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settles down into dark bodies like the earth, it will have to pass through stages in which its condensing materials will form bright sun-like bodies.

Fig. 4.—Crab Nebula.

It seems as if this process of condensation might almost be witnessed at the present time in some parts of the great object.

There are also some very striking nebulæ which are often spoken of as planetary. They are literally balls of bluish-coloured gas or vapour, apparently more dense