1. Standard classification. This is the Mt. Wilson classification as used by Hubble between 1925 and 1935; it has been so often described [A, B, D] that only a short summary will suffice here. It is illustrated by Hubble's well known ``tuning-fork'' diagram (Fig. 1).

(i) Elliptical nebulae (E), range from circular or globular objects, such as NGC 3379, to elongated, lenticular objects, such as NGC 3115. As a rule they show no structural details, besides a small, bright and strongly condensed nucleus around which the textureless nebulosity decreases smoothly outwards in all directions to an indefinite edge where it fades into the general luminosity of the night sky.

Fig. 1. Standard classification: Hubble's tuning-fork diagram (1925).

Sub-types are defined by the index n = 10(1 - b/a), if a, b are the apparent major and minor axes measured on photographs. The most strongly elongated objects, type E7, such as NGC 3115, depart notably from a geometrical ``elliptical'' shape, being pointed near the ends of the major axis.

(ii) Normal spirals (S) show the characteristic spiral arms when seen pole-on and, as a rule, a ``spindle'' shape with heavy absorption lanes of dark matter when seen edge-on. In the normal spirals the arms emerge tangentially from a bright central nucleus at opposite points on its indefinite edge and vanish after about one complete turn of the best fitting logarithmic spiral [12]; (cf. Sect. 6). In the more regular or classical examples only two main arms, very nearly symmetrical with respect to the nucleus, are present. In most cases, however, additional or secondary arms may exist and the spiral pattern is often far from regular.

Sub-types, noted a, b, c, are defined by the relative importance of the nucleus (decreasing from a to c) and the degree of unwinding and resolution of the arms (increasing from a to c). According to Hubble [25]:``the arms appear to build up at the expense of the nuclear regions and unwind as they grow; in the end the arms are wide open and the nuclei inconspicuous. Early in the series the arms begin to break up into condensations, the resolution commencing in the outer regions and working inwards until in the final stages it reaches the nucleus itself'' [25], p. 326). The resolution referred to is into blue supergiants and emission objects characteristic of a Type I population. The gradual decrease of the axial ratio nucleus/spiral arms is best seen in edgewise systems (see Plate VIII), while face-on systems show more clearly the increasing resolution and irregularity from ``early'' types (Sa) to ``late'' types (Sc) (Plate V)¹.

Intermediate types: Hubble also introduced the notion of ``lateral'' extension or width of the classification sequence in its intermediate section, giving M81 with ``large nuclear region and thin, rather open arms'' and M94 ``having smaller nuclear region with closely coiled arms'' as extreme cases. This distinction has been little used in practice.

Of more importance was the recognition of objects intermediate between normal and barred spirals, such as M83 and M61 which have been classified alternatively as Sc or SBc. Their intermediate characteristics first noticed by Hubble ([B], p. 46) and Lundmark [43], [44] have been discussed by Lindblad and Langebartel [41] ². They more or less fill the gap between the two branches of the tuning-fork diagram.

(iii) Barred spirals (SB), include the ``pin-wheel'' or ``-type'' first described by Curtis [9]. In it a very bright central nucleus is crossed diametrically by a bar at the extremities of which spiral arms start at right angles (in ``late'' sub-types) or tangentially from the rim of a continuous ring of which the bar is a diameter (in ``early'' sub-types). Additional or secondary arms may exist, but as a rule the symmetry of the pattern is more regular than in normal spirals (see Plate VI).

Sub-types, noted a, b, c, are defined as for the normal spirals by the relative size of the nucleus and the degree of resolution and opening of the spiral structure. In Hubble's original system the ring, closed in the SBa and SBb sub-types, opens at SBc, producing the aspect sometimes described as ``S-shaped'' spirals. SBa objects observed under various angles give rise to singular ``Saturn-like'' shapes (Plate X).

(iv) Irregulars (I), were described originally by Hubble [25] as a class of objects ``lacking both dominating nuclei and rotational symmetry'' and of which ``the Magellanic Clouds are the most conspicuous examples'' ([25], p. 328). These were termed more specifically ``Magellanic nebulae'' by Lundmark [43], [44]. However, the class was broadened by Hubble to include peculiar or chaotic objects which ``do not find a place in the sequence of classification'' since ``the remaining irregulars might be arbitrarily placed in the regular sequence as highly peculiar objects, rather than in a separate class ... Others, such as M82, are merely nondescript'' ([B], p. 47).

In fact, the symbol I has often been used as almost equivalent to the subscript p for peculiar; such an extension of the notation is both confusing and unwarranted. As a result, the relation of ``irregulars'' to other sections of the classification sequence was not clear, some of them being clearly related to late-type spirals and others to early-type spirals.

From 600 bright galaxies in the Lick and Mt. Wilson plate collections Hubble [25] found the following apparent relative frequencies:

Hence, the spirals constituted about 80% of the sample, with (S + SB) / E = 4 or 5 and, further, a ratio S / SB = 2 or 3.

However, Shapley and Ames³ found that in the Coma-Virgo region for m < 12 spirals comprise only 46% of the population and for m < 14, S = 48%, E = 47%, I = 5%.

These early statistics, although not necessarily inconsistent, were affected by selection effects arising from clustering and different interpretations of some nebular types not included in the standard classification. Frequencies based on revised types are given in Sect. 3c.