The following lines give a summary of the work of Kapteyn’s Plan of Selected Areas from January 1938 up to September 1947.

Since the last meeting of the Union there has been considerable development in the field covered by the Commission, and the following report, which has been drawn up on the basis of reports by the members of the Commission, can only summarize very briefly some important lines of investigation in the recent development.

Photometry and Spectral Investigations in Various Regions of the Galaxy. In addition to the work in Kapteyn’s Selected Areas, systematic investigation is in progress for a great number of galactic regions, and it seems to be an urgent need that such investigations be co-ordinated as effectively as possible in the future.

This Commission was organized at the Stockholm meeting of the Union in 1938, and now after ten years has the opportunity to make its first report. The rearrangement of the fields of the different Commissions at Copenhagen in 1946 gives Commission 34 some responsibility for the entire volume of space between the stars, if not between the galaxies. Since the presence of interstellar material in any region of space may be revealed by emission, reflection, scattering, or absorption of radiation, the study of this material falls under or overlaps the activities of half a dozen other Commissions of the Union. Because of this extent of the relations of the Commission we may consider again the directions in which international co-operation may function to advantage. As pointed out by Schlesinger when the Union was organized, such co-operation is valuable in matters of convention; in avoidance of duplication; and especially in investigations too large for one institution or even one country to undertake.

The study of the internal constitution of the stars has advanced rapidly in the past few years, especially for those of the main sequence. The present summary is necessarily brief.

For a star of uniform composition, whose interior is in radiative equilibrium, with or without a convective core, the constitution can be completely determined by successive approximations by quadratures, if there are given

(a)

1. the atomic composition

2. (b) the equation of state, defining the mean molecular weight μ

3. (c) the equation defining the opacity for the outflowing radiation

4. (d) the equation defining the rate of liberation of energy per gram.

By publishing at each meeting a report on the progress of astronomical spectrophotometry, our Commission has done in the past work which has been appreciated. Such a report may be expected to be of special usefulness in the present circumstances, after the disruption of normal communications for so many years. For this purpose, an attempt has been made to give a list of the most important publications, as a bibliographical appendix to this report. The titles are quoted in abridged form. My thanks are due to many members of the Commission, who contributed materially to this list and helped me with important information. I apologize for omissions, nearly inevitable in this post-war period. In future some restriction of the field of our activity ought to be considered; spectrophotometry is nowadays so general an astrophysical method, that it is nearly impossible to give a more or less complete survey of its applications to all different fields of our science. This report was finished on January 1, 1948. A few recent publications have been included in the bibliographical list.

Unlike the investigations in many fields of astronomy, the work on star clusters offers few problems for practical international or inter-observatory co-operation. But there are two useful projects that the International Commission can undertake: the reporting of the researches in progress at the various observatories, and the co-operative listing of useful categories of research on star clusters. The list may serve as a guide for new workers in the field. Also there is, of course, much value in the mutual discussion of researches. Such interchange of ideas may be the most useful outcome of the Commission’s conferences at the Zürich meeting of the International Astronomical Union.

1. At the Meeting of the Executive Committee of the Union held at Copenhagen in March 1946 the following resolutions were adopted:

(a)

1. That a standing Commission be appointed for facilitating the exchange of astronomers.

2. (b) That the attention of U.N.E.S.C.O. be drawn to the existence of this Commission as an advisory body to whom U.N.E.S.C.O. could refer questions of the exchange of astronomers.

At this time (March 15, 1948), it is possible to make only a preliminary report on the development of the project of International Astronomical Observatories. Excellent discussions of the proposals, in response to an outline by the President of the Commission, have been received from Dr Ira Bowen and Dr Walter Baade (of Mount Wilson and Palomar); Sir Harold Spencer Jones (Greenwich); Dr R. O. Redman (Cambridge, England); Dr Otto Struve (Yerkes and McDonald); Dr Bertil Lindblad (Stockholm); Dr Enrique Gaviola (Córdoba); Dr M. Waldmeier (Zürich); Dr G. A. Shajn (Simeis); Dr Joseph Witkowski (Poznań); Dr B. Lyot (Paris). At my request Dr J. S. Paraskevopoulos, Superintendent of the Harvard South African Station, has reported on South African sites. All these reports should eventually be printed or at least fully summarized and mimeographed for the use of the International Astronomical Union. Of special value are Lyot’s consideration of atmospheric conditions; the arguments of Bowen, Baade, Spencer Jones, and others in favour of a modest beginning with provision for growth when needs and directions are clearer, and Witkowski’s reconsideration of our earlier plan for an International Observatory and Laboratories in southern Europe.

The observations were made with a stellar spectrometer containing a Cashman PbS cell as receiver constructed in 1946 for the 82-inch telescope. The radiation is interrupted 1080 times per second; the small A.C. signal produced by the cell as a result of the fluctuating resistance and 90 V. D.C. voltage is amplified up to 30 million times, rectified, and recorded on a D.C. milliammeter.

This report deals mainly with the study of spectra secured with a 4° objective prism attached to the 24–36-inch Schmidt-type telescope of the Warner and Swasey Observatory of the Case Institute of Technology. The dispersion at Hγ is 283 A./mm. Spectra obtained with a 2° objective prism will be dealt with briefly. We will consider first spectra secured by means of long exposures intended to be used for statistical studies; second, spectra of relatively short exposures for detailed study of individual stars and, third, infra-red spectra for the classification of low-temperature stars.

The analysis of the Sun by spectroscopic methods refers to an exceedingly thin layer, just above the photosphere, which has a thickness of only a few hundreds of kilometers. The deeper layers are inaccessible to our investigations, because there the matter becomes too opaque. The higher layers of the chromosphere and the corona again are less well known, because they are too transparent, so that deviations from thermodynamic equilibrium conditions may occur. So it is understandable that just the layer where the equilibrium is nearly established must be the only layer which can be fully investigated. Probably this is the best analysed sample of the universe. Whether the composition here found is really representative for the Sun as a whole depends on the importance of the convection, which tends to stir the gases into one homogeneous mixture.

A priori it would seem possible to determine the solar abundances of the H, C, N and O atoms on the basis of the solar abundances of the CN, CH, C2, NH and OH molecules. Such a determination entails two distinct problems:

1. (1) The determination of the molecular abundances with the help of the measured intensities of individual rotational lines.

2. (2) The calculation of atomic abundances on the basis of the equations of dissociation equilibrium.

The quantitative analysis of stellar atmospheres requires—if we may put it somewhat paradoxically—already considerable knowledge of their composition and structure. In practice it is therefore necessary to begin with the simplest problems and even there to proceed in successive approximations.

Minnaert explained the latest results on the measurements of the darkening at the sun’s limb obtained from plates taken with the Lick refractor in two different wavelengths.

UNSÖLD gave a new theory on the variation centre-limb of the intensity of the faint lines in the solar spectrum, and of the variations of the wings in the intense lines. He deduced a new law for the variation of the temperature near the limb. By means of this new theory it is possible to interpret the results of Miss Adam and Redman (published in the Memorie Società Astronomica Italiana, 20, 49, 1949).

1. 1. L’Union Astronomique Internationale a pour but:

   1. (1) de faciliter les relations entre les astronomes des divers pays lorsqu’il est utile ou nécessaire d’avoir recours à une coopération internationale;

   2. (2) de favoriser l’étude de l’astronomie dans toutes ses branches.

2. 2. L’Union adhère au Conseil International des Unions Scientifiques.