June 17^ 1920] 



NATURE 



489 



Recent Researches on Nebulae.^ 



By Major William J. S. Lockyer. 



T^HE latest volume (No. xiii.) in the series of 



J- Publications of the Lick Observatory, situ- 

 ated on Mount Hamilton, California, is completely 

 devoted to a series of well-laid-out investigations 

 of the study of the forms, distribution, velocities, 

 and spectra of the nebulae. The volume is one of 

 extreme interest and importance, and will become 

 a classic for a considerable time on those interest- 

 ing objects scattered throughout the heavens. 



In these days, when the study of the evolution 

 of the stars is occupying a position in the front 

 rank, the more detailed information of the nebulae, 

 their composition, structure, and movements, is 

 of fundamental importance, for these bodies are 

 criteria in the evolutionary stages of stars. 



Considerations of space will not permit here 

 more than an outline of the contents of this sub- 

 stantial volume, which includes six separate con- 

 tributions, each devoted to a special research, and 

 a large number of beautifully reproduced plates. 



Part i. is contributed by Mr. H. D. Curtis 

 (pp. n-42), and deals with the descriptions of 

 762 nebulae and clusters photographed with the 

 Crossley reflector. It comprises all photographs 

 of these objects which have been taken with this 

 instrument since the year 1898, when systematic 

 work was commenced, forming, therefore, a 

 valuable homogeneous research. 



It is interesting to note the types of the 762 

 entries, which Mr. Curtis divides as follows : 

 513 spiral, 56 diffuse, 36 globular, 24 sparse, 

 78 planetary, 8 dark, and 47 unclassified. Mr. 

 Curtis is led to believe that all the many thou- 

 sands of nebulae not definitely to be classed as 

 diffuse or planetary are true spirals, and that "the 

 very minute spiral nebulae appear as textureless 

 discs or ovals solely because of their size." 



In estimating the probable total number of the 

 spiral nebulae, Mr. Curtis concludes that at least 

 700,000, and very probably 1,000,000, small spirals 

 are within the reach of large reflecting telescopes. 

 A chart showing the distribution of regions on 

 which small nebulae were counted indicates also 

 the position of the galactic plane, and the paper 

 concludes with reproductions of a few typical 

 nebulae. 



The second part, by the same author, is devoted 

 to a study of occulting matter in the spiral nebulae 

 (pp. 45-54),- and its object is to show that the 

 occurrence of such dark bands running down the 

 length of spiral nebulae seen edgewise is a rela- 

 tively common feature. While a description of 

 these appearances is not necessarily satisfactory 

 to those who have not had occasion to observe 

 them or to see the original photographs, Mr. 

 Curtis includes seventy-seven reproductions. By 

 the kindness of Prof. W. W. Campbell, repro- 



1 Univers'ty of California Publications. Publications of the Lick Ob- 

 servatory. Vol. xiii. Pp. 268 + 50 plates. (Berkeley : Univenity of Cali- 

 fornia Press, igiS.) 



NO. 



1642, VOL. 105] 



ductions of some of these spirals are here given 

 (Figs. I, 2, and 3). 



References are made to other evidences of 

 occulting matter in the sky, such as the cutting 

 off in the number of stars round a nebula, "coal 

 sacks " or starless regions, dark nebulae, etc. 

 (see Fig. 4). The fact that many spectroscopic 

 binaries indicate a constant radial velocity for the 

 H and K lines, different from the periodic shift of 

 the other lines in the spectrum, suggests, according 

 to the author, the interposition between us and the 

 binary of a cloud of non-luminous matter, though, 

 as he says, there are some difficulties in this hypo- 

 thesis. The subject of the peculiar grouping of the 



N.G.C. 



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5866 



Fig. I.- Spiral nebulae seen almost exactly edgewise and show- 

 ing indubitable evidence of dark lanes. (H. D. Curtis.) 



spiral nebulae about the galactic poles is also 

 mentioned. 



Part iii. is entitled "The Planetary Nebulae," 

 and in it Mr. Curtis brings together the results 

 of a research on a series of photographs of all 

 the planetary nebulae north of 34° S. declination. 

 Seventy-eight of these objects are dealt with, and 

 they are all reproduced either by photographs or 

 by drawings (with scale). Drawings were resorted 

 to only when the objects were so small that 

 they could not be reproduced by the process of 

 photo-engraving, or when great differences in 

 brightness between the central and the 

 faint outlying portions were encountered, which 



