1. STRUCTURES IN THE UNIVERSE

Antiquity and Middle Ages have seen attempts of visualizing worlds beyond our world. The concept of the plurality of solar systems marks the beginning of modern times. Yet, the first to actually picture - in the literal sense - a cosmos of organized stellar systems appears to be Thomas Wright of Durham. Fig. 1 is taken from his book ``An Original Theory of the Universe'' (1750).

Figure 1. From the 9th letter of Thomas Wright's ``An Original Theory of the Universe'' (1750) Plate XXXI, about which he writes: ``. . . that as the visible Creation is supposed to be full of sidereal Systems and planetary Worlds, so on, in like similar Manner, the endless Immensity is an unlimited Plenum of Creations not unlike the known Universe. See Plate XXXI. which you may if you please, call a partial View of Immensity, or without much Impropriety perhaps, a finite View of Infinity ... That this in all probability may be the real Case, is in some Degree made evident by the many cloudy Spots, just perceivable by us, as far without our starry Regions, in which tho' visibly luminous Spaces no one Star or particular constituent Body can possibly be distinguished; those in all likelyhood may be external Creation, bordering upon the known one, too remote for even our Telescopes to reach.''

During the same century first attempts were made to catalogue nebulous objects, especially those which are not resolved into stars (the five original ******************** - nebulous stars - of Ptolemaios were clusters or loose groups of stars). Within little more than a century the ``Catalogue of Nebulae and Clusters'' (J. Herschel 1864) had been assembled: the work of a single family - William, Caroline and John Herschel. The first complete picture of the distribution of nebulae, which are not obviously associated with the Milky Way, based on the New General Catalogue and the two Index Catalogues (Dreyer 1888, 1895, 1908), was published by Charlier (1922). Fig. 2 shows his presentation of 11 475 nebulae. The inhomogeneity in the distribution, by then long recognized (W. Herschel 1811), is clearly apparent. Among the counts of nebulae made in the early 20th century Fath's list (1914) obtained from photographs of 139 selected areas is mentioned here, because of the extensive interpretation of the data by Seares (1925) and his comment ⁽¹⁾:

``Further, the Selected Areas are too widely spaced for a satisfactory determination of the effect of local irregularities in distribution; but, in spite of the limitations, the data merit special attention because of the freedom from any selection favoring regions in which nebulae were known to exist.''

Both considerations are important because they are still disputed in connection with modern surveys. More detail on the early history of mapping nebulae is given e.g. by Lundmark (1927) and Flin (1988).

By the mid-twenties about 10,000 mostly faint galaxies had been accumulated in the Heidelberg nebular lists (No. 1-15, Wolf 1901, 1914, 1916; continued by Reinmuth, 1916, 1940).

A sample of 44,000 galaxies was available by the mid-thirties. Excesses and deficiencies of galaxies in certain areas were discussed (Hubble 1934); Fig. 3 is taken from his paper.

The largest total sample - before the advent of the Lick Survey - was accumulated at the Harvard College Observatory under the leadership of Harlow Shapley. A plot of 78,000 from a total of 392,780 is shown in Fig. 4. Based on this material Shapley (1938) first claimed that structures on such large scales suggest ``gradients'' rather than clustering.

Figure 2. Charlier's map of the nebulae (Charlier 1922). ``A glance at this plate suffices for stating how the Milky Way, which is designed by the great axis of the chart is systematically avoided by the nebulae. A remarkable property of the image is that the nebulae seem to be piled up in clouds (as also the stars in the Milky Way). Such a clouding of the nebulae may be a real phenomenon, but it may also be an accidental effect. . .''

Figure 3. Hubble's distribution of 44,000 galaxies (Hubble 1934). ``Distribution of extra-galactic nebulae when observed data are corrected for the latitude effect. Small crosses represent normal distribution (log N = 1.82 - 2.11); small disks and circles, moderate excesses (log N = 2.12 - 2.26) and deficiencies (log N = 1.67 - 1.81); large disks and circles, considerable excesses (log N = 2.27 - 2.56) and deficiencies (log N = 1.37 - 1.66), with crosses added for log N > 2.56 and log N < 1.37. Feilds with no nebulae are omitted.''

Figure 4. Part of the Harvard Survey (Shapley 1957). ``Plot of 78,000 individual galaxies in the Canopy area.'' ``. . . the far extension of the `Cepheus flare' or cloud of absorbing material... comes out of the Milky Way. This flare of absorption covers the north celestial pole. Supported by this survey is the evidence that from galactic latitude +40° to the north galactic pole there is no appreciable net increase of population density with latitude.''

Figure 5. First section of the Lick Survey (Shane and Wirtanen 1954). ``Total area available. Contour map, showing smoothed numbers of galaxies per square degree.''

Figure 6a. Princeton presentation of Lick Catalogue (Seldner et at. 1977). ``Map of galaxy counts in the northern galactic hemisphere. The north galactic pole is at the center, the galactic equator is at the edge, and galactic latitude is a linear function of radius. Galactic longitude increases in the clockwise direction with lII = 0° at the bottom of the map.''

Figure 6b. Princeton presentation (cont.) ``Map of galaxy counts in the southern galactic hemisphere. Galactic longitude increases in the counterclockwise direction from lII = 0° at the bottom of the map.''

The last one of the catalogues assembled without the use of automatic procedures is the Lick Survey, described by Shane and Wirtanen in 1950 and completed 17 years later (Shane and Wirtanen 1967). Even the first results presented by the authors in 1954 made an immense impact, stimulating theoretical work by Limber, Neyman and Scott (Sect. 2.3.1). The first Lick survey map is shown in Fig. 5. Also important because of its far reaching influence was the Princeton presentation of the Lick catalogue and its analysis made 10 years later (Seldner et al. 1977). It is shown for the northern and southern hemispheres in Fig. 6.