The so-called nebulous stars consist of a faint nebulosity, usually circular, surrounding a bright and sharp star, which generally occupies its centre. The nebulosity surrounding these stars varies in brightness as well as in extent, and while, in general, its light gradually fades away, it sometimes terminates quite suddenly. Such nebulosities are usually brighter and more condensed towards the central star. The stars thus surrounded do not seem, however, to be distinguished from others by any additional peculiarity. Some nebulæ of this kind are round, with one star in the centre; others are oval and have two stars, one at each of their foci. The nebulous star, _τ_ Orionis, represented at the upper part of Plate XV., above the great nebula, has a bright star at its centre and two smaller ones on the side. The association of double stars with nebulæ is very remarkable, and may in some cases indicate a mutual relation between them. The so-called planetary nebulæ derive their name from their likeness to the planets, which they resemble in a more or less equable distribution of light and in their round or slightly oval form. While some of them have edges comparatively sharp and well defined, the outlines of others are more hazy and diffused. These nebulæ, which are frequently of a bluish tint, are comparatively rare objects, and most of those known belong to the southern hemisphere. When seen through large telescopes, however, they present a different aspect, and their apparent uniformity changes. The largest of these objects, No. 2,343 of the General Catalogue, is situated in the Great Bear, close to the star _β_. Its apparent diameter is, according to Sir J. Herschel, 2', 40", and "its light is equable, except at the edge, where it is a little hazy." In a study which I made of this object in 1876, with a refractor 6⅓ inches in aperture, I found it decidedly brighter on the preceding side, where the brightest part is crescent shaped. In Lord Rosse's telescope its disk is transformed into a luminous ring with a fringed border, and two small star-like condensations are found within. Another planetary nebula, near _χ_ Andromedæ, has also shown an annular structure in Rosse's telescope.