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THE POPULAR SCIENCE MONTHLY. 



an accumulation of matter toward the centre, with a gradual increase 

 of density, and a rotatory movement. But, if we combine with this 

 motion the supposition of an opposing medium, it is difficult to har- 

 monize such a system with the known laws of statics. Accurate meas- 

 ures are, therefore, of the highest interest for the purpose of showing 



Annulab Nebula in Lyba. 



whether actual rotation or other changes are taking place in these 

 uebulse; but, unfortunately, they are rendered extremely difficult and 

 uncertain by the want of outline, and by the remarkable faintness of 

 these nebulous objects. 



Pio. 8. 



Nebula with Seveeal Rings. 



The transition state from the spiral to the annular form is shown 

 in such nebulae as the one represented in Fig. 6 ; and they then pass 



