Clusters and Nehulce. 449 



year, there is something in the idea of a simultaneous variation 

 in a whole assemblage of stars, as we have already pointed out 

 in the case of the nebula in Orion, which seems to pass, the 

 bounds of probability, and inclines us rather to the alternative 

 of luminous mist. The question, however, as to whether this 

 nebula — so curiously situated among the Pleiades — is actually 

 variable, can hardly be considered as decided, notwithstanding 

 the evidence which has been adduced. Schmidt, the present 

 director of the observatory at Athens, is strong on the affirma- 

 tive side. He states that he has been in the habit of watching 

 the Pleiades more or less since 1841, and commenced drawing 

 them in 1844, and that between that date and 1860 he had ob- 

 served them on thirty-two nights ; his comparisons of bright- 

 ness, several hundred in number, being intermitted only in 

 1846 and 1859; but that during all this time he had never 

 seen the nebula. He thinks it could not have escaped him 

 had it been visible in 1861 with a 4-feet Dollond; February 5th 

 in that year he saw it for the first time with the refractor at 

 Athens (probably a Dialyte by Plossl, of 7i [French ?] inches 

 aperture ; but not, as it seems, of the highest quality). The air 

 being quite clear, it appeared very large, very pale, and quite 

 shapeless, Merope lying in its N. corner, so as to appear a nebu- 

 lous star in comparison with its brilliant neighbours. Between 

 this date and the end of 1861 he saw it two or three times. — 

 1862, March 26th, he finds "the great triangular nebula in the 

 Pleiades easily visible ; its extension towards the west is, how- 

 ever, much greater than I had previously believed." On the 

 other hand, Schonfeld at Mannheim doubts the fact of variation. 

 He saw it, 1862, September 20th, not fainter than in 1860, as 

 Chacornac had done at Paris two nights previously; and he 

 thinks that this and the other suspected nebulae, being very 

 feeble, large, and diffused, are influenced in visibility by mag- 

 nifying power, varying transparency of air, and practice of the 

 eye, so that aperture is less concerned in their case than in that 

 of minute stars. Auwers, of Gottingen, argues on the same 

 side. It has often, this observer says, been remarked — Encke's 

 comet being an instance of it — that large, ill-defined, faint 

 objects are best seen with small instruments, and that probably 

 this nebula, having 15' of extent, filled D' Arrest's field under a 

 considerable magnifier, and so became inconspicuous ; he found 

 it an easy object in a comet-finder of 2 feet focus, and saw it 

 repeatedly; — 1860, September 23rd and 24th, when only 16* 

 high; 1861, January 14th; 1862, February 19th and 21st. 

 From its size it can be distinguished with only twenty-one lines 

 of aperture (If French inches) . — Winnecke, again, the assist- 

 ant at Poulkowa, saw it with 4 French inches in March, 1862, 

 large and ill -defined, yet easily visible, as it was also in a 



