The secular perspective acceleration in the proper motion of a star is directly proportional to the parallax p, proper motion μ, and radial velocity V (van de Kamp, 1967):

The largest effect is known is for Bernard’s star, for which

The predicted perspective acceleration is:

The Wavelength Dependence of Polarization as observed in 32 stars, for which the Henry Draper numbers are given, is shown in figure 1. Details of some of these observations are presented in reference 1.

The equipment is now being used with the new 154-cm Catalina reflector of the Lunar and Planetary Laboratory at the University of Arizona. The instrumental polarizations are nearly zero. The data processing and observing techniques have been further improved; the precision is mainly determined by statistics such that the internal probable error in the percentage polarization is ±0.03 percent (±0.0006 magnitude) for a half-hour observation per filter on objects brighter than about 7 magnitudes. The wavelength λ ranges from 0.33 to 0.95 μ covered by seven filters of bandwidth of about 0.05 μ. The wavelength range is being extended to 1.2, 1.6, and 2.2 μ and, with high-altitude ballooning, to 0.28 and 0.22 μ.

In the recent Yale Zone Catalogues star charts have been included showing 20,000 year changes in star positions indicated by the proper motions of the catalogued stars. An inspection of such charts (e.g. Figure 1) reveals some widely scattered stars which seem to share large proper motions. In the Zone Catalogue for -40° to -50°, just published as Vol. 30 of the Transactions of the Yale University Observatory, we have indicated some fifty pairs or groups of stars appearing to have closely the same proper motions exceeding 0.”100. The numbers of stars represented in each group range from two to fifty. In general, the largest groups correspond to the smallest motions, where the uncertainties are also the greatest. The average number of stars per group decreases from eleven for proper motions between 0.”100 and 0.”200, to three between 0.”300 to 0.”500, to two for proper motions exceeding 0.”500.

The Interstellar Band at λ4430 has been observed in over 100 stars by using the Lick Observatory scanning spectrograph mounted on both the 120-inch telescope and on the Crossley telescope. Although in some instances the feature is badly blended with stellar lines, particularly with early B supergiants, in general the central depth of λ4430 has been determined to an accuracy exceeding 0.5 percent of the stellar continuum.

The feature has been found to have a fairly symmetrical profile, although there may be changes in the profile from star to star. The extended wings reported by other observers were not detected, and also the feature does not seem to be polarized.

The correlation between the absorption band at A.4430 and color excess is not one-to-one. Even field stars lying in the galactic plane show much intrinsic scatter in the ratio of this absorption band to color excess.

Regions Perturbed By Early-Type Stars are characterized by the occurrence of high Ca II velocities and an increase in the abundance ratio Ca II/Na I (ref. 1), by a decrease in the λ4430 absorption to color excess (ref. 2), and possibly by a deficiency of small relative to large grains (large values of the ratio of total extinction to color excess, A/E). According to reference 3, the grain temperatures in the same regions are expected to be higher, and therefore may lead to selective evaporation of the smallest grains. Since these observations may be connected by way of λ4430 absorption originating in very small grains, it is of interest to compare the value of nf (number of absorbers times flvalue) of the λ4430 absorbers with the number of molecules nmol in the grains responsible for extinction in the visible.

One characteristic of Schmidt telescopes is that the focal surface is spherical. This causes a flat photographic plate to be buckled and under strain while it is in the plate holder. In the early days of the space program, it was proposed to use Schmidt telescopes for the calibration of tracking stations because they were photographically fast. The writer opposed this proposal because they are not of high astrometric quality, for the reason indicated above.

For a Long Time the Davis-Greenstein Theory (ref. 1) has been generally considered an adequate explanation of the mechanism responsible for the alinement of interstellar grains and thus for the observed interstellar polarization. The magnetic field required to orient paramagnetic grains is apparently somewhat greater than 10-5 gauss, a relatively large field, but according to reference 2 ferromagnetic grains, proposed by the authors of reference 3, can be oriented by a field of only 10-7 gauss. These results indicate that orientation of interstellar grains is easily explained, even if the magnetic field in interstellar space is relatively weak.

