16 ANNUAL KECORD OF SCIENCE AND INDUSTRY. 



hundred and five nebulae occur in this zone. Of these many- 

 have been previously well determined by D'Arrest and others, 

 and Dr. Vogel accordingly devoted his labor to the nebulae 

 whose positions were uncertain, one hundred and forty in 

 number. Each nebula was observed on two difierent nis^hts 

 at least, and Vogel finds the probable error of his final posi- 

 tions to be in R. A. = 0.09", in ^ := 1.1". Between Vogel 

 andSchonfeld the same personal difierence remains which ex- 

 isted in 1867. Supposing both observers to have the same 

 accuracy, the probable errors of a concluded position do not 

 exceed 0.14^ in R. A., and 1.5" in declination. It will be 

 noted here that these determinations are surprisingly accu- 

 rate, and that the observations of D'Arrest, Laugier, Schultz, 

 Schonfeld, and Vogel will serve to determine the proper 

 motions of nebulae if these are of important magnitude. 

 Three star-clusters (G. C. 905, 1361, and 4440) have also been 

 studied, and twelve nebulae drawn. The nebula G. C. 2211 

 = h. 761 is probably variable in brightness. 



SUPPOSED CHANGES IN THE OMEGA NEBULA. 



Professor Holden has recently published the results of a 

 careful study of all published drawings and observations 

 of this nebula (R. A. 18^ 12^, N. P. D. 106.2). The method 

 adopted in the discussion of the drawings was peculiar, al- 

 though perfectly simple and elementary. All the published 

 drawings Avere photographed on the same scale, and in such 

 a way that they represented the aspect of the nebula as seen 

 in a refractor. They became thus strictly comparable, and 

 any variation between the several drawings was at once evi- 

 dent. In all such delineations such differences exist, and the 

 only difiiculty in their discussion is to distinguish between 

 real changes in the shape of the object itself, and apparent 

 changes due to lack of fidelity in the drawing. 



Professor Holden seeks to avoid this great difiiculty by in- 

 quiring how the nebulosity in each drawing is situated with 

 regard to lines joining certain pairs of stars, and terminated 

 by these stars. It is probable that in a careful drawing no 

 observer would lay down a certain star outside of a nebula 

 when his telescope showed it inside. In this way the suc- 

 cessive drawings of this nebula, from 1833 to 1875 (seven in 

 all), have been studied, and the conclusion is arrived at that 



