our ideas of the scale and variety of the universe, has been warmly asserted on behalf of a number of individuals. Holland maintains the rights of Jansen, Lippershey, and Metius; while our own country produces evidence that Roger Bacon had, in the thirteenth century, 'arrived at theoretical proof of the possibility of constructing a telescope and a microscope' and that Leonard Digges 'had a method of discovering, by perspective glasses set at due angles, all objects pretty far distant that the sun shone on, which lay in the country round about.'

All these claims, however, whether well or ill founded, are very little to the point. The man to whom the human race owes a debt of gratitude in connection with any great invention is not necessarily he who, perhaps by mere accident, may stumble on the principle of it, but he who takes up the raw material of the invention and shows the full powers and possibilities which are latent in it. In the present case there is one such man to whom, beyond all question, we owe the telescope as a practical astronomical instrument, and that man is Galileo Galilei. He himself admits that it was only after hearing, in 1609, that a Dutchman had succeeded in making such an instrument, that he set himself to investigate the matter, and produced telescopes ranging from one magnifying but three diameters up to the one with a power of thirty-three with which he made his famous discoveries; but this fact cannot deprive the great Italian of the credit which is undoubtedly his due. Others may have anticipated him in theory, or even to a small extent in practice, but Galileo first gave to the world the telescope as an instrument of real value in research.

The telescope with which he made his great discoveries was constructed on a principle which, except in the case of binoculars, is now discarded. It consisted of a double convex lens converging the rays of light from a distant object, and of a double concave lens, intercepting the convergent rays before they reach a focus, and rendering them parallel again (Fig. 1). His largest instrument, as already mentioned, had a power of only thirty-three diameters, and the field of view was very small. A more powerful one can now be obtained for a few shillings, or constructed, one might almost say, for a few pence; yet, as Proctor has observed: 'If we regard the absolute importance of the discoveries effected by different telescopes, few, perhaps, will rank higher than the little tube now lying in the Tribune of Galileo at Florence.'

Galileo's first discoveries with this instrument were made in 1610, and it was not till nearly half a century later that any great improvement in telescopic construction was effected. In the middle of the seventeenth century Scheiner and Huygens made telescopes on the principle, suggested by Kepler, of using two double convex lenses instead of a convex and a concave, and the modern refracting telescope is still constructed on essentially the same principle, though, of course, with many minor modifications (Fig. 2).

The latter part of the seventeenth century witnessed the introduction of telescopes on this principle of the most amazing length, the increase in length being designed to minimize the imperfections which a simple lens exhibits both in definition and in colour. Huygens constructed one such telescope of 123 feet focal length, which he presented to the Royal Society of London; Cassini, at Paris, used instruments of 100 and 136 feet; while Bradley, in 1722, measured the diameter of Venus with a glass whose focal length was 212-1/4 feet. Auzout is said to have made glasses of lengths varying from 300 to 600 feet, but, as might have been expected, there is no record of any useful observations having ever been made with these monstrosities. Of course, these instruments differed widely from the compact and handy telescopes with which we are now familiar. They were entirely without tubes. The object-glass was fastened to a tall pole or to some high building, and was painfully manuvred into line with the eye-piece, which was placed on a support near the ground, by means of an arrangement of cords. The difficulties of observation with these unwieldy monsters must have been of the most exasperating type, while their magnifying power did not exceed that of an ordinary modern achromatic of, perhaps, 36 inches focal length. Cassini, for instance, seems never to have gone beyond a power of 150 diameters, which might be quite usefully employed on a good modern 3-inch refractor in good air. Yet with such tools he was able to discover four of the satellites of Saturn and that division in Saturn's ring which still bears his name. Such facts speak volumes for the quality of the observer. Those who are the most accustomed to use the almost perfect products of modern optical skill will have the best conception of, and the profoundest admiration for, the limitless patience and the wonderful ability which enabled him to achieve such results with the very imperfect means at his disposal.

The clumsiness and unmanageableness of these aerial telescopes quickly reached a point which made it evident that nothing more was to be expected of them; and attempts were made to find a method of combining lenses, which might result in an instrument capable of bearing equal or greater magnifying powers on a much shorter length. The chief hindrance to the efficiency of the refracting telescope lies in the fact that the rays of different colours which collectively compose white light cannot be brought to one focus by any single lens. The red rays, for example, have a different focal length from the blue, and so any lens which brings the one set to a focus leaves a fringe of the other outstanding around any bright object.

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