Calculations have been carried out of single light scattering by Rayleigh type spheroidal dust particles. It can be shown that elliptical polarized radiation with e = v/p ≈ 0.15 can be produced from unpolarized incident radiation, in the case of dirty quartz particles with a refractive index m = 1.54 ± 0.4i and a geometrical axial ratio c: a ≈ 1:2.

An elliptical Surface polarimetry of the Night Sky has been carried out at the Boyden Observatory, South Africa. The Milky Way is linearly polarized by p = 1 to 2 % (Figure 1); 11 regions also show a circular polarized light contribution (Table I).

The amounts of circular polarization near v ≈ 0.1 % or less, observed within the Zodiacal Light, are generally not exceeding twice the error of measurement: see Staude and Schmidt (1972). (v > 0 is defined by clockwise rotation, looking against the light source).

A review of observational data on interstellar polarization is given. The degree of fluctuation in the direction of the galactic magnetic field is discussed. An empirical formula is given which fits all the observations of the wavelength dependence of interstellar polarization when they are normalized with the wavelength λmax of maximum polarization. Well-defined regions on the sky with values of λmax deviating from an average are found. Values of λmax are correlated with the ratio of total to selective interstellar extinction. A unified wavelength dependence of interstellar extinction is obtained by normalizing the observations with the wavelength λmax of maximum interstellar polarization.

Photometric and Polarization measures of fainter stars (m > 11) are still very few, especially in the red to infra-red region. In order to exploit large Schmidt cameras to remedy this shortage, improved methods of photographic calibration are needed. This note reviews some of the problems associated with photographic calibration of direct images, and presents a progress report on the development of the ring polarimeter.

This paper shows that optical observations of circular polarization produced by aligned interstellar grains could yield valuable information about the grain material. The interstellar medium is known to be linearly dichroic from observations of interstellar linear polarization; many different grain models using a large variety of compositions can be found to reproduce these observations. Since the same aligned grains make the medium linearly birefringent, a small component of circular polarization can result from incident linearly polarized light if the position angle of the linear polarization does not coincide with either principal axis of the medium. Here calculations are presented to demonstrate that the wavelength of the circular polarization is sensitive to the imaginary part of the complex refractive index of the grain material. This provides an opportunity of investigating whether the grains are characteristically dielectric or metallic. Some possible observations are suggested.

A search in many small regions of the Crab Nebula has resulted in the detection of a small component of circular polarization. The variation of the sign and magnitude with position in the Nebula indicates that the polarization is of interstellar origin. On the basis of the polarity, strength, and colour dependence, it is concluded that the composition of the aligned grains causing this polarization is dielectric. Metallic particles are clearly ruled out.

Circular polarization of interstellar or perhaps circumstellar origin has been clearly established in two reddened stars in the Upper Scorpius region, o-Sco and σ-Sco A; and detected preliminarily in a third, HD 154445. In the Corning 5–60 filter band λλ 3800–4600, measured values of 104q were: o-Sco, –(2.9 ± 0.2); σ-Sco A, –(1.7 ± 0.2); and HD 154445, +(2.8 ± 0.8). Complete wavelength dependences q(λ) for o-Sco and σ-Sco A are under study, but so far are known to have the following similarity. Each shows a broad maximum (of negative q) centered in the range λλ 3800–4300; and the values are smaller by a factor ~ 4, but still negative, in the range λλ 5000–6000. The effect here is most likely due to a twisting grain alignment along the line of sight. In that case a simple model predicts q(λ) ∝ (n′l – n′r) · (n″l – n″r) θ0z02, where n = n′ +i n″ is a formal, complex refractive index (in van de Hulst's forward-scattering approximation), l and r referring to longitudinal and transverse grain axes, and θ0 is the total twist angle over the interaction length z0. An alternate mechanism would be multiple scattering (at large angles, not along the line of sight) within asymmetrical, well-localized clouds around the individual stars. The similar sign and magnitudes of q for o-Sco and σ-Sco A, which are 1.5° apart in the sky, argue for the line-of-sight mechanism – for example through a large cloud in front of the entire local region. Comparative studies of many stars in Upper Scorpius will be of obvious interest.

