Zones of Galactic Average number of Star North Polar Distance. per Field of 15'. 0° to 15° 4.32 15° to 30° 5.42 30° to 45° 8.21 45° to 60° 13.61 60° to 75° 24.09 75° to 90° 53.43 Zones of Galactic Average number of Stars South Polar Distance. per Field of 15'. 0° to 15° 6.05 15° to 30° 6.62 30° to 45° 9.08 45° to 60° 13.49 60° to 75° 26.29 75° to 90° 59.06 In these tables the Milky Way itself is taken as occupying two zones of 15° each, instead of one of 20° as in Professor Newcomb's tables, so that the excess in the number of stars over the other zones is not so large. They show also a slight preponderance in all the zones of the southern hemisphere, but this is not great, and may probably be due to the clearer atmosphere of the Cape of Good Hope as compared with that of England. [Illustration: DIAGRAM OF STAR-DENSITY. From Table in Sir J. Herschel's _Outlines of Astronomy_ (10th ed., pp. 577-578).] It need only be noted here that this diagram shows the same general features as those already given, of a continuous increase of star-density from the poles of the Galaxy, but more rapidly as the Galaxy itself is more nearly approached. This fact must, therefore, be accepted as indisputable. CLUSTERS AND NEBULÆ IN RELATION TO THE GALAXY An important factor in the structure of the heavens is afforded by the distribution of the two classes of objects known as clusters and nebulæ. Although we can form an almost continuous series from double stars which revolve round their common centre of gravity, through triple and quadruple stars, to groups and aggregations of indefinite extent--of which the Pleiades form a good example, since the six stars visible to the naked eye are increased to hundreds by high telescopic powers, while photographs with three hours' exposure show more than 2000 stars--yet none of these correspond to the large class known as clusters, whether globular or irregular, which are very numerous, about 600 having been recorded by Sir John Herschel more than fifty years ago. Many of these are among the most beautiful and striking objects in the heavens even with a very small telescope or good opera-glass. Such is the luminous spot called Praesepe, or the Beehive in the constellation Cancer, and another in the sword handle of Perseus.