Page:Popular Science Monthly Volume 58.djvu/32

24 seems to have a special actinic power, as the star is more conspicuous on the photographs than to the eye.

There are several other annular nebulæ which are fainter than than of Lyra. The one best visible in our latitudes is known as H IV. 13, or 4,565 of Dreyer's catalogue. It is situated in the constellation Cygnus which adjoins Lyra. Both Herschel and Lord Rosse have made drawings of it. It was photographed by Keeler with the

Crossley reflector on the nights of August 9 and 10, 1899, with exposures of one and two hours, respectively. Keeler states that the nebula, as shown by these photographs, is an elliptical, nearly circular ring, not quite regular in outline, pretty sharply defined at the outer edge." The outside dimensions are: