June 17, 1920] 



NATURE 



491 



refractor at Mount Hamilton and with the 37-in. 

 Mills reflector at Santiago, Chile, and they were 

 commenced in the year 191 3. The list includes 138 



•^ ' '^^i■ 





Jig. 4.— a "dark nebula" (i7h. 57m., -27°5o') visible through its projection upon the 

 background of stars, and not considered to be "a hole" in the Milky Way. Note 

 the circular protuberance at the South-west corner, as clear-cut as an ink-drop and 

 perfectly dark. (H. D. Curlis.) 



nebulae with bright lines in their spectra. In the 

 earlier part details are given with respect to the 

 spectroscopes employed, the probable errors of 

 the results, and a description of the observations 

 made at the two stations. 



The detailed results of each object are then 

 given in the order of right ascension. Attention 

 may be directed to the fact that the lengths of 

 the slits of the ' spectroscopes employed were in 

 most cases more than sufficient to cover the width 

 of the images of the objects photographed, so 

 that the spectra of the central and outlying por- 

 tions of the nebulae should both be recorded. 



In a further table the final deduced radial veloci- 

 ties of each object observed are given. The 

 observations recorded are evidence of most 

 extensive and arduous work, and the numerous 

 observations of each object considerably emphasise 



displayed from which the internal motion of the 



nebula was deduced. 



The average magnitudes of the derived values 

 for the velocities are as follows : Calling 

 the nebulae less than 5 sees, in diameter 

 "stellar," and those greater than 5 sees, 

 "non-stellar," the mean velocity for 

 thirty-one " stellar " nebulae is 28 km. 

 per sec, and for sixty-five non-stellar 

 nebulae 31 km. per sec, with reference 

 to the stellar system. For evidence of 

 rotation or internal motion in the 

 planetary nebulae, the lines in the 

 spectra of forty-six such objects have 

 been examined in detail and are here 

 discussed. Of these, twenty-five gave 

 evidence of internal effects, while nine- 

 teen, and possibly two more, indicated 

 rotations about axes roughly perpen- 

 dicular to the line of sight. It is worthy 

 of note that the most elongated plane- 

 tary nebulae showed the highest rota- 

 tional speeds. 



The study of the radial velocities of 

 numerous parts of the Orion nebula 

 shows a range in velocities from 

 -1-9-7 km, to +24-9 km., and, as the 

 authors state, the results do not favour 



the hypothesis of a rotation as a whole, but the 



observed differences appear to be local or regional 



in character. 



Fig. 5. —On the left a photograph of N.G.C. 2392, a double-ring planetary 

 nebula ; and on the rlgiit a composite drawing of the same made from 

 several photographs t) show details of the structure not attainable from 

 any single, photograph. (II. D. Curtis.) 



this fact. The accompanying illustration (Fig. 6) 

 shows a photograph of the chief nebular line in 

 N.G.C. 2392, and exhibits the kind of structure 

 NO, 2642, VOL. 105] 



Mr. R. E. Wilson 

 contributes part v. 

 of this volume 

 (pp. 187-90), which 

 deals with the radial 

 velocity of the 

 greater Magellanic 

 cloud. In 1914 it 

 was pointed out that 

 several gaseous 

 nebulae in this region 

 exhibited very large 

 and approximately 

 equal radial velo- 

 cities, so Mr. 

 Wilson presents the 

 results of his study 

 of th<i cloud as a 

 whole. The author 

 upholds this view 

 after his survey, for 

 he finds that, observ- 

 ing seventeen planet- 

 ary nebulae in this 

 region, the radial 

 velocities lie between 

 + 251 km. and 

 -f 309 km., an aver- 

 age of -1-276 km. 

 Correcting this mean 

 for solar motion, 

 the mean velocity is -4-261 km. per sec. This 

 average, compared with the mean velocity deduced 

 in part iv. for other planetary nebulae, points to 



Fig. 6.— a photograph 



of the chief 

 nebular line in N.G.C. 2393. The 

 slit of the upectroscope was placed 

 alonu the major axis of the nebula. 

 (W. W. Campbell and J. H. Moore.) 



