SCIENCE 



Fig. 28. The Spectrum of the Plauetaiy Neljula N.G.O. 76(32, photographe.l l.y Wright with the 

 Draper slitless quartz spectrograph and the Crossley reflecting telescope of the Lick Observatory. 



[The images of the nebula given by the radiations of different elements and different wave-lengths 

 show a great variety of diameters and structural details. The two ellipses at the extreme right, strongly 

 overlapping, are the two green nebulium images of the nebula. The adjoining smajler ellipse, slightly 

 overlapping, is the hydrogen Beta image. The small images at the left end of the figure are far out in 

 the ultra-violet region of the spectrum.] 



criticallj' examined was found to extend 

 out farther than do the other chemical ele- 

 ments. Such was the ease in the great 

 Orion nebula, in the Trifid nebula, and in 

 one of the small planetaries, N. G. C.2 418. 

 Viewed in the telescope, the latter was ob- 

 served to be a disc of greenish-blue light, 

 about fourteen seconds of arc in diameter, 

 with a well-defined star near the center of 

 the disc. Viewed in the spectroscope, the 

 spectrum of the central star was a continu- 

 ous line of light, and the composite disc 

 was resolved into three separate discs (see 

 Fig. 27) ; the largest one of hydrogen, 

 fourteen seconds of arc in diameter; a 

 smaller one, corresponding to the first green 

 line of the element nebulium, eleven seconds 

 of arc in diameter; and a still smaller one, 

 corresponding to the second line of nebu- 

 lium, nine seconds of arc in diameter. The 

 nebulium did not extend out so far from 

 the central star as the hydrogen. Wolf and 

 Burns applied photographic methods to a 

 similar study of the Ring Nebula in Lyra, 

 in 1908 and 1910, respectively, and found 

 differences both in the sizes and in the de- 

 tailed structure of the spectral rings. 

 Wright has in the past two years carried 

 the development of the subject much 

 further, by photographic methods applied 

 to the principal planetary nebulse (see Fig. 

 28). He finds, for example, that the dis- 

 tribution of the helium in the structure of 

 the planetary nebulfe always favors the cen- 



tral nucleus or star more than the hydrogen 

 and nebulium distribution do. In some of 

 the planetaries, the helium has apparently 

 been drawn entirely into the central nu- 

 cleus. In one of the planetaries the hy- 

 drogen globes seem to persist brilliantly 

 after the nebulium images have become re- 

 duced in size, or have become exceedingly 

 faint. We can scarcely doubt that in these 

 phenomena we are witnessing certain stages 

 of progress in the gradual evolution of the 

 planetary nebiilaj into the stars which we 

 see at their centers, or, possibly, into sys- 

 tems of stars and planets. The materials, 

 for the most part, seem to have been drawn 

 -^possibly are still moving — into the cen- 

 tral stars, into suns that are forming ; and a 

 very little of the materials shown by ob- 

 servation to be revolving around the central 

 suns may ultimately be left to form planets 

 of the systems. Let us recall that in our 

 own solar system 99 6/7 per cent, of the 

 materials are in the sun, and that only one 

 seventh of one per cent, of the materials 

 exists outside of the sun, in the eight major 

 planets and their moons. 



The different sizes of the elliptic images 

 of different chemical elements in the spec- 

 trum of a planetary nebula give some basis 

 for the speculative thought that the chem- 

 ical composition of the large outer planets 

 of our solar system may be quite' different 

 from that of the small inner planets. 



While a strong case can be made out for 



