Astronomy of To-day

doubt soon noticed that, with the exception of the moon and those brilliant wandering objects known to us as the planets, the individual stars in the heaven remained apparently fixed with re

hey imagined these to be rough representations of ancient heroes and fabled beasts, whom they supposed to have been elevated to the heavens as a reward for great deeds done upon the earth. We know these groupings of stars to-day under the name of the Constellatio

speak, in the celestial vault. They regulated their hours, their days, and their nights by the changing positions of the sun, the moon, and the stars; and recognised the periods of seed-time an

espect to each other; and, from observations thus made, they constructed charts of the stars. The earliest complete survey of this kind, of which we have a record, is the gre

oved; but, had they even been perfect, they would have been utterly inadequate to reveal those minute displacements, from which we have learned the actual distance of the nearest of the celestial orbs. From the early times, therefore,

eoples embraced the science in earnest; Ulugh Beigh, grandson of the famous Tamerlane, founding, for instance, a great observatory at Samarcand in Central Asia. The Mongol emperors of India also established large astronomical instruments in the chie

nvention of the Telescope. It is to this instrument, indeed, that we are indebted for our knowledge of the actual scale of the celestial distances. It penetrated the depths of space; it b

struction of an instrument based upon this principle. The result was what was called an "optick tube," which magnified distant objects some few times. It was not much larger than what we nowadays contemptuously refer to as a "spy-glass," yet its employment upon the leading celestial objects instantly sent astronomical science onward with a bound. In rapid succession Galileo announced world-moving discoveries; large spots upon the face of the sun; crater-like mountains

hich they called the planets, were, after all, globes of a size comparable with the earth, and peopled perchance with sentient beings. Even to us, who have been accustomed since our early youth to such an idea, it still re

we native to that s

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ease in the size of object-glasses. A method was, however, discovered of diminishing this colouration, or chromatic aberration as it is called from the Greek word χρ?μα (chroma), which means colour, viz. by making telescopes of great length and only a few inches in width. But the remedy was, in a way, worse than the disease; for telescopes thus became of such huge proportions as to be too unwieldy for use. Attempts were made to evade this unwieldiness by constructing them with skeleton tubes (see Plate II., p. 110), or, indeed, even without tubes at all; the object-glass in the tubeless or "aerial" telescope being fixed at the top of a high post, and the eye-piece, that smal

r they employed simple tubes, blackened on the inside, in order to obtain a clearer view of distant objects. It is sa

eat Telescop

constructed by the celebrated seventeenth century astronomer, H

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ociety of London by the Dutch astronomer Huyghens. Hevelius of Danzig constructed a skeleton one of 150 feet in length (see Plate II., p. 110). Bradley used a t

of a highly polished concave metallic mirror, or speculum, as it was called. It is to Sir Isaac Newton that the world is indebted for the reflecting telescope in its best form. That philosopher had set himself to investigate the causes of the rainbow-like, or prismatic colours whic

, be pointed directly towards the mirror, for the observer would then have to place his head right in the way of the light coming from the celestial object, and would thus, of course, cut it off. In order to obviate this difficulty, the following device was employed by Newton in his telescope, of which he constructed h

"flat," as the small mirror is called, had the further effect of excluding from the great mirror a certain proportion of light. But the reflector had the advantage, on the other hand, of costing less to make than the refractor, as it was not necessary to procure flawless glass for the purpose. A disc of a certain metalli

ubeless, or "A

in the Opera Varia

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pes are pointed in the same direction; that is to say, the ra

e reflected rays, and reflected them back again through the hole in question into the eye-piece, which was fixed just behind (see Fig. 8, p. 113, "Gregorian"). The Gregorian had thus the sentimental advantage of being pointed directly at the object. The hole in the big mirror did not cause any loss of light, for the central portion in which it was made was anyway unable to receive light through the small mirror being directly in front of

slight slant, so that the light coming from the object is reflected straight into the eye-piece, which is fixed facing it in the side of the tube (see Fig. 8, p. 113, "Herschelian"). This telescope has an advantage over the other forms of reflector through the saving of light c

. At first, however, he rejected it; but in 1786 he seems to have found that it suited the huge instruments which he was then making. Herschel's largest telescope, constructed in 1789, was about f

f different kinds of glass, i.e. crown glass and flint glass, the troublesome colour effects could be, to a very great extent, removed. Hall's investigations appear to have been rather of an academic nature; and, although he is believed to have constructed a small telescope upon these lines, yet he seems to

purposes (see Fig. 8, p. 113, "Achromatic"). The expense of making large instruments of this type is very great, for, in the object-glass alone, no less tha

been made, the object-glasses of which were composed of various transparent liquids placed between thin lenses; but leakages,

, covered with a thin coating of silver and highly polished, have been used since about 1870 instead of metal mirrors. They are very much lighter in weight and cheaper to make than the old specula; and though

nd which as bottom. Such an inversion would, of course, be most inconvenient when viewing terrestrial objects. In order to observe the latter we therefore employ what is called a terrestrial telescope, which is merely a refractor with some extra lenses added in the eye portion for the purpos

escope must be specially borne in mind with rega

United States, i.e. the forty-inch of the Yerkes Observatory (see Plate IV., p. 118), and the thirty-six inch of the Lick. The object-glasses of these and of the thirty-inch telescope of the Observatory of Pulkowa, in Russia, were

Dr. Common, of Ealing, and the other by the American astronomer, Professor G.W. Ritchey. The latter of these is installed in the Solar Observatory belonging to Carnegie Institution of Washington, which is situated on Mount Wilson in California. The former is now at the Harvard College Observatory, and i

our time, it has nevertheless keen competitors in Sir Howard Grubb, of Dublin, and such well-known firms as Cooke of York and Steinheil of Mu

id. Each kind of instrument has, indeed, its special advantages; though perhaps

e Great Yerk

est refracting telescope in the world. Diameter of object-glass, 40 inches; length of telescope, about 60 feet. The object-glass was made by the

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t those portions of the sky which are to be observed, into the tube of a telescope kept fixed. Such too are the "Equatorial Coudé" of the late M. Loewy, Director of the Paris Observatory, and the "Sheepshanks Telescope" of the Observatory of Cambridge, in which a telescope is separated into two portions, the eye-piece portion being fixed upon a downward slant, and the object-glass portion jointed to it at an angle an

ays of light. It will be remembered that we made allusion to loss of light in this way, when pointing out the advantage in lig

large as nine feet in diameter might now be made. As regards refractors there are several strong reasons against augmenting their proportions. First of all comes the great cost. Secondly, since the lenses are held in position mere

nsteadiness of the air will be increasingly magnified. And further, the larger the tubes become, the mor

w feeling rather than seeing our way about the universe. Up to the present, for instance, we have not the slightest proof that life exists elsewhere than upon our earth. But who shall say that the twentieth century has not that in store for us, by which the presence of life in other orbs may be perceived through some for

hirteenth century, describes it very clearly; and several writers of the sixteenth century have also dealt with the idea. Even Lippershey's claims to a practical solut