Page:Popular Science Monthly Volume 58.djvu/30

22 use the term, is far less conspicuous than in the older drawings, and is, in fact, scarcely recognizable.

Among the most curious of the nebulæ 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 nebulæ which are brighter round their circumference than in the center. The most striking of the annular nebulæ is that of Lyra. It may easily be found from being situated about half-way between

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 complicated structure. It seems to be made up of several narrower bright rings, interlacing somewhat irregularly, the spaces between them being filled with fainter nebulosity. One of these rings forms the outer