Baikie Through the Telescope

1. Auflage 2017
ISBN: 978-3-7364-2070-0
Verlag: anboco
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 423 Seiten

The main object of the following chapters is to give a brief and simple description of the most important and interesting facts concerning the heavenly bodies, and to suggest to the general reader how much of the ground thus covered lies open to his personal survey on very easy conditions. Many people who are more or less interested in astronomy are deterred from making practical acquaintance with the wonders of the heavens by the idea that these are only disclosed to the possessors of large and costly instruments. In reality there is probably no science which offers to those whose opportunities and means of observation are restricted greater stores of knowledge and pleasure than astronomy; and the possibility of that quickening of interest which can only be gained by practical study is, in these days, denied to very few indeed. Accordingly, I have endeavoured, while recounting the great triumphs of astronomical discovery, to give some practical help to those who are inclined to the study of the heavens, but do not know how to begin. My excuse for venturing on such a task must be that, in the course of nearly twenty years [pg viii] of observation with telescopes of all sorts and sizes, I have made most of the mistakes against which others need to be warned. The book has no pretensions to being a complete manual; it is merely descriptive of things seen and learned. Nor has it any claim to originality. On the contrary, one of its chief purposes has been to gather into short compass the results of the work of others. I have therefore to acknowledge my indebtedness to other writers, and notably to Miss Agnes Clerke, Professor Young, Professor Newcomb, the late Rev. T. W. Webb, and Mr. W. F. Denning. I have also found much help in the Monthly Notices and Memoirs of the Royal Astronomical Society, and the Journal and Memoirs of the British Astronomical Association.

Autoren/Hrsg.

Baikie, James

Leseproben

CHAPTER I

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.'

FIG. 1.—PRINCIPLE OF GALILEAN TELESCOPE.

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).

FIG. 2.—PRINCIPLE OF COMMON REFRACTOR.

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 212¼ 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 manœuvred 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.

In 1729 Mr. Chester Moor Hall discovered a means of conquering this difficulty, but his results were not followed up, and it was left for the optician John Dollond to rediscover the principle some twenty-five years later. By making the object-glass of the telescope double, the one lens being of crown and the other of flint glass, he succeeded in obtaining a telescope which gave a virtually colourless image.

This great discovery of the achromatic form of construction at once revolutionized the art of telescope-making. It was found that instruments of not more than 5 feet focal length could be constructed, which infinitely surpassed in efficiency, as well as in handiness, the cumbrous tools which Cassini had used; and Dollond's 5-foot achromatics, generally with object-glasses of 3¾ inches diameter, represented for a considerable time the acme of optical excellence. Since the time of Dollond, the record of the achromatic refractor has been one of continual, and, latterly, of very rapid progress. For a time much hindrance was experienced from the fact that it proved exceedingly difficult to obtain glass discs of any size whose purity and uniformity were sufficient to enable them to pass the stringent test of optical performance. In the latter part of the eighteenth century, a 6-inch glass was considered with feelings of admiration, somewhat similar to those with which we regard the Yerkes 40-inch to-day; and when, in 1823, the Dorpat refractor of 96/10 inches was mounted (Fig. 3), the astronomical world seemed to have the idea that something very like finality had been reached. The Dorpat telescope proved, however, to be only a milestone on the path of progress. Before very long it was surpassed by a glass of 12 inches diameter, which Sir James South obtained from Cauchoix of Paris, and which is now mounted in the Dunsink Observatory, Dublin. This, in its turn, had to give place to the fine instruments of 14·9 inches which were figured by Merz of Munich for the Pulkowa and Cambridge (U.S.A.) Observatories; and then there came a pause of a few years, which was broken by Alvan Clark's completion of an 18½-inch, an instrument which earned its diploma, before ever it left the workshop of its constructor, by the discovery of the companion to Sirius.

FIG. 3.—DORPAT REFRACTOR.

The next step was made on our side of the Atlantic, and proved to be a long and notable one, in a sense definitely marking out the boundary line of the modern era of giant refractors. This was the completion, by Thomas Cooke, of York, of a 25-inch instrument for the late Mr. Newall. It did not retain for long its pride of place. The palm was speedily taken back to America by Alvan Clark's construction of the 26-inch of the Washington Naval Observatory, with which Professor Asaph Hall discovered in 1877 the two satellites of Mars. Then came Grubb's 27-inch for Vienna; the pair of 30-inch instruments, by Clark and Henry respectively, for Pulkowa (Fig. 4) and Nice; and at last the instrument which has for a number of years been regarded as the finest example of optical skill in the world, the 36-inch Clark refractor of the Lick Observatory, California. Placed at an elevation of over 4,000 feet, and in a climate exceptionally well suited for astronomical work, this fine instrument has had the advantage of being handled by a very remarkable succession of brilliant observers, and has, since its completion, been looked to as a sort of court of final appeal in disputed questions. But America has not been satisfied even with such an instrument, and the 40-inch Clark refractor of the Yerkes Observatory is at present the...