NEB 



7. Nebulae that have a cometic appearance. 



8. Nebulae that are suddenly much brighter in the middle. 



9. Round nebulae increasing gradually in brightness up to a nucleus 

 in the middle. 



10. Nebulae that have a nucleus. 



11. Round nebulae that are of an almost uniform light. 



12. Nebulae that draw progressively towards a period of final conden- 

 sation. 



13. Planetary nebulae. 



14. Stellar nebulae. 



15. Stellar nebulae nearly approaching to the appearance of stars. 



Clusters rf stars. 



We have seen according to Herschel's doctrine, that extensive ne- 

 bulosities are in process of time broken up into separate and distinct 

 nebulae ; and that these last again, after becoming gradually more and 

 more condensed, form stars. Upon the same principle he accounts for 

 the formation of clusters of stars. He conceives that in rich portions of 

 the heavens, as for instance the milky way, various centres of attraction 

 are formed, to which the neighbouring stars gravitate ; that thus the 

 whole is broken up into separate systems or clusters of stars. That these 

 clusters at first are of various irregular figures, and consist of stars coarse- 

 ly and unequally scattered over the mass ; that by the progress of con- 

 densation they become more insulated and detached from the neighbour- 

 ing stars, their figures are more regular and spherical, and the stars more 

 rich and closely connected ; till they at length form those minute and 

 beautiful phaenomena which are undoubtedly the most interesting ob- 

 jects for our finest telescopes. He arranges them as follows, according 

 to their degree of condensation. 



1. Aggregation of stars, or patches of stars, which seem beginning to 

 form clusters. 



2. Irregular clusters of various unascertained sizes. 



3. Clusters variously extended and compressed. 



4. Considerably compressed clusters of stars. 



5. Gradual concentration and insulation of clusters of stars, 



6. Globular clusters of stars requiring a very fine telescope 



7. More distant globular clusters of stars. 



8. Still more distant globular clusters. 



194 



