The need for observing at least four different categories of optical objects for a satisfactory definition of optical spiral structure is emphasized. A sequence of optical form-types for the description of spiral structure is outlined. A pronounced difference in the nature of the spiral structure of the outer and inner parts of Messier 33 is illustrated. Striking local differences between spiral arms defined by differing criteria are noted.

Time and again Dr. B. Bok publishes his reviews on the state of optical searches of the spiral arms in our Galaxy. A great optimist at the beginning, concerning the agreement of different results, he became later more critical in his conclusions. But in his paper of 1967 he claims anew, as well established, that we can trace the spiral arms up to a distance of 14000 pc from the centre.

Apparent relative frequencies of various types of spirals are given for 900 spirals with the best revised Hubble classifications. Mean diameters of inner ring structures vary from 2.1 kpc in ordinary spirals (SA) to 4.4 kpc in barred spirals (SB) with a total range of 10 to 1 within each type. The probable morphological type of our galaxy is estimated from 6 criteria (multiplicity of spiral pattern, inner ring diameter, broken ring structure, radio structure of nucleus, Yerkes type, HI diagram); arguments advanced in 1963 for an SAB(rs) structure of the inner regions of the galactic system are strengthened by this analysis. Several examples of galaxies in this area of the classification plane are discussed.

Seventeen Sc- galaxies have been studied in order to determine the distribution of dark nebulae within them. Hα plates were used to compare the distribution of obscuring material with the location of H II regions, and it was found that the H II regions are always tangent to or imbedded in regions of high obscuration. Sandage's conclusions are confirmed that strong, regular dust lanes exist in the central regions of these galaxies and wind outward on the inside edge of the two prominent arms. Characteristic dimensions of the dust lanes are noted, and secondary dust characteristics are described.

The angular momenta of galaxies may be evaluated from their photometry. The mass to light ratio in NGC 224 is discussed and the calculation is applied to two late-type spirals.

The characteristic μ angles for the spirals of 6 multi-arm galaxies (NGC 1232, 5247, 4303, 4321, 3938, 3184) have been determined. It has been found that they vary along spiral arms within a rather large interval, with mean square deviations ranging from 4° to 20°. The mean μ for each arm deviates in a relatively small degree from the mean μ of the whole galaxy.

The authors apply the normal logarithmic spiral model in estimating the number of arms in our Galaxy. On the basis of this model the number of spiral arms for the above galaxies have been determined and compared with the actual number. It is shown that the normal logarithmic spiral model is unsuitable for our purposes. The number of spiral arms calculated in this way is not reliable.

The density distribution and the radial velocity field in the Andromeda galaxy, M 31, have been studied on the basis of the 21-cm radio-line data from Jodrell Bank and Green Bank. The true density has been obtained from the observed one by solving a two-dimensional integral equation. As the resolving power of the radio telescopes is too low to locate all spiral arms separately, optical data on the distribution of ionized hydrogen clouds have been also used. The mean radial velocities have been derived by solving a two-dimensional non-linear integral equation with the help of hydrogen densities, and a model radial velocity field.

The inner concentrations of hydrogen form two patchy ringlike structures with mean radii 30′ and 50′, the outer concentrations can be represented as fragments of two leading spiral arms.

The rotational velocity, derived from the radial velocity field, in the central region differs considerably from the velocity curves obtained by earlier authors. The difference can be explained by the fact that in this region the correction for the antenna beam width is much greater than adopted by previous investigators.

A model for the Andromeda galaxy, M 31, has been derived from the available radio, photometric, and spectroscopic data. The model consists of four components – the nucleus, the bulge, the disc, and the flat component.

For all components the following functions have been found: the mass density; the mass-to-light ratio; the velocity dispersions in three perpendicular directions (for the plane of symmetry and the axis of the galaxy); the deviation angle of the major axis of the velocity ellipsoid from the plane of symmetry; the centroid velocity (for the plane of symmetry).

Our model differs in two points from the models obtained by other authors: the central concentration of mass is higher (in the nucleus the mass-to-light ratio is about 170), and the total mass of the galaxy (200 × 109 solar masses) is smaller. The differences can be explained by different rotation curves adopted, and by attributing more weight to photometric and spectroscopic data in the case of our model.

(1) From new radial velocities of 67 H II regions in M 31, rotational velocities and a mass model of M 31 are derived, and compared with the rotation curve and Schmidt mass model of our galaxy. (2) It is shown that in M 31 the distribution of H II regions as identified by Baade agrees with the distribution of neutral hydrogen determined from 21-cm observations. Also, the rotation curve derived from the H II velocities outside of the nucleus is similar to the rotation curve derived from 21-cm H I observations.

