314 



NATURE 



\yuly 1 8, 1878 



and Picard. Legentil compared its outline to that of the 

 open mouth of an animal. Messier was the first who gave 

 a catalogue of the stars seen in the nebula. Schroter, 

 in Lilienthal observed it, 1794-1799. It was this 

 eminent astronomer who discovered that this chaotic 

 mass is not in perfect equilibrium, and several of the changes 

 he pointed out have been verified by modern observers. 

 Sir W, Herschel watched the nebula during thirty-seven 

 years. He believed that changes were taking place 

 in Nebula Mairani. The most important fact con- 

 nected with the discovery of changes was, that the 

 three conspicuous stars e, C, and 17 (J, Herschel, 1825), in 

 most of thcie old maps, are represented as inside the 

 bright nebulosity, while they are now seen far from it. 

 D'Arrest showed that no changes have occurred here by 

 aid of a drawing, which Lefebvre published, 1783, in 

 Roziers, " Observations de la Physique," He also 

 remarked the characteristic circumstance that in this 

 figure Sinus Magnus is represented as running right 

 across the trapeze, which, in consequence, is lying alto- 

 gether outside the nebula. Lefebvre's drawing is, how- 

 ever, executed in the style of those that preceded Messier. 

 It is of uniform brightness and sharp outlines. 



Sir J. Herschel appears to have been the first who 

 understood that in order to ascertain changes, it was 

 required to give faithful drawings of all the minute 

 parts of the nebula. His first drawing was executed 

 in 1824, and that was, in 1847, followed by the beautiful 

 figure founded on micrometric measurements made at 

 the Cape, 1834- 1837. He attributes hardly any weight 

 to the first drawing, which had been made with freehand, 

 compared with the last one. 



Lamont published, 1837, an image of the brightest 

 part of the nebula which Herschel criticised. He 

 found, for instance, Regio Hugeniana more uniform, 

 and marked with certain channels, while Lamont repre- 

 sented it as consisting of rounded masses running into 

 each other. Later authorities agree with Herschel, but 

 it deserves to be remarked that he had not himself, 1824, 

 remarked these channels, nor are they laid down on 

 Cooper's map. It so happens that the refractor in Copen- 

 hagen is exactly similar to that in Munich, and in 

 consequence a comparison of the respective drawings 

 made at an interval of thirty-five years could not but 

 be of importance. There is no trace of the sharp outline 

 in the north-west corner, which the Danish drawing 

 shows, and it is so much more likely that here great 

 alterations in brightness have taken place, as all the old 

 drawings, for instance Cooper's, support Lamont, while 

 the later ones in this respect agree among themselves. 

 Amongst the most remarkable differences d' Arrest classed 

 Pons Schroteri in Sinus Magnus. Lamont has of this 

 bridge only the small piece, which, like a promontory, is 

 attached to the north side, while d' Arrest saw the brightest 

 patches about midway. On the above-mentioned old 

 drawing by Cooper, Pons Schroteri is only represented as 

 three small pieces emanating from the north side, while 

 the same is now in the large refractor of the Markree 

 Observatory only noticed as a little spot in the middle 

 of the bay. Such changes were already alluded to by 

 Schroter, and modern diagrams support this hypothesis. 



Liapounov*s diagram, drawn after most careful micro- 

 metric measures,' represents the object as seen about 1850, 

 He agrees with Lamont about Regio Hugeniana, and also 

 about the east point, which he found well defined against 

 the far fainter Proboscis Major. He observed Sinus 

 Lamontii, which he surrounded by the bright nebulosity, 

 since called Hemicyclium Liapounovii. The darkness of 

 this Sinus varied considerably, and thus it was explained 

 why it was not noted by Herschel, though indicated on 



From a discussion of his own and W. Struve's observations, Liapounov 

 concluded that three stars of the trapeze were moving with respect to the 

 fourth, the most southern star. An investigation, on the whole confirmative 

 of this, was read by Prof. Nobile, last year, before the Reale Accademia of 

 Naples. 



