NOTES ON NEBULAE. 125 



lies in the constellation of Andromeda, where, on a clear 

 and dark night, a faint spot of light can just be discerned 

 by a good eye. But a mere glimpse gives us really no 

 adequate notion of the true character of the object. It 

 might only, so far as the naked eye discloses its nature, be 

 a cluster of stars like that we have already discerned in 

 Perseus, or like the similar group that, under the name of 

 the Beehive, is comparatively familiar in the constellation 

 of Cancer. With the single exception of the nebula in 

 Andromeda, all the objects so called are entirely telescopic, 

 yet how important a constituent the nebulae form in the 

 contents of the heavens will be shown by a look at some of 

 the lists of these objects. There are now several thousands 

 of nebulae known, and their positions in the sky, as 

 well as the details of their appearances, are set forth in 

 the catalogues. 



It will therefore be proper that during our evenings at 

 the observatory more opportunity should be taken to 

 examine these mysterious nebulae. An exceptionally fine 

 night should be chosen for this purpose. The sky should 

 be clear and bright, and the moon should be absent. In- 

 deed, when the moon is present the light it scatters over 

 the sky is sufficient entirely to extinguish the faint nebulae, 

 and greatly to impair the lustre and the beauty of the 

 brighter ones. A good test of the suitability of a night 

 for such purposes is found in the visibility of the Milky 

 Way. If it be seen clearly spanning the sky, then the 

 night will usually be favourable for such observations. 

 From the same considerations we may infer that it will 

 not do to choose nights in the middle of summer. Then 

 the twilight glow over the sky in our northern latitudes 



