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: