176 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



was stated by him at the time to be quite 

 large and in brightness equal to that of a 

 beautiful telescopic comet. For this nebula 

 M. D'Arrest looked repeatedly on clear nights 

 in August, 1862, with his fine refractor, but 

 saw no trace of it. M. Schmidt, of Athens, 

 learning of the circumstance, announced (Sept. 

 20, 1862) that the nebula could not have be- 

 come visible more than a very short time be- 

 fore the date of its discovery by Tempel, or 

 that at least it must have been very faint for 

 some time previous ; as he had since 1841 made 

 many and most careful observations of the stars 

 in the Pleiades, estimating several hundred 

 times their apparent magnitude; and he had 

 not seen the nebula in question until Feb. 5, 

 1861, when it appeared large, pale, and of no 

 definite form. After that date, he observed it 

 several times, the last on March 26, 1862, when 

 it was easily visible. M. Auwers suggests a 

 doubt in regard to the variability of this nebula, 

 thinking that its apparent disappearance may 

 perhaps be accounted for by the greater ease 

 with which faint, ill-defined objects are detect- 

 ed with small than with large telescopes. 



A third instance of variability is that of a 

 nebula not far from the two already named, 

 observed at Bonn, and afterward at Cambridge, 

 and which is now almost invisible. These being 

 the only three nebulse whose variableness ap- 

 pears established, it is a curious fact that they 

 are all situated in the same celestial region, in 

 the vicinity of the Pleiades. 



A few years since, the resolution, by aid of 

 Lord Rosse's powerful telescope, of one or 

 more of those singular, fixed, and hazily lumin- 

 ous patches in remote space into congeries of 

 actually separated and individual stars, a res- 

 olution confirmed subsequently by other in- 

 struments and on other nebula?, proved suf- 

 ficient to shake the "nebular hypothesis" of 

 Laplace to its foundation ; until certain physical 

 explorers . bethought themselves that Laplace's 

 diffused or nebulous condition of primitive 

 matter could still be a fact, as so many of the 

 phenomena of our planetary system and of the 

 geological constitution of the earth seemed to 

 require, even though all the existing nebula) of 

 the heavens should successively prove to be in 

 reality clusters of stars. Now, however, as- 

 suming that the three nebulae referred to have 

 actually disappeared or faded, new and strange 

 questions are raised; and the facts seem likely 

 to prove as irreconcilable with the doctrine 

 that all nebulas are clusters, as the resolution 

 of some of them was with the hypothesis of 

 . Laplace. Is such disappearance due to the 

 interposition of some vast, dense, and invisible 

 body, but which, not being wholly opaque, does 

 not. suffice to hide observed stars? Or, in the 

 first of the cases above given, did the star illu- 

 minate the nebula, which in that case could 

 not have been itself a cluster? and did the 

 visibility of the nebula diminish with the de- 

 creased light of the star? Several variable 

 stars have, in fact, been detected in the region 



of the great nebula of Orion ; and in 1860, a 

 star suddenly shone out in the middle of the 

 well-known nebula, Messier 80, in Scorpio, 

 and vanished again in a few days. Up to 

 the present time, however, the subject remains 

 one of considerable mystery. 



Stars. Some facts relative to variable stars 

 have necessarily appeared in the preceding 

 section. The star which has become known as 

 the "companion of Sirius," was discovered 

 Jan. 31, 1862, by Mr. A. Clark, of Cambridge, 

 Mass., with his new achromatic object-glass of 

 18^ inches aperture. Sirius being in our lati- 

 tude low, and the new star being in contrast 

 with its extreme brightness but faint, it is at 

 first difficult to catch sight of the latter. It 

 remains for the present a question, whether this 

 forms with Sirius a binary star ; and whether, 

 accordingly, it is the hitherto invisible body 

 which has disturbed the motions of Sirius, and 

 the existence of which had been surmised by 

 Bessel and Peters ; as also whether it is merely 

 an opaque body illuminated by the brighter 

 star, or itself, though large, only feebly self- 

 luminous. Mr. L. M. Rutherford, of New 

 York, on the same evening on which he first 

 learned of the discovery of the companion to 

 Sirius (March 8), readily detected it with his 

 equatorial telescope of 11 J inches aperture and 

 14 feet focal distance, the workmanship of Mr. 

 Fitz, of the same place. Though admitting 

 that the new star may be a variable one, he 

 judges that the difficulty of seeing it most likely 

 arises solely from its nearness to an object so 

 bright as Sirius. He also finds in the facts of 

 its discovery now given, a probable illustration 

 of such a principle as that it requires a far 

 higher telescopic power to make a new dis- 

 covery than it does to redetect the same object 

 when its existence has become known to the 

 observer. 



Just as, in 1851, Dr. Peters found that the 

 irregularities in position of Sirius could be ex- 

 plained by motion of the bright star about an 

 invisible one (since found), so recently M. Au- 

 wers find* that Procyon moves about a dark 

 star, and in an orbit whose plane is that of the 

 visible heavens, the distance of the companion 

 being about 1J". He concludes that the mass 

 of the dark "body equals % that of the sun, or 

 at least 100 times that of Jupiter. Thus Bes- 

 sel's idea of the existence of dark stars in the 

 heavens, controlling the motions of some of 

 those visible to the eye, an idea ridiculed by 

 Humboldt, appears now to be established in 

 one instance, if not in two. 



Refraction. Prof. Challis read before the 

 meeting of the British Association, 1862, a 

 paper on the "Augmentation of the Apparent 

 Diameter of a Body, by its Atmospheric Re- 

 fraction." For reasons which he gave in a 

 previous discussion on the earth's atmosphere, 

 he assumed that, generally, the atmospheres 

 of the bodies possessing them have definite 

 boundaries, at which their densities have very 

 small but finite values. Two cases of refraction 



