The available binary thermal diffusion data for a number of gas systems as a function of temperature and composition have been critically examined and interpreted using the kinetic theory of Chapman and Enskog. In the computation of the thermal diffusion factor ${\alpha }_T$ two familiar central intermolecular potentials, viz., Lennard-Jones (12-6) and modified Buckingham exp-six have been considered. The theoretical expressions for ${\alpha }_T$ employed are of known and tolerable accuracy so that the uncertainty associated on this account in many early interpretations is avoided in this study. The scope of work is confined mostly to inert gas mixtures as the kinetic theory is rigorously developed so far only for such systems. This comprehensive effort brings to light many interesting points. A general deficiency is realized in the available ${\alpha }_T$ data. Even for some systems where data exist as a function of composition, it is found that the composition range which is theoretically interesting is not investigated. This study also leads to the unpleasant prospect that some of the available data may be in error. Thus, many indications are provided for planning future work on ${\alpha }_T$ measurements. Further, the L-J (12-6) potential is found to be inadequate for representing the force field of these molecules and especially for such systems where He is one of the two components. To be specific, the choice of a repulsive index as twelve is found to be a bit large. The exp-six potential has succeeded in general to explain the ${\alpha }_T$ data of many of the systems discussed in this article.

The measurement of time, space, and space-time intervals, and the related geometries in any gravitational field are analyzed carefully. It is pointed out that synchronization is arbitrary and very complicated operationally; and a method for doing away with it is given. Then it is shown how space geodesics can be used to construct several useful coordinate systems in an arbitrary field, and the associated coordinate conditions are derived. These coordinate systems are then used to find a unique physically meaningful solution to the field equations in the cases of zero, linear, rotation, and spherically symmetric fields.

This article presents a review of coherence properties of electromagnetic fields and their measurements, with special emphasis on the optical region of the spectrum. Analyses based on both the classical and quantum theories are described. After a brief historical introduction, the elementary concepts which are frequently employed in the discussion of interference phenomena are summarized. The measure of second-order coherence is then introduced in connection with the analysis of a simple interference experiment and some of the more important second-order coherence effects are studied. Their uses in stellar interferometry and interference spectroscopy are described. Analysis of partial polarization from the standpoint of correlation theory is also outlined. The general statistical description of the field is discussed in some detail. The recently discovered universal "diagonal" representation of the density operator for free fields is also considered and it is shown how, with the help of the associated generalized phase-space distribution function, the quantum-mechanical correlation functions may be expressed in the same form as the classical ones. The sections which follow deal with the statistical properties of thermal and nonthermal light, and with the temporal and spatial coherence of blackbody radiation. Later sections, dealing with fourth- and higher-order coherence effects include a discussion of the photoelectric detection process. Among the fourth-order effects described in detail are bunching phenomena, the Hanbury Brown-Twiss effect and its application to astronomy. The article concludes with a discussion of various transient superposition effects, such as light beats and interference fringes produced by independent light beams.

This article is a résumé, intended for experimentalists and other nonspecialists who have a need to know of that part of group representation theory which is employed in the spectroscopic classification of energy levels. A summary of the general properties of semisimple Lie groups, sufficient to make the treatment self-contained, is included. Attention is then focused on Racah's important invariant operators (such as $J^2={J_1}^2+{J_2}^2+{J_3}^2$ for the rotation group), whose eigenvalues provide the essential connection between group theory and physics by serving as the "good quantum numbers" which label the multiplets and energy levels of the system. The rotation and isospin groups are used as examples throughout the discussion. The final section describes the way in which, in entirely analogous fashion, S${\mathrm{U}}_3$ and larger groups are being employed at present in the classification of strong-interaction multiplets.

The procedure for setting up projection operators to derive symmetrized wave functions in crystals is discussed. This includes a description of the projection operator method, discussion of irreducible representations, basis