ASTRONOMY 



the nebulas belonging to the other two classes, the 

 numbers within the reach of modern telescopes 

 being estimated to be about a million. 



The typical form is a central globular con- 

 densation, from two diametrically opposed points 

 of w r hich proceed arms which coil round the 

 nucleus in spiral form. Along these arms are 

 numerous knots and condensations, some of which 

 are almost of stellar aspect. 



The appearance of the spirals depends upon 

 the angle of presentation, some being seen full face, 

 so to speak, while in others the plane of the spirals 

 passes through the earth, and the nebula takes on 

 a spindle-shaped form. In these latter forms the 

 darker and presumably cooler matter of the arms 

 is seen projected against the more luminous central 

 nucleus. 



The spectra of these nebulas are quite different 

 from those of the irregular and planetary types. 

 They are continuous, crossed by dark lines, and 

 so resemble those of the stars. 



The main problem to be solved in connection 

 with the spiral nebulae is whether they are a part 

 of our own sidereal system, or are to be regarded 

 as island universes, systems coequal with our own, 

 situated far beyond its limits. 



The evidence which supports the latter view 

 may be briefly summarised. 



In the first place, nearly all appear to be moving 



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