A.STKUNOMY. 127 



out that upon (lie hypothesis that stars of higher temperature will last 

 longer than stars of lower, we must (H)nclu(le, from the testimony of the 

 si)e<'troscope, that kSirius, Vega, and the greater luimber of the stars 

 visible to the naked eye are in their youth, while Aldebaran, Areturus, 

 and our own sun have long since i)assed their period of greatest activity. 



NEBULAE AND STAR-CLU.STElt.S. 



Dr. Dreyer's new general catalogue of nebuhe and clusters of stars is 

 essentially a new edition of Sir John Uerschel's catalogue, revised, cor- 

 rected, and brought down to December, 1887, It therefore forms a com- 

 plete list of all known nebuhe, and is of the greatest value to observers. 

 iJerschel's (Jeneral Catalogue was published in ISG-i and contained 5,079 

 objects. D'Arrest's work, published three years later, gave the means 

 of correcting many of the errors in the earlier observations, and in 187G 

 Dr. Dreyer com[)iled a supplement from the material at that time avail- 

 able. Itecent discoveries have given rise to a demand for a second sui)- 

 l)Iement which has been wisely met by recasting the whole work. •The 

 piesent catah)gue contains 7,840 objects, the positions being given in 

 right ascension i:o seconds of time and in declination to tenths of a min- 

 ute of arc. The epoch of the first general catalogue and of D'Arrest's 

 tinal [)ositions — 18G0— has been retained ; precessions are given for 1880. 

 There is an index to published tigures of nebuhe and clusters, and an 

 a[)[)endix giving the places of new nebuhe published too late to be in- 

 corporated in the catalogue itself. Further additions in numbers or in 

 accuracy of positions might, perhaps, now be made from the recently 

 published lists of Bigourdan, von Engelhardt, Giuzel, Stone, and Swift. 

 The largest refractor devoted almost exclusively to the observation of 

 lU'buhe is the 2(>-inch ecpuitorial of the Leander McCormick Observatory. 

 Professor Stone's object is to obtain as accurate positions as possible 

 and thus to establish the means of dete(;ting the proper nu)tion of these 

 objects if any exists. His working list embraces all nebuhe north of 

 — oOo which are as bright as the fourteenth magnitude. 



Dr. Dreyer has submitted to a rigid examination all re])orte(l cases 

 of variability or proper motion in nebuhe, and concludes that in not 

 one case can either be considcired as well established. It seems that 

 the only well inithenticated changes are changes of brightness only, 

 while we so far do not [mssess any clear evidence of change of form or 

 change of [)lace. 



Detection of new nchuhc hy photography. — Professor Pickering in order 

 to test the efficacy of photografdiy in the discovery of new nebuhe has 

 compared the number of nebuhe shown in a series of photographs of 

 the regions about the great nebuhe in Orion with the number in the 

 same region given by Dreyer's catalogue. The instrument employed 

 was the Rache telesco[)e, whi(rh has a photographic; doublet with an 

 aperture of 8 inches and local length of 44 inches. Each plate covered 

 a region 10 degrees square, the great nel>uhc being about the center j 



