The classification used in the present investigation is essentially the detailed formulation of a preliminary classification published in a previous paper.[6] It was developed in 1923, from a study of photographs of several thousand nebulae, including practically all the brighter objects and a thoroughly representative collection of the fainter ones.[7] It is based primarily on the structural forms of photographic images, although the forms divide themselves naturally into two groups: those found in or near the Milky Way and those in moderate or high galactic latitudes. In so far as possible, the system is independent of the orientation of the objects in space. With minor changes in the original notation, the complete classification is as follows, although only the extra-galactic division is here discussed in detail: CLASSIFICATION OF NEBULAE I. Galactic nebulae: Symbol Example A. Planetaries P N.G.C. 7662 B. Diffuse D 1. Predominantly luminous DL N.G.C. 6618 2. Predominantly obscure DO Barnard 92 3. Conspicuously mixed DLO N.G.C. 7023 II. Extra-galactic nebulae: A. Regular: / N.G.C. 3379 E0 1. Elliptical En / 221 E2 (n = 1, 2, ..., 7 indicates the \ 4621 E5 ellipticity of the image \ 2117 E7 without the decimal point) 2. Spirals: a) Normal spirals S (1) Early Sa N.G.C. 4594 (2) Intermediate Sb 2841 (3) Late Sc 5457 b) Barred spirals SB (1) Early SBa N.G.C. 2859 (2) Intermediate SBb 3351 (3) Late SBc 7479 B. Irregular Irr N.G.C. 4449 Extra-galactic nebulae too faint to be classified are designated by the symbol “Q.” REGULAR NEBULAE The characteristic feature of extra-galactic nebulae is rotational symmetry about dominating non-stellar nuclei. About 97 per cent of these nebulae are regular in the sense that they show this feature conspicuously. The regular nebulae fall into a progressive sequence ranging from globular masses of unresolved nebulosity to widely open spirals whose arms are swarming with stars. The sequence comprises two sections, elliptical nebulae and spirals, which merge into each other.