Mat 25, 1917] 



SCIENCE 



523 



the nebular and the stellar spectra. These 

 facts have led Slipher to favor the view 

 that the two nebula are not shining by their 

 own light, but by the reflected light of the 

 brilliant stars in their midst. One of these 

 is the nebula in which the Pleiades cluster 

 is immersed (Fig. 13), and the other is 

 around the bright star Rho Ophiuchi (Fig. 

 14). There are in the Milky Way many 



scores of nebulas such as these, though for 

 the most part fainter; but whether their 

 spectra are of the , bright-line type or re- 

 fleeted-light type, we do not know. 



Our photographic telescopes are confirm- 

 ing William Hersehel's observation that 

 these large and formless nebulse are in or 

 are bordered by regions of sky showing 

 fewer stars than abound in the surround- 



FiG. 12. The Ti-ifid Nebula, in Sagittarius, photographed by Keeler with the Crossley Eeflector 



Lick Observatory. 



