ASTKONOMY. 127 



out tbat ui)on (he hypothesis that stars of higher temperature will last 

 longer than stars of lower, we must conclude, from the testimony of the 

 spectroscojie, that Sirius, Vega, and the greater number of the stars 

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

 aiul our own sun have long since passe<l their period of greatest activity. 



NEBULAE AND STAR-CLUSTERS. 



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

 essentially a new edition of Sir John Herschel'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 observ^ers. 

 Herschel's General Catalogue was published iu 1864 aud contained 5,071) 

 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 compiled a supplement from the material at that time avail- 

 able. Recent discoveries have giveu rise to a demand for a second sup- 

 plement which has been wisely met by recasting the whole work. "The 

 piesent catalogue contains 7,840 objects, the positions being given in 

 right iiscension co seconds of time and iu declination to tenths of a min- 

 ute of arc. The epoch of the lirst general catalogue aud of D'Arrest's 

 final i)ositions — 18G0— has been retained ; precessions are given for 1880. 

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

 appendix giving the places of new uebuhc published too late to be in- 

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

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

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



The largest refractor devoted almost exclusively to the observation of 

 nebuUe is the 2tJ-iuch equatorial of the Leauder McCormick Observatory. 

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

 and thus to estal)lish the means of detecting the proper motion of these 

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

 —30° which are as bright as the fourteenth magnitude. 



Dr. Dreyer has submitted to a rigid examination all reported cases 

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

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

 the only well-authenticated changes are changes of brightness only, 

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

 change of place. 



Detection of new nebuhv by liliotoffraphi/. — Vioi'essov Tickeriug in order 

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

 compared the number of nebuhe shown in a series of photograi)hs of 

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

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

 was the Bache telescope, which has a photographic doublet with an 

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

 a region 10 degrees square, the great ucl)ula' t)eing about the center; 



