Clusters, Nebulae, and Occultations. 347 



about the middle of the last century, was led to conclude that 

 it must have varied in form and brightness ; an inference 

 which, however, seems to have had no foundation, except in 

 the inadequacy of the instruments then in use. It stands No. 

 31 in the catalogue of Messier, communicated to the Academy 

 of Sciences in 1771. Sir W. Herschel estimated its dimensions 

 at about 1|° in length by upwards of 16' in breadth, and con- 

 sidered it undoubtedly the nearest of all the great nebula?. 

 His illustrious son in 1826 described it as " very bright and 

 of great magnitude, and altogether a most magnificent object," 

 its brilliancy increasing gradually from the edges to the centres 

 where he found a kind of indistinct nucleus of stronger light, 

 about 10" or 12" in diameter. Lamont observing it ten year, 

 afterwards at Munich, with a power of 1200 on a very large 

 achromatic, gave this magnitude about 7". On the mounting 

 of the great achromatic by Merz, of 14f inches aperture and 

 23 feet focal length, at the Cambridge Observatory, Harvard 

 College, U.S., in 1847, nothing was remarked at first but a 

 vast number of stars of various magnitudes scattered over its 

 surface, an almost star-like nucleus, and a sudden termination 

 of the light all along the np side. But it was subsequently 

 perceived that this abrupt bordering was occasioned by the 

 presence of a long dark streak, 1¥ broad, beyond which the 

 hazy light recommenced, though with less intensity, for about 

 4', when it was a second time interrupted by a similar dark 

 band, and again continued till it faded entirely away. Thus 

 one side of the great mass appeared subdivided lengthways 

 by two dark canals, the innermost nearly 1° in length, straight 

 for half that distance, but subsequently bent a little inwards ; 

 the other not so extended in length, and inclined to the first 

 at an angle of about 3\ A truly wonderful structure is thus 

 indicated, but, as it would seem, not without a parallel in the 

 heavens, many other instances of arrangements more or less 

 analogous having been pointed out by Sir J. Herschel and the 

 Earl of Rosse ; and unless, as the former has suggested, we 

 can conceive the interposition of imperfectly transparent 

 matter nearer to our eyes (which seems very difficult), we 

 must suppose it to be a kind of stratification of the nebulosity 

 presented edgeways to our sight, and giving us but little 

 opportunity of conjecturing what might be the aspect of the 

 luminous mass viewed in a transverse direction. The Earl 

 of Rosse, indeed, seems to incline to the opinion that there 

 may be an extreme fore-shortening of some kind of annular 

 formation, the dark streak being the perspective projection 

 of an interior void ; but he has not referred to the complication 

 introduced by the presence of the second streak, which would 

 seem to require . a kind of figure-of-eight arrangement ; and, 



