148 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



from the earth, and doubtless also as to their absolute position in 

 space, a marked antipathy to the plane of the milky way, the galactic 

 plane. 



This fact was noticed long ago by the jihilosopher and sociologist, 

 Herbert Spencer. It is shown by the often-published figure con- 

 structed by Proctor. The principal catalogued nebulae are indicated 

 by so many points. The white spot near the south pole corresponds 

 to the Magellanic Clouds, a small region where nebulee and clusters 

 abound. A place of similar nature, though less important, lies in the 

 northern hemisphere close to the milky way. Apart from these two 

 exceptions, the milky way traverses, throughout nearly all its whole 

 extent, regions poor in nebulas which cluster chiefly near the north 

 pole of the milky way. 



But is this law of distribution the same for the spiral nebulae? 

 For some years it was generally admitted that it was not, that the 

 spirals were iiTegularly distributed as regards the milky way. We 

 might therefore treat them as strangers and keeping in mind their 

 circumvolutions, bifurcations, gaps and the fact that they inclose so 

 many stars and clusters of stars, consider each one as an mdependent 

 milky way. 



To-day that conclusion does not seem so assured since Keeler has 

 pomted out that many of the famt nebulre, showing to the naked eye 

 no trace of a central nucleus or spiral structure, reveal on long- 

 exposure photographs both these characteristics. Now we are begin- 

 ning to ask whether the greater number of nebulae are not spiral and 

 whether statistics, including all of them, would not show that the 

 great majority of these objects are related to the milky way. A pho- 

 tographic exploration of the entii-e sky with a powerful instrument is 

 necessary to solve this problem. 



Apart from their structure, which too often escapes us, is there no 

 other easily determinable chanicteristic which may serve to classify 

 the nebulae ? Could we not, for example, group them, as we have the 

 stars, according to the richness of their spectra in absorption lines? 



Huggens, trying to do this, noted that they readily fall into two 

 classes. One shows a spectrum composed of bright lines like that of 

 a gas made luminous electrically. These are often called the green 

 nebulae because the greater part of their light is concentrated in a 

 bright green line in their spectrum which has never been identified 

 with any known terrestrial element. Provisionally it is considered 

 as an indication of an unknown element wliich has been named 

 nehulium. Of the four lines to which the spectrum of a nebula of 

 this class is usually limited, the tliird in order of mtensity is the only 

 one upon whose origin we are agreed. It belongs to the spectrum of 

 hydrogen. 



