260 Clusters and Nebulce. 



Herschel lias cautioned us that "very great differences will 

 occur in the descriptions of one and the same nebula taken on 

 different nights, and under different atmospheric circumstances, 

 as well as in different states of the mirror and the eye ; nor 

 will it at all startle one accustomed to the observation of 

 nebulae to see such an object described at one time as faint, 

 small, round, and at another as bright, pretty large, pretty 

 much extended, resolvable, etc.;" but after making every 

 allowance on this score, as well as for the great difficulty of 

 representing such an object well, it must be admitted that 

 the variations are more considerable and perplexing than, 

 considering the character of the observers, we might have 

 expected. 



Are we then thus forced upon the supposition of actual physical 

 change ? Such an idea would be sufficiently astonishing, when 

 the enormous magnitude of so remote an object is considered; 

 but it has been seriously entertained. Herschel I. had been 

 led to suppose it fully established, from the disappearance of a 

 misty envelope round certain stars adjacent to the nebula, and 

 from an alteration in the direction of one of its branches ; and 

 though his son had at one time come to the conclusion that 

 there were no differences which would not admit of explanation 

 upon the grounds already mentioned, and that there was no 

 reliable evidence of change, he subsequently felt obliged to alter 

 his opinion, from the dissimilarity between his own designs in 

 England and at the Cape ; other discrepancies he could account 

 for, from practice and a superior instrument and climate ; but 

 as to one part, he says, " it seems hardly possible to avoid the 

 conclusion of some sensible alteration having taken place. No 

 observer now, I think, looking ever so cursorily at this point 

 of detail, would represent the broken, curved, and unsym- 

 metrical nebula in question ... as it is represented in the 

 earlier of the two figures ; and to suppose it seen as in 

 1837, and yet drawn as in 1824, would argue more negligence 

 than I can believe myself fairly chargeable with." This would 

 be indeed most weighty testimony, even if it stood alone; 

 but such is by no means the case. In 1852, Lassell, after 

 observing the nebula carefully with his 24-inch speculum, says, 

 "& comparison of Sir J. Herschel' s, Mr. Bond's, and my own 

 drawings of this wonderful object must, I think, suggest the 

 idea of change in the nebula, or variability in the stars, or 

 otherwiso a less uniformity of delineation of the same things 

 than might have been hoped for." In 1 856 W. Struve was led 

 by the observations of Liapounov, who had during four years 

 been most scrupulously measuring and delineating the nebula 

 at Kazan with a 9^-inch achromatic, to infer that it may be 

 subject to changes of form and relative brightness in its different 



