4. Description of typical examples. The following eleven plates illustrate the revised classification system and will serve to support the description of the main morphological characteristics used as classification criteria. Since the revised three-dimensional classification recognizes at least 16 stages along the sequence from E to Im and 9 or more pure or mixed types at each stage across it (at least in the spiral sequences), a fully illustrated description, allowing also for tilt and orientation effects, would require several hundred examples. However, the main criteria used to define the position of any normal galaxy along and across the classification volume can be illustrated with only a fraction of the total number of possible cases.

Plates I and II describe the main criteria used to distinguish between families and varieties in a central cross-section near the stage Sb.

Plates III and IV illustrate the ellipticals and principal stages of the lenticular sequences.

Plates V and VI illustrate the four main stages along each of the four principal sequences of ordinary and barred spirals.

Plate VII shows the (magellanic) irregular later stages of the spiral sequences.

Plates VIII and IX illustrate the appearance of edgewise system of the lenticular, spiral and irregular classes.

Plate X illustrates the effects of tilt and orientation on the appearance of SB (s) a or SB (r) a spirals.

Plate XI shows some peculiar lenticulars and (non-magellanic) irregulars of the M 82 type.

In order to minimize as far as possible the complications arising from differences in scale and resolution, several sources of illustrations were used as follows:

1. Mt. Palomar 200-inch reflector (P 200") and 48-inch telescope (P 48").

2. Mt. Wilson 100-inch and 60-inch reflectors (W 100"; W 60") ¹.

3. Mt. Stromlo 74-inch and 30-inch reflectors (S 74"; S 30").

4. Isaac Roberts 20-inch reflector (IR 20") ².

For technical reasons all photographs are reproduced here as inverted negative prints, i.e. are mirror images of the objects as they appear on the sky, and the orientation with respect to the celestial coordinates is arbitrary.

The classification criteria are described in relation to the three main parts of a galaxy defined by Hubble:

a) the nucleus, i.e. the very small, very bright central condensation often sharply defined as the center of symmetry of the structure; it is round in SA, elliptical in SB.

b) the lens, smooth, bright and sharply defined in the lenticulars and early spirals, being crossed by the bar in SB and brighter at the edge in S (r).

c) the envelope, generally faint and smooth with indefinite outer boundary in the lenticulars, brighter and occupied by the spiral arms in spirals. The outer (R) structure appears often in its outer parts near the stage S0/a.

The distinction between these three regions is most definite near the middle of the classification sequence (S0/a) and vanishes at E and Im.

The spirals of the ordinary family SA are above, of the barred family SB below; the ringed variety S (r) is to the right, the S-shaped variety S (s) to the left. The mixed types SAa (rs) are in the center. There is a continuous transition between each type and only the main forms are illustrated; actual objects can occupy any intermediate position in the plane (volume).

The main classification criteria are as follows:

SAB (s) : Has a fairly small, bright, elongated nucleus crossed by a weak, twisted dark lane in the center of a weak and broad bar marking the major axis of a faint, diffuse and elongated lens with much spiral pattern of dark matter having a tendency to run parallel to the bar. Two main arms emerge near the extremities of the bar along a smooth, curved path turning sharply just outside the lens; their faint extremities tend to return to the lens along an almost circular outer loop, simulating an (R) structure in the early stages. Examples are NGC 1566 (illustrated from S 30"), NGC 5236 (M83), Sbc, NGC 4579, Sab and NGC 7392.

SB (s) : This is the typical ``S-shaped'' barred spiral; it has a small, very bright and elongated nucleus distorted by a strong, twisted dark lane as it crosses over from one side to the other of the strong, narrow bar marking the major axis of a much elongated lens. Two strong, main arms start sharply at right angles to the bar at both ends and return faintly to it after completing a turn of a quasi-circular loop. Examples are NGC 1097, NGC 1300, NGC 1365 (illustrated from S 30"), NGC 5383. Later stages in this sequence are markedly asymmetrical as NGC 7479, Sc and NGC 7741, Scd (31). Some, like NGC 1300, have a third, fainter arm forming a half-ellipse close to the lens and indicating a transition toward the SB (rs) type.

SB (rs) : has a fairly small, very bright and elongated nucleus crossed by a twisted dark lane, as in SB (s), in a very strong, very narrow bar marking the major axis of an elongated lens whose rim is brighter, especially near the extremities of the bar; this produces a characteristic ``dash-dot in brackets'' pattern, thus (- o -) from which emerge two main spiral arms with faint additional branches near the rim of the lens. Examples are NGC 4548, NGC 4593, NGC 7124 (illustrated from S 74"). Some, like NGC 4349, Sab, having a brighter lens and more continuous rim mark the transition toward SB (r).

SB (r) : This is the typical ``a-shaped'' barred spiral. It has a fairly large, elongated nucleus with weak spiral dark lane in a strong, narrow bar along the major axis of an elliptical ring marking the edge of the lens. Two main arms start tangentially from the ring near the extremities of the bar (i.e. at right angles to the bar); one or two fainter arms branch out from breaks in the ring near its minor axis. In early stages the main arms tend to form an outer, circular (R) structure, as in NGC 1433 (25); in later types the breaks in the ring tend to produce slightly hexagonal shapes. Examples are NGC 1433 (illustrated from S 74"), Sa or Sab, NGC 3185, NGC 3351, Sab (26), NGC 2523, Sbc (27).

SAB (r) : has a fairly small, little elongated nucleus in a fairly broad and faint bar marking the major axis of a little elongated ring from which several spiral arms branch out. The main arms have a slight tendency to ``return'' inwards. Examples are NGC 1832, NGC 7531, Sab, NGC 6744, Sbc (illustrated from S 30"). Some, like NGC 6902, NGC 6935, Sa, NGC 6937, have only very faint traces of a bar and mark the transition towards SA (r).

SAB (rs) : this is the most general mixed type involving all possible transitions between the main typical patterns. An almost infinite variety is possible here, but for classification purposes the main characteristics of this hybrid type are: a small bright nucleus in a broad, diffuse bar with some spiral structure in the lens. The bar crosses a nearly circular or often hexagonal pseudo-ring formed by the inner sections of the spiral arms. A very good example is NGC 4303 (M61), Sbc (illustrated from IR 20"); other examples are NGC 3145, NGC 6814. At a later stage this mixed structure is well illustrated by the central regions of NGC 5457 (M101) and NGC 6946, both Scd.