458 Clusters and Nebulae. — Double Sta?°s. 



way we get the incidental result, that though a reflector when 

 compared with an achromatic of equal light, is, under ordinary 

 circumstances, at some disadvantage, from including in its 

 larger aperture more atmospheric confusion, still, when the 

 air is really steady, its optical definition will surpass that of its 

 rival.) In the case, however, of the nebula from which w T e> 

 have been wandering so far, the effect of larger aperture is 

 twofold in producing a greater contrast, diminishing at once 

 the spurious discs of the stars, and enlarging the real diameter 

 of the nebula, whose fainter edges come into view. The object 

 thus to be at length "swept up" is termed by Sm. a small 

 pale blue planetary nebula, diam. 8". With my smaller instru- 

 ment it was exactly like a star out of focus, bearing 300 well ; 

 with the larger one it was a bright ball with woolly edges; 65 

 seemed to show its colour best; with 111 it was encompassed 

 with a glow, not so evident with higher or lower powers. Secchi 

 saw it with 1500 as a sparkling group of stars. Schultz at 

 Upsala, with a 9h (Paris ?) in. achromatic, has since described 

 it as a very curious object, 9*"o diam., almost planetary, yet 

 distinctly granulated ; it w T ould, he remarked, be an interesting 

 object tor Muggins's analysis ; and so it has proved. With 8- 

 in. and powers up to lOOl), that observer found it had an uniform 

 disc, intensely bright, and decidedly blue, surrounded with a 

 faint nebulous halo; the spectroscope showed three bright 

 lines, with glimpses of a very faint continuous spectrum: as 

 to the greater part, therefore, of its extent, and possibly with 

 slight condensation in some places, this is a ball of incandescent 

 gas, magnitude and distance utterly unknown ! 



DOUBLE STARS. 



Having learned to find the guide-star to M5, we should 

 examine it in its individual character, as it is really a pretty 

 pair. It will stand in our list as 



100. 5 Serpentis. 10'"3. 39°'8. 5J, 10J. Pale yellow, 

 light grey. 



We will also include, for another reason, 



liil. a Serpentis. 50". 1°'5. 2^,15. Pale yellow, fine 

 blue. The attendant, which is called extremely delicate by 

 Sm., forms an excellent test for light, alike from minuteness, 

 distance, and position. With my ( J£ in. speculum it was quite 

 obvious. I do not. however, profess to be able, in general, to 

 detect any colour in these faint points, which were considered 

 by 2 also, if below his ( J m. (=9i Sm.) to show no certain hue. 

 In achromatics, if near enough to the larger star, they might, 

 perhaps, acquire an adventitious tint from being involved in 

 its outstanding fringe, the freedom from which constitutes one 



