300 RECENT RESEARCHES RELATIVE TO THE NEBULA. 



to a very exact (Iptermiuation, by a process of triangulation, of the positions 

 of all the stars which his instruments have enabled him to see in that nebula, 

 and to a most careful delineation of all the parts of that remarkable celestial 

 object, of which more than one chart had been already constructed, while 

 assi'>-ninf^ particular names to its several regions. M. Struve, in comparing the 

 results of Liapounoff with those of Sir John llerscbel, Lamont, and Bond, has 

 expressed the opinion that this nebula must be subject to changes of form and 

 relative brightness in its different parts. 



M. Otto Struve has continued, at the observatory of Poulkova, the labors of 

 M. Liapounoff', and has reported the first results of his researches in a commu- 

 nication, of the date of May 1, 1857, presented to the Astronomical Society by 

 M. Airy, June 12 of the same year, and published in the seventeenth volume 

 of the M. N., pp. 225-230. 



In this, M. Struve begins by describing the variableness of the lustre of 

 different small stars situated in the nebula of Orion — a variableness which he 

 has verified as well by a comparison of his observations with those of other 

 astronomers as by different observations of his own.* "The existence of so 

 many variable stars," he continues, "in so limited a space of the central part 

 of the most curious nebula of the heavens must naturally lead to the supposition 

 that these phenomena are intimately connected with the mysterious nature of 

 this body. * * Admitting that the rapid changes of light observed in these 

 small stars, whether in the region called Huygens or in that called Suhnehulosa, 

 are connected with the nature of Irhc nebula, it might be presumed that changes 

 would be equally observed in the appearance of the nebula and in the distribu- 

 tion of the nebulous matter. But observations of this kind are subject to so 

 many illusions, that we can scarce be sufficiently reserved in the conclusions 

 drawn from them. I cannot think that the course commonly pursued by 

 astronomers in this species of researches — the comparison, namely, with one 

 another of graphic representations made at different epochs by different 

 observers — ever conducts to results which can be regarded as indubitable. 

 The optic power of the telescope, the transparency of the atmosphere, varying 

 with different stations, the peculiarities of the observer's eye, the measure of 

 skill and of experience in graphic representations of the kind — all this, joined 

 with the influence of the imagination of the observer, forms obstacles which it 

 will always be difficult to overcome in proceeding after this manner. It might 

 perhaps be possible, by following this method for centuries, to discover pro- 

 gressive changes, if any exist ; but those can never be thus verified which take 

 place in short intervals of time. Now, the rapid variations of light in the stars 

 may well cause us to expect similar, and perhaps periodical, variations in the 

 appearances of the nebulous matter. It is therefore to rapid changes of this 

 sort that we should particularly direct our attenti(to, and we shall be better able 

 to verify their existence by comparative observations on the degree of light and 

 the forms of some prominent portions of the nebula than by representing it in 

 its entireness. It was in this way that I endeavored to proceed during last 

 winter, and the impression produced upon me was a strong one that, at different 

 points, considerable changes occurred within the short period of my observa- 

 tions. I do not venture, however, to regard them as positive facts until they 

 shall have been corroborated, especially by observers stationed in more favorable 

 climates and provided with optical instrumentalities sufficient for the purpose." t 



* I have heretofore had occasion to speak of this work of M. O. Struve, in a Notice on the 

 Stars of Variable Brightness, published in the numbers of the Bibliotheque UnieerselU 

 {Archives, vol. xxxvi, pp. 5-8U) for September and October, 1857. M. Otto Struve has 

 receutly succeeded his fatlier in the direction of the great Russian observatory of Poulkova. 



tThe memoir of M. 0. Struve on this subject has been published, I believe, in vol. ii of a 

 collectioa entitled, Melanges MaUiematiques ct Astronomiques, 



