ASTRONOMICAL PROGRESS AND PHENOMENA. 



51 



1815, expected now at any time, is the most 

 interesting of those unverified by return. When 

 it appears and it may be delayed until 1887 

 it will add another to the list of known peri- 

 odics, and increase to three the number of 

 those of long - period, viz., Halley's, Pons- 

 Brooks's, and Olbers's, the average period of 

 the three being about 73 years . The next 

 shorter is Tuttle's, of 13f years, and the short- 

 est of all is Encke's of 3 - 3 years. 



Celestial Photography. The interest attaching 

 to this subject, which has had a trial long 

 enough to establish its merits, is unabated. 

 Since the great and unexpected success of the 

 brothers Henry, in photographing stars and 

 nebulae invisible with the visual telescope, 

 other astronomers have entered upon the 

 work, and still others, armed with telescopes 

 larger than that used by those gentlemen, are 

 preparing to follow. Its scope is wide, and 

 promises rich return. 



The detection of a nebula by the telescope, or 

 by photography, in this age of astronomical ac- 

 tivity, is an unimportant event, but the finding 

 of one in the Pleiades, in close contact with one 

 of the principal stars of the group, which the 

 closest scrutiny of the great telescopes of the 

 world had failed to reveal, is certainly wonder- 

 ful, and raises the hope that by this method 

 many of these bodies, otherwise unknowable, 

 may be detected. The priority of this discovery, 

 however, really belongs to Prof. E. C. Picker- 

 ing, of Harvard College Observatory, as it was 

 on his photographic plate that it was first de- 

 picted, though he then as, at its first appear- 

 ance, did afterward the Henrys ascribed it to 

 an accidental stain, or to the intrusion of false 

 light. However, after three exposures, on as 

 many different occasions, when the intruding 

 blur presented itself each time in the same 

 place and with similar contour, they were 

 forced to the conclusion that it was the deline- 

 ation of a real object. This conclusion has 

 been visually verified by Prof. Struve with the 

 30- inch refractor, and by others possessing 

 large telescopes. The shape of this nebula is 

 unique. It appears to be spiral, and is almost 

 in contact with Maia, one of the six naked-eye 

 stars of the Pleiades. Merope, also of this 

 constellation, is surrounded with a nebula, 

 which extends to a considerable distance south 

 of the star. The exceedingly faint, nebulous 

 body between Merope and Alcyone, discovered 

 by Mr. Commons, has also been photographed, 

 and its existence, so long doubted, is proved. 

 At the November meeting of the Royal Astro- 

 nomical Society, Isaac Roberts read a paper on 

 photographing the Pleiades, wherein he says, 

 " After an exposure of 89 minutes, the plate 

 showed clearly that the stars Alcyone, Maia, 

 Electra, and Merope, are surrounded by nebu- 

 lae." On another plate, exposed for three 

 hours, the nebulosity is seen to extend in 

 streamers and in fleecy masses nearly filling the 

 spaces between the stars named, and in places 

 far beyond. He expresses the opinion that the 



entire cluster is situated in one vast nebula. 

 Hyperion, the last-discovered and the faintest 

 satellite of Saturn, and the satellite of Nep- 

 tune in any part of its orbit presented to us, 

 are somewhat easy subjects for the celestial 

 photographer. The Henry brothers have given 

 to the Paris Academy a single plate, of an 

 hour's exposure with their 13-inch reflector, 

 on which may be counted five thousand stars 

 between the sixth and fifteenth magnitudes. 

 The plate embraces 2 15' in right ascension, 

 and 3 in declination of a region near the Milky 

 Way, which is rich in stars. Mr. Roberts has 

 succeeded in greatly reducing the time of ex- 

 posure, having photographed with his 20-inch 

 reflector small stars in fifteen minutes, while 

 Admiral Mouchez recommends an exposure of 

 from one to three hours, which is largely in 

 accord with the practice of the brothers Henry. 

 One grand desideratum in this unspeakably 

 useful adjunct to astronomical science remains 

 to be achieved, viz., the invention of a process 

 for printing directly from the photographic 

 plates, thus absolutely perfecting cartography. 

 Besides the construction of star-charts, pho- 

 tography opens up an avenue to the discovery 

 of satellites and asteroids, and, not impossibly, 

 a trans-Neptunian planet. That the stars are 

 stationary is well known, and consequently 

 their images on the photographic plates will be 

 points ; but the impression of all moving ob- 

 jects will be lines, whose length will depend 

 upon the time of exposure of the plate and the 

 velocity of the object. Should, therefore, the 

 photographer find a short line recorded on his 

 plate, out of the immediate neighborhood of a 

 planet, he may be quite sure that an asteroid 

 (perhaps a new one) has impressed its image 

 there. It is doubtful if the motion of a planet 

 beyond Neptune would be rapid enough to pro- 

 duce a line unless the exposure were greatly 

 prolonged. 



Mars may have more moons than Deimos 

 and Phobus; Saturn is supposed to possess 

 three or four undiscovered satellites ; and so, 

 also, though too faint for the visual telescope, 

 photography may yet reveal the existence of 

 several more satellites about Uranus and Nep- 

 tune. It is worthy of notice that M. Gothard's 

 attempt to photograph the Swan nebula was a 

 failure, not a trace being visible; whereas 

 every star, even the faintest, was clearly and 

 accurately revealed. This is remarkable, as 

 the nebula is one of the brightest. On the 

 other hand, the great nebula in Andromeda, 

 and the ring nebula in Lyra, show very strong- 

 ly with shorter exposure. 



A lively controversy has been carried on be- 

 tween Mr. Huggins and Prof. W. H. Pickering, 

 as to the reality of the faint markings found 

 by the former on the plates after his attempts 

 to photograph the corona in full sunshine. 

 The latter, unable to obtain any coronal indi- 

 cations, expressed doubts of the genuineness 

 of those produced by Mr. Huggins. No dis- 

 cussion of the merits of the case can be at- 