Van den Bergh (1964) has selected and mapped OB associations in M 31 on the basis of plates taken with the Tautenburg 52 inch Schmidt camera. The selection was done by the blinking method. The task of the present investigation is to find if there exist OB stars (single or in association) on the outermost borders of this stellar system. For this purpose we measured the brightness of all stellar objects in UBV down to the magnitude 20m.0 (B) on Tautenburg plates in a special test field (Figure 1). It includes 0.26 square degrees and its centre has a distance of 104′ = 22 kpc from the centre of M 31. This test field includes too Baade's field IV with the photoelectric standards in UBV observed by Arp (Baade and Swope, 1963) and the OB associations OB 184, OB 185 and OB 186 of Van den Bergh. The total number of stars brighter than 20m.0 (B) was 996. The stars within the associations OB 184 and OB 185 are not included in these statistics. From this number, 704 objects could be measured in all 3 colours. We found among them 23 blue objects with U–B ≤ U− ≤ −0m.25. The mean error of the brightness on the basis of 4 plates in each colour does not exceed ±0m.0.7 for stars of 19m.0 (B).

Observations of continuum emission from M 31 have been made with the Cambridge One-mile radio telescope (Pooley, 1969). Two observing frequencies were used; maps of the whole of the visible nebula were obtained at 408 MHz, and spectral data for the central region at 1407 MHz. The results show that the radiation from the disc is confined to the nucleus and to the population I spiral arms.

The nuclear region may be described in terms of two spherical radio components, with diameters of 200 pc and 1 kpc. Any more compact source at the nucleus has a luminosity less than 1/20 of that of the source Sgr A in our Galaxy.

The intensity of the radio emission from the spiral arms is closely correlated with the number of H ii regions visible. The main spiral arms, corresponding to Baade's arms 4 and 5, cross the major axis at about 8 and 12.5 kpc from the nucleus. The spectrum of the radiation shows that it is non-thermal in origin; the spectral index is 0.8. The intensity is less than that which would be observed from our own Galaxy at the same distance.

First interferometric results on M 33 in the Hα line at a dispersion of 20 Å/mm, involving 1048 radial velocities, have already been published (Carranza et al., 1968). A new analysis with 3000 measures more will be published soon.

Radial velocities have been measured from the absorption lines on two image tube spectra of M 51. These velocities show large deviations from the ‘smoothed’ rotation curve for that object. The measurements seem to indicate that both the stars and the gas move in the same way.

Multicolour observations of galaxies are being carried out at the prime focus of the 2.6 m Schajn telescope using an image converter and 6–9 colour filters. The effective wavelengths are approximately 3600, 3730, 4400, 4680, 5090, 5280, 6090, 6600 and 7400 Å. The filters for 3730, 5090 and 6600 Å are centred on emission lines. For absolute calibration extrafocal star images are used.

The associations of NGC 6946 outline its spiral arms. There is no relation between the colour or magnitude of the associations and their distance from the centre. Their mean absolute magnitude is −11m.1 and their mean colour index near zero.

This paper discusses the evidence on the hydrogen spiral structure which is available from 21-cm observations. The problems involved in deriving a hydrogen map are discussed, and one interpretation of the spiral pattern is presented. Some of the major characteristics of the H I in spiral arms are discussed, and a comparison is made between the kinematics of H I and H II.

The distribution of optically observed H II regions and OB stars with galactic longitude indicates that it is primarily determined by extinction by interstellar dust. Thus optical observations can, at the best, reveal the local structure in the vicinity of the sun. Radio observations, on the other hand, are not affected by dust. Thus the distribution of galactic radio sources, which peaks in the northern part at about lII = 17°.5, must be related to the large-scale structure of our Galaxy. Two radio recombination line surveys of the northern and southern sky yield kinematic distances. If only the ‘giant H II regions’ are retained, the following distribution is obtained: (1) Only 5 giant H II regions are found within the 4 kpc arm. (2) The bulk of the giant H II regions is concentrated in a ring between 4 and 6 kpc from the galactic center. (3) There are other concentrations of giant H II regions indicating the existence of the Sagittarius and Perseus arm. (4) The three features revealed by optical observations of H II regions in the vicinity of the sun cannot be matched with the large-scale distribution outlined by giant H II regions. This is particularly true for the so-called Orion arm. (5) At distances beyond 13 kpc from the galactic center virtually no giant H II regions are found. (6) The surface density of giant H II regions attains its maximum between 4 and 8 kpc; the surface density of neutral hydrogen (H I) attains its maximum between 11 and 15 kpc, but the actual space density of H I in the region 4 to 8 kpc may still be rather high.