Unfortunately, this relatively satisfactory state of affairs has been upset by two developments. Firstly, the basic mechanism of magnetic relaxation proposed by Davis and Greenstein has been questioned by Dr. C. Kittel of the University of California, who has pointed out that the disorienting effect associated with thermal fluctuations of magnetization within the solid particles is ignored in the formulation of this mechanism.

The recomputation of the plate constants for an astrographic zone requires a general catalogue of the positions of all of the comparison stars. Then on each plate the ideal coordinates, Xik and Yik, of each star, insofar as they depend upon its individual position inthe comparison star catalogue.

Of all the fields of stellar astronomy, the study of white dwarfs has been most influenced by work done on proper motions. From proper motion studies, particularly by Luyten and Giclas in recent years, many white dwarf candidates have been found. These candidates have been studied intensively, particularly by Greenstein who has obtained spectra and Eggen who has measured magnitude and colors. As a result, many new white dwarfs have been discovered, and we now have many examples of most types of white dwarfs. It is, therefore, possible to study each group separately and to find relationships between the various types of white dwarfs.

A Survey of the Polarization of Light by the nebulosity within 67 regions of the Pleiades cluster was made in early 1962. The survey was made without color filters. On two nights, however, some preliminary polarization measures were made with color filters; the results obtained in this investigation seemed to be promising enough to justify a more thorough study of polarization in three colors. The polarimeter used in 1962 was not suited for observations in the UV, however, because its polaroid analyzer would have absorbed most of the ultraviolet light.

In early 1963 color measures of nebulosity were made within the Pleiades cluster in the blue and yellow spectral regions. Later that year a few more observations of polarization were made in three colors with a polarimeter which contained a Glan-Thompson analyzer. An entirely new polarimeter with a Glan-Foucault prism was built in 1964.

A Few Observations Were Made in NGC 7023 with filters at 0.36, 0.56, and 0.74 μ The wavelength dependence of polarization fitted with a straight line shows an appreciable rise with longer wavelengths. (See refs. 1 and 2.) The observations at the three filters were compared with Mie calculations obtained from Dr. B. M. Herman and General S. R. Browning of the University of Arizona for various refractive indices and particle sizes. Only single particle sizes have been considered, and therefore the results are preliminary. The present work represents a first reconnaissance to learn the techniques and some of the geometrical conditions.

A firm conclusion, however, is reached in ruling out purely graphitic and purely metallic interstellar grains. The reasoning is as follows: The polarizations in various reflection nebulae (observed by Elvius and Hall) are “positive” and strong, and the color dependence is strong, with a fairly steep rise toward longer wavelengths; this behavior occurs for scattering angles from 20° to 70°. Such behavior is found in the Mie calculations at 2<a/λ ≅ 1.5. (See ref. 3.)

The establishment of a truly inertial reference system is a problem that has defied solution for many years. However, with the completion of the Lick proper motion survey (Wright 1950) and the USSR program (Deutsch 1954) the situation for the northern hemisphere should be satisfactory.

A Feasibility Study For an Observational Program was made to investigate models of reflection nebulae. Specifically, for a relatively simple model of an interstellar dust cloud with a source of illumination nearby, predictions of the color of the nebula, relative to the source color, at points along any radial direction from the source are sought. Furthermore, from these color differences, the possibility is studied of discriminating between the effects produced by the cloud-source-observer geometry and those due to the assumed optical properties of the scattering particles. If such discrimination is feasible, the types of observations that would be most useful in separating the effects must be known to the investigator.

Plane-parallel slab models similar to those of reference 1 in which the scattering particles are distributed isotropically and homogeneously were used. The scattering properties of the particles were assumed to be those obtained by using the Mie theory of electromagnetic scattering.

Quasistellar objects with and without radio emission have recently given unforeseen and vigorous impetus to astrometry. Great effort has been made in the search for the optical counterparts of radio sources. For the purpose of identifications accurate positions were demanded, optical as well as those by radio techniques. Optical positions of radio sources are also needed to serve as calibrators for radio positions. In all cases the objects under consideration are optically much fainter than those ever used previously in high precision astrometry. Finally, methods have been developed for the absolute measurement of radio positions to an accuracy which competes with that known from fundamental observations of bright stars.

