492 



NATURE 



[June 17, 1920 



exceptional conditions in this region. Mr. Wilson 

 refers to the spiral appearance of this great 

 cloud and to the high velocities observed in spiral 

 nebulae, nebulae which may be considered as 

 isolated island universes similar to our Milky Way 

 system, suggesting that the great cloud may 

 afford an opportunity for the study of detailed 

 characteristics of spiral nebulae. 



Part vi., the last of the series of the important 

 contents of this volume, is contributed by Mr. 

 W. H. Wright, and deals with the subject of the 

 wave-lengths of the nebular lines and general 

 observations of the spectra of the gaseous nebulae 

 (pp. 193-268). The matter falls under three head- 

 ings : (i) The measurement of wave-lengths and 

 the intensities of the nebular lines ; (2) the study of 

 the nebular nuclei ; and (3) the investigation of 

 the distribution of nebular radiations throughout 

 the nebulae ; and is accompanied by a series of 

 plates, which demonstrate, more than text can 

 do, the fine definition and great scale of the photo- 

 graphs of the spectra of the nebulae which served 

 as his data. Fig. 7 is an illustration of the 



3426 3727 3869 



career increasing in temperature, reaching a maxi- 

 mum of development and temperature, and after- 

 wards cooling until the invisible stage is reached. 

 In the light of these hypotheses Mr. Wright, as 

 the result of his research, expresses his view as 

 follows : — 



There are at present two general conception? as to 

 the nature of stellar evolution, one of which assumes 

 a falling temperature throughout the period of a star's 

 development, while the other predicates a rise to 

 maximum and a subsequent decline; both of these 

 views assume the nebula as the primordial state. As 

 between these two hypotheses, the present observa- 

 tions undoubtedly favour the first, since they add 10 

 the proof that the gaseous nebulee are associated only 

 with the hot stars 



While the above is one of the main conclusions 

 derived by Mr. Wright from this research, there 

 are many other points of particular interest to 

 which limitations of space forbid reference in this 

 article. 



It is interesting to compare a direct photo- 

 graph of a nebula with its 'Spectrum taken with 

 a slitless spectrograph. Nebulae when photo- 



Hy 4686 Ni— 2 



Fig. 7. — The spectrum of N.G.C. 6818, which records images of a variety of shapes and sizes, most of them having the appearance of a horse-shoe, the open 

 end of the shoe lying to the north. Some of the images show mottlings or condensations scattered along the shoe or ring. (W. H. Wright.) 



spectrum of N.G.C. 6818, taken with the slitless 

 spectrograph with an exposure of four hours. It 

 does not seem that the statement could be con- 

 tradicted that the wave-lengths and intensities of 

 the nebular lin^' deduced will be used as a 

 standard in this branch of physical astronomy for 

 some time. 



This research is very opportune, because more 

 detailed facts were required to help in the unravel- 

 ling of the relationships between nebulae as such, 

 nuclei of nebulae, and bright-line stars such as 

 Wolf-Rayet stars. As the whole problem of the 

 nature of stellar evolution is that of the solution 

 of the relationship between nebulae and stars, the 

 study of the question is of vital importance. The 

 idea of a falling temperature continuing through- 

 out the whole life-history of a star has more 

 recently given place to the hypothesis, appar- 

 ently a very natural one, of a star in its early 



graphed with the latter instrument present 

 remarkable varieties of form and size corre- 

 sponding to different nebular lines in the 

 spectrum, while the direct photograph shows only 

 a form resulting from the integration or the fitting 

 together of the component images of the different 

 forms and sizes. The prismatic images afford a 

 means, therefore, of detecting the differences in 

 distribution of the component gases of the nebula, 

 and indicate that the view of a nebula in a tele- 

 scope or on a direct photograph is not the best 

 means of studying the complex structure of these 

 bodies. 



In conclusion, it may be stated that this addi- 

 tion to the University of California Publications is 

 a valuable contribution, and sustains the high 

 standard of the researches which emanate from the 

 Lick Observatory under the able directorship of 

 Prof. W. W. Campbell. 



The Importance of Meteorology in Gunnery. 



By Dr. E. M. Wedderburn 



AT the commencement of the war the know- 

 ledge of the effect of wind and of the 

 density of the air on the flight of a shell was 

 elementary. It was assurned by the gunners that 

 the wind was of the same direction and strength 

 at all heights reached by the projectile, and that 

 NO. 2642, VOL. 105] 



the density of the air decreased with altitude 

 according to an artificial convention. The cor- 

 rections for wind and density which the gunner 

 was taught to apply were supposed to be refer- 

 able to the meteorological conditions observed by 

 him at the battery, but he was not taught how 