This paper is devoted to a comparison between results obtained by Purcell and Spitzer (1971) using a Monte-Carlo method and by the author (1971) using a Fokker-Planck equation. It is shown that there is a good agreement between the results within the dispersion expected from the Monte-Carlo method.

Observations of infrared polarization have been tried in the galactic center. A preliminary result is that the polarization at K-band (2.2 μ) is less than 5 %, much less than that expected from interstellar polarization.

We have considered the cooperative effects of photon and magnetic alignment on plausible models for interstellar dust grains. Neither effect, either in cooperation or alone, appears able to produce the observed alignment.

We have plotted the differential polarization as a function of distance and galactic longitude for 1732 stars between galactic latitudes −5° and +5°. The data were obtained from several published sources. A fairly clearcut distinction is seen between the effects of spiral arms and inter-arm regions indicating that starlight is systematically polarized only when passing through spiral arms.

Several galaxies were observed for polarization during April and May 1972. Preliminary results are presented concerning those galaxies where polarization can be expected because of selective extinction or scattering of light by interstellar dust.

Special photographic observations reveal silhouetted dust patterns as close as 6″ from the nucleus. The spatial distribution and kinematics of the dust are discussed.

As a first step towards an empirical estimation of the minimum size of interstellar dust formation units (clouds etc.) the correlation between angular separation and difference in colour excess has been studied for a selection of close star couples with coinciding magnitude and spectral type. Principally, the result indicates that there is no palpable correlation between the difference in colour excess and the lateral separation of the stars. This and other properties speak in favour of the conception that obscuring matter in the Milky Way is concentrated in extremely small units of high density.

A 21-cm survey at Dwingeloo of a 5° square containing IC 1396 shows several neutral hydrogen concentrations that correspond in shape and position with dust clouds, particularly the bright-rimmed clouds involved with the H II region.

The hypothesis that interstellar dust is associated with cold gas is advanced. To test this, such observational features of the gas that depend predominantly on the cold gas have to be isolated. Two such features are discussed, the 21-cm line strength in absorption and the intensity of narrow emission line components in intermediate galactic latitudes. It is shown that both features have a good correlation with the dust extinction. Various measurements of the gas to dust ratio are collected and some implications for the interstellar medium are discussed.

Using the Shklovsky-Minkowski-Aller constant mass method of distances to planetary nebulae, a model of the Galactic dust distribution in the vicinity of the Sun has been determined. Distances are determined in part from extinctions derived from radio continuum to Hβ flux ratios for an assumed electron temperature of 7000 K. The distance scale is based on the brightness of planetary nebulae in the Magellanic Clouds (Seaton, 1968).

High frequency resolution 21-cm line observations of almost 100 interstellar dust clouds showed that about one-third of them contain significant amounts of cold neutral hydrogen. Simple models for the velocity distribution of hydrogen in the clouds were fitted to the observations, and spin temperatures ranging from 16 to 40 K and column densities from 7 × 1019 to 6× 1020 atom cm–2 were found. The ratios of H i to OH and H i to dust in the clouds were found to have about 1 to 10 % of their value in the general interstellar medium.

For 61 stars whose interstellar Lα absorption was observed by OAO-2, the average relation of H I column density to B – V color excess is equal to 5 × 1021 atoms cm–2 mag–1. For a recent dust grain model this value implies that the density ratio 〈ρHI/ρdust〉 ≈ 100. A good correlation was found for deviations in the measurements of NHI and E(B – V) from their mean values expected over the distances to the different stars, indicating an association of H I gas with dust.

Detailed measurements have been made of the H, OH and H2CO lines formed in the L1534 dense dust cloud; some H and H2CO observations have been made of the dust cloud lying behind the Orion cluster. These indicate a similar distribution of the 3 species throughout the dust clouds. Studies of H, OH and H2CO absorption features in clouds lying in front of bright radio sources provide estimates of the physical conditions in regions of molecule formation.