ANNULAR nebulæ can no longer be regarded as a class completely apart. They should rather be described as planetaries in which one special feature predominates over the rest. The progressive improvement of telescopes has tended to assimilate the two varieties by bringing into view peculiarities common to both. It is only when they are ill seen that planetary nebulæ appear really such. The uniformity of aspect at first supposed to characterize them disappears before the searching scrutiny of a powerful and perfect instrument. The usually oval surfaces which they present are then perceived to be full of suggestive detail. They are broken up by irregular condensations, or furrowed by the operation of antagonistic forces; betray here the action of repulsive, there of attractive, influences; and bear as yet undeciphered inscriptions of prophetic no less than of commemorative import. Among the various modes of diversification visible in them, however, two are especially conspicuous—first, the presence of a nucleus; next, the emergence of a ring, or even of a system of rings.