Nebulce. 279 



enlarged diameter with the same power ; it could not possibly 

 be overlooked here by any imaginable amount of carelessness. 

 It is, however, very probable that this effect is due in part to a 

 greater decrease of light towards the edge than would be 

 made sensible in any other way. It is singular how little 

 power the most delicate vision has of detecting, at least under 

 certain circumstances, gradual variations in brightness. This 

 is undeniably and curiously exhibited in the case of Jupiter, 

 whose disc, sensibly uniform in brilliancy, is shown to be much 

 more brilliant in the centre by the frequent change in the 

 aspect of the satellites during their transits. It is also exem- 

 plified in the telescopic discs of stars, which are known, from 

 theory, to degrade rapidly in light from the centre, though 

 this is only manifested to the eye by the different sizes of the 

 discs in proportion to their luminosity. And it is more than 

 probable that the uniformity of light ascribed by Sm. to this 

 nebula and Jupiter is as illusive in the one instance as in the 

 other. With this large aperture the edges were extremely 

 woolly; the elliptic form and blue tint were not conspicuous, 

 but there was some moonlight, and, probably, haze, and the 

 meridian was long passed. With 170 and about 600, I fancied 

 the light not very equable ; but I was aware that Secchi had 

 broken up the supposed uniformity of aspect : not in conse- 

 quence of greater light, having only 9-iV-in. achrom. against 

 18-Uin. front view reflection, but from higher power; H/s 180 

 being the most suitable for his purpose of surveying the whole 

 heavens ; while Secchi, having a special design, was able to 

 carry his even up to 1000 with great distinctness, and thus he 

 perceived, within a circular nebulosity, two clusters connected 

 by two semicircular arcs of stars into one sparkling ring, with 

 a single star upon the hazy central area. Yet, as in the case 

 of the annular nebula in Lyra, the appearance must have been 

 deceptive, at least as to the existence of matter in a solid or 

 even a liquid state. At such a distance the eye can take no 

 direct cognizance of the nature of what it sees ; but the pris- 

 matic analysis of light will still give information which, though 

 indirectly obtained, seems fully to be trusted ; and in this way 

 Huggins, through his 8-in. object-glass, with powers of 600 and 

 920, detects the oval ring, which he thinks is seen obliquely 

 by us within a globular mass of faint nebulosity, yet finds, 

 from the quality of its light, decomposed into three bright 

 lines, that it consists almost entirely of gaseous matter. With 

 a wider opening of the spectroscope than would define the 

 lines properly, he iC suspected a faint and broad continuous 

 spectrum," indicating the presence of some feebly luminous 

 solid or fluid materials (or possibly small stars involved in the 

 mass?). We have here evidently a structure not very dissi- 