Reflection Nebulae are suitable objects for studying the physical nature of interstellar grains. Unfortunately, there exist only a few sufficiently bright objects, the most widely studied being nebula NGC 7023. This nebula has been studied photographically and photoelectrically by many authors, and therefore it is best suited for the verification of some theoretical conclusions. Earlier measurements (refs. 1 to 3) have already shown unambiguously that almost all nebulae exhibit a negative color excess in comparison with the color of the illuminating star. This fact has been verified photoelectrically in reference 4. In the B – V system the average difference between the color of the star and that of a nebula is – 0.2m. These facts have been known for a long time, but the distribution of the color excess in the nebula itself has not been studied very intensively.

In Hamburg we have the catalogue AGK 3 and a revised AGK 2 for the declination zones +1° to +18°, +24° to +89° on magnetic tapes, in manuscript and on punched cards. The remaining zones are in preparation. At the present time (March 1970) the catalogue, on account of some deficiencies, is not in the proper shape for a universal publication. A. Günther and H. Kox (1970) since 1967 investigated the zones Greenwich, Rome-Vatican, Catania and Helsingfors of the Astrographic Catalogue (AC) using the AGK 2/3 data as source for reference stars’positions. The original rectangular coordinates x, y for each of the plates together with AGK2 numbers are on tapes. By applying the systematic corrections derived by Günther and Kox and by using the new, definitive plate constants derived in course of the work on the AC-zones the x, y can be transformed into spherical coordinates reffered to the equinox 1950.0 and in the FK4 system. This was done in the last few months, and arithmetic means were produced for the individual stars with AGK 2 numbers for reference number. Simultaneously the mean square errors (mse) were derived when two or more plates went into the means. Table 1 gives values for average values of the mse, as an example for always two observations.

One of the major projects in positional astronomy now in progress is the cooperative effort of 18 transit circles throughout the world to relate the positions of 20,000 reference stars in each hemisphere of the sky to the fundamental system. It is intended that these reference stars will serve as intermediaries in concurrent photographic surveys to relate the positions of upwards of 300,000 stars to the 9th magnitude to the fundamental system.

In Some of the Plots of the Interstellar Polarization (for example, fig. 1 of first paper by Gehrels in the present compilation) a striking feature is noted in a discontinuity that frequently occurs near λ-1 = 1.3. The following explanation of the shape of the curve of interstellar polarization as a function of wavelength is proposed in terms of physical optics.

Far in the infrared, light passes the particle with relatively little interaction. At shorter wavelengths (red light), scattering by the molecules at the particle skin increases and consequently the amount of polarization rises. The rise with increasing values of λ-1 is steep (although not necessarily with λ-4 for polarization).

Prediction of the wavelength at which the rise becomes evident would necessitate a knowledge of the optical depth or scattering cross sections. These factors have not yet been evaluated. Qualitatively, the larger the particles, the larger the wavelength at which the polarization rise becomes manifest because of greater molecular optical depth near the skin.

With the imminent completion of AGK3 in the northern hemisphere, and the prospect of results from the SRS programme in the southern hemisphere becoming available within the next few years, we may regard the problem of the establishment of a fundamental reference frame for proper motions down to about mpg = 11 as, in principle, solved. This reference frame, through the large number of stars involved, will be readily accessible to photographic observers, with carte-du-ciel type, and even longer focus telescopes, for the reduction of relative proper motions to an absolute system.

The Possible Role of Interstellar Particles in the determination of the ionization structure of H II regions, the transfer of the Lyman line radiation, and the early stages of stellar contraction have stimulated an investigation of the properties and relative abundance of the scattering particles that exist within the boundaries of a number of bright galactic diffuse nebulae. It is interesting to note that the conclusion reached in reference 1, based on crude observations, was that there does not seem to be any scattered light in the bright H II regions; this conclusion is in contradiction to the results of the study presented herein. Results of the present investigation show that, in general, dust is an important constituent of the diffuse nebulae; that certain anomalies in the gas-to-dust ratio can be explained in a straightforward manner through the interaction of the particles and stellar radiation; and that the anomalies are closely related to the deviations known to exist in the interstellar reddening law.