488 PHOTOGRAPHY IN THE SERVICE OF ASTRONOMY. 



lices to recall the mysterious rectilinear cauals which Messrs. Schiapa- 

 relli and Perrotiu have pointed out on the surface of Mars. In refer- 

 ence to sketches sent from Nice, Mr. Jansseu has made the statement 

 that he was urgently seeking to obtain, with the aid of our great in- 

 strument, photographic images sufficiently i^erfect to replace these de- 

 signs. " I know," says he, " that when i^henomena are concerned, as 

 delicate as those which have been discovered at Milan and at Nice, 

 photography unhappily can no longer strive with sight; but it is nec- 

 essary to enter resolutely into this path, to prepare for the future." If 

 in place of designs, we could have photographic images even less de- 

 tailed, we wduld already derive from them, in regard to the changes 

 which have occurred on the surface of Mars, notions incomparably 

 more certain than those with which we are obliged to content ourselves. 



In order to judge of the difficulty which is experienced in confronting 

 designs of a diverse origin, we have only to pass in review the long 

 series of sketches of the nebula of Orion, made through two centuries 

 by observers such as Huyghens, Mairan, Messier, De Vico, Lamont, 

 J. Herschel, Lassell, O. Struve, the two Bonds, Lord Eosse, Father 

 Secchi. In 1882, Mr. Holden devoted to this nebula a monograph where 

 he gives the results of his own observations, at the same time also 

 copies of the more celebrated drawings of this famous object. These 

 copies, notwithstanding they are very imperfect, cause no less the 

 growth of the conviction that it would be rash to invoke unbiased tes- 

 timony to prove whether it is true that the appearance varies thus from 

 one sketch to another. Mr. Holden has also reproduced a jihotograph 

 of the nebula, obtained by H. Draper in 1882. It has been since photo- 

 graphed by Mr. Common, by Mr. Roberts, and by other astronomers. 

 Messrs. Holden and Struve think that the contour of the nebula of 

 Orion has not changed since it has been observed with care, but that 

 the brightness of certain portions has undergone variations which con- 

 tinue to re-produce themselves before our eyes. Photography alone 

 will be able, some day, to give us in this regard a complete certainty, 

 as it j)ermits us already to watch rapid changes in comets, in outlines 

 so variable. 



Meanwhile, it has already called up from the bosom of the darkness 

 unknown nebulae which the human eye had not perceived. Upon a 

 plate of the Pleiades, which Messrs. Henry had obtained November 16, 

 1885, the star Maia was shown accompanied with a small cometarj' tail, 

 very brilliant ; it was discovered that this was a nebulosity. It has been 

 found that it also impressed itself upon a negative of Mr. Pickering, 

 which dated November 3; but in America it had been taken for an ac- 

 cidental spot. Once informed, astronomers in possession of very i)Ower- 

 ful telescopes have been able to verify directly its existence; it has 

 been observed successively at Pulkova, Nice, Vienna, Washington, 

 Geneva, and other i)laces with more or less facility. Since then, Messrs. 

 Henry have continued to perfect their processes, and they repeat each 