Cooper's map. Liapounov represented Pons Schroteri as 

 emanating from the north side of Sinus Magnus, but he 

 made it end with a bright spot, and his representation 

 is, therefore, the midway between older and later 

 drawings. Few astronomers have conducted similar 

 researches so earnestly and faithfully as the Russian 

 professor, and his merits have not been so highly appre- 

 ciated as they deserve. It appears to me that this is even 

 the case from the side of his Danish colleague. 



Lassell published in 1854 a steel engraving, which was 

 badly executed, the regions round Sinus Magnus in 

 particular. Nebula Mairani was Miade brightest of all the 

 nebulae, while it only holds the third or fourth place. All 

 these drawbacks have, however, been removed from the 

 second drawing made in Malta, 1862 and 1863, which is 

 one of the best extant. 



The drawing Secchi published in 1868 is not to be 

 trusted, and even the central region is wrongly drawn. 

 D'Arrest had made a similar remark about an earlier 

 figure by Secchi, to which the Papal astronomer 

 answered : " Che la figura litografica pubblicata, benchfe 

 esatta in generale, ha alcune inesattezze non trascurabili." 

 The possibility of a similar explanation in the present 

 case was excluded by the remark : " Cosi siamo sicuri che 

 I'incisione rappresenta la nebulosa come vedesi da noi nel 

 nostro strumento." 



George Bond's drawing, of about i860, is in d' Arrest's 

 opinion, more like the nebula than any that has been 

 drawn from a refractor, and the characteristic calm- 

 ness over the whole has been successfully imitated. 

 He only saw the northern boundary, and the parts about 

 Palus Bondii somewhat different from Bond. The divi- 

 sions in the south-east corner of the nebula, so prominent 

 in the drawings made of late with gigantic telescopes, 

 do not appear so distinct in d' Arrest's refractor as in 

 Bond's. In Markree it is not possible to trace them at all. 

 On this point Rosse's drawing contains more particulars 

 than any that I have seen. 



The most complicated drawing of the nebula was pub- 

 lished in 1868, by the present Earl of Rosse, D'Arrest 

 found this drawing to be very accurate. The dark chan- 

 nels in Regio Hugeniana are, however, rather broad, and 

 two large spots north of the bright mass too prominent, the 

 boundaries are generally considered too sharp, and the 

 contrast between the stronger and feebler parties rather 

 strong by those accustomed to other telescopes ; but it 

 does not appear that the limits to which nebulosity was 

 traced are much farther than in the refractors of Cam- 

 bridge, United States, and Copenhagen. The feeble 

 streams of nebulosity which connect the 6 with the south- 

 ern t nebula have been well studied at Birr Castle, while 

 the faint northern branches were more attended to in 

 Cambridge, United States, where Bond first traced the 

 connection with the c nebula. The connection between 

 6 and i was known to d'Arrest since autumn, 1865. 



On Rosse's drawing the east point of the main part is 

 bent somewhat and does not go smoothly over in Pro- 

 boscis Major. Thus far this agrees with d'Arrest, but the 

 image at present seen in the Markree refractor is more 

 like Bond's figure. D'Arrest evidently gives the almost 

 straight south-eastern outhne of Regio Hugeniana too 

 great concavity. Rosse, d'Arrest, and Holden agree well 

 about the part west of Sinus Gentilii. Hereabout the 

 Roman drawing does not correspond to nature at all. 

 Nor are the diagrams of Liapounov and Cooper in ac- 

 cordance with d'Arrest. Now this might arise from the 

 different quality of their telescopes, but it is not unlikely 

 that some change has taken place here, though d'Arrest 

 does not offer this explanation. But he declares Sinus 

 Lamontii and Hemicyclium Liapounovii to be very 

 changeable. The agreement of the different diagrams of 

 Lacus Lassellii is striking ; it was remarked already m 

 1795, by Schroter, and notwithstanding possible fluctu- 

 ations in brightness, no alteration in the form has taken 



