Critical Summary

If at the proper time a Source Book of Astronomy is prepared for the interval 1950 to 1975, or 1950 to 2000, the galaxies will receive a much greater share of attention than that accorded to them in this volume. The reason is that several large telescopes, both optical and radio, have come into operation since 1950, and others soon will be built. Also the metagalactic picture is growing in interest, and the measures thereof in accuracy. In fact, this section on galaxies might have been considerably enlarged through further selections from the many contributions published by Mount Wilson, Lick, and Harvard astronomers.

The Magellanic Clouds, which must be rated as the most fruitful of all external galaxies, are not here discussed as such, although the Harvard Observatory has published scores of papers on this pair of exciting systems. The next Source Book will be more generous to the Clouds, for not only Harvard, but a half-dozen other well-equipped observatories will give the Clouds serious attention; they will turn on them the analytical power of radio, fast spectroscopes, polarimeters, and electronic photometers. Indirectly the Clouds make their debut in this Source Book through Miss Leavitt’s classic investigations of the Small Cloud’s cepheids (selection 32).

Among other significant contributions not included in this part on galaxies is the work of Sinclair Smith on the mass of the Virgo Cloud, and of K. Lundmark and his associates at the Lund Observatory on the classification and distribution of galaxies. The identification of spiral nebulae as star-composed galaxies was the work of many from Immanuel Kant in 1755 to H. D. Curtis, K. Lundmark, E. P. Hubble, and others. No specific account is given here of this activity. Also not included are the Spitzer-Baade speculations on the collisions of galaxies (their paper was published in 1951); the census-taking by Max Wolf and by C. W. Wirtz in Germany; the early descriptive photographic studies by Curtis and visual surveys by Bigourdan; the work of Shapley and colleagues on metagalactic population gradients; the examination by Carpenter, Page, Holmberg, and Zwicky of double galaxies and their connectors; the classification scheme by Hubble; the beginning of large-scale survey work by Zwicky and colleagues with the Palomar schmidt telescopes and by Shane at the Lick Observatory with the Carnegie telescope; and the early radial velocity work by Slipher at Flagstaff

and the later programs by Mayall on Mount Hamilton. The practical reasons for not including some of these have been noted in the general introduction to this volume. Certainly the next Source Book of Astronomy, which may be more technical and specialized, will have much to say about galaxies.

The articles which appear in this part deal with the discovery of the center of the galaxy, galactic rotation, and the expanding metagalaxy. Since 1950, additional work has also been done on these three problems, but, in general, the conclusions here presented stand, notwithstanding some numerical improvements.

In the three papers from Mount Wilson the term "extragalactic nebula" is used instead of the now more common term "galaxy."