Besides the seven nebulæ visible to the naked eye, a great number of similar objects are visible through the telescope. In Sir John Herschel's catalogue of nebulæ and clusters, are found 4,053 irresolvable nebulæ, and with every increase of the aperture of telescopes, new nebulæ, invisible in smaller instruments, are found. Notwithstanding their irresolvability it is probable, however, that many among them have a stellar structure, which their immense distance prevents us from recognizing, and are not therefore true nebulæ. The giant telescope of Lord Rosse has shown nebulæ so remote that it has been estimated that it takes their light 30 million years to reach the Earth. The nebulæ are very far from being uniformly distributed in space. In some regions they are rare, while in others they are numerous and crowded together, forming many small, irregular groups, differing in size and in richness of aggregation. The grouping of the nebulæ does not occur at random in any part of the heavens, as might naturally be supposed, but, on the contrary, it is chiefly confined to certain regions. Outside of these regions nebulæ are rare and are separated from each other by immense intervals; so that these isolated objects appear as if they were lost wanderers from the great nebulous systems. The regions where the nebulæ congregate in great number are very extensive, and in a general view there are two vast systems of nebular agglomeration, occupying almost opposite points of the heavens, whose centres are not very distant from the poles of the Milky-way. In the northern hemisphere, the nebulous system is much richer and more condensed than in the southern hemisphere. The northern nebulæ are principally contained in the constellations Ursa Minor and Major, in Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and Virgo. In this region, which occupies about ⅛ of the whole surface of the heavens, ⅓ of the known nebulæ are assembled. The southern nebulæ are more evenly distributed and less numerous, with the exception of two comparatively small, but very remarkable centres of condensation which, together with many star-clusters, constitute the Magellanic clouds.