Clusters and Nebulce. 261 



parts. The year following", a very niiimte and continuous ex- 

 amination of the central portion with the great achromatic 

 at Poulkova induced O. Struve to entertain the impression that 

 the light of its several parts was in a state of perpetual change. 

 Even with the best definition, he says that its appearances were 

 to him on no evening entirely agreeing with those on the next 

 or any other night, and he points out several spots as open to 

 the greatest amount of suspicion, adding, however, unfortu- 

 nately we fear for our readers, that for the greater part such 

 changes in the gradation of light can only be seen with very 

 high optical means, and would require as much as ten inches 

 of aperture, unless in extraordinary states of the atmosphere. 

 D' Arrest, the Copenhagen observer, who is paying especial 

 attention to the subject of nebulae, and in a manner which 

 promises most satisfactory results, thought, in May 1862, that 

 these variations, especially the bridging over of the large dark 

 opening with a misty filament, were the first established facts 

 with regard to the variability of nebulae. 



But should any of these suspected changes be deemed suffi- 

 ciently ascertained, we see at once on what a marvellous field of 

 speculation we have entered. Hitherto we have been assuming 

 that all nebulous appearances are, what the majority unques- 

 tionably are, nothing but the light of excessively remote and 

 closely compacted masses of stars. But greatly as our power 

 of belief has been enlarged by the study of sidereal wonders, 

 it still remains inconceivable that any aggregation of stars 

 could possibly experience such changes either of form or bright- 

 ness as we should thus be compelled to admit. We cannot 

 imagine a multitude of suns thus displaced collectively through 

 distances, which upon any reasonable computation must be of 

 enormous extent ; no known force exists to which such a result 

 could be ascribed ; and the idea of simultaneous variation of 

 light affecting uncounted individuals is equally inadmissible. 

 Are we then sure that our original inference is in this instance 

 correct ? Is that misty brightness composed of stars at all ? 

 Is it not possible that it may be merely a luminous haze, as 

 Halley had surmised long ago, of exceeding tenuity, and there- 

 fore capable of receiving impulses, which, however obscure their 

 origin may be, would yet imply no such irreconcileable con- 

 tradictions to all our experience ? And have we not in comets, 

 especially if they are conceived to be self-luminous (and the 

 contrary has not been proved), an instance of a material exactly 

 of such a nature ? Ideas of this kind had gradually opened 

 themselves to the mind of the great discoverer of the nebulae and 

 their starry composition, the elder Herschel; and in 1811 he 

 announced this essential change in his earlier views. He had 

 once supposed that even the most irresolvable or c c milky" nebulae 



VOL. IV. — NO. IV. t 



