22 



POPULAR SCIENCE MONTHLY. 



use the term, is far less conspicuous than in the older drawings, and is, 

 in fact, scarcely recognizable. 



Among the most curious of the nebula? are the annular ones, which, 

 as the term implies, have the form of a ring. It should be remarked 

 that in such cases the interior of the ring is not generally entirely 

 black, but is filled with nebulous light. We may, therefore, define these 

 objects as nebula 1 which are brighter round their circumference than 

 in the center. The most striking of the annular nebulae is that of 

 Lyra. It may easily be found from being situated about half-way be- 



Fig. 12. The Triphid Nebula, Photographed at the Lick Observatory. 



tween the stars Beta and Gamma. Although it is visible in a medium 

 telescope, it requires a powerful one to bring out its peculiar features 

 in a striking way. Recently it has been photographed by Keeler with 

 the Crossley reflector of the Lick Observatory, who found that the best 

 general impression was made with an exposure of only ten minutes. 



The ring, as shown by Keeler's photographs, has a quite compli- 

 cated structure. It seems to be made up of several narrower bright 

 rings, interlacing somewhat irregularly, the spaces between them be- 

 ing filled with fainter nebulosity. One of these rings forms the outer 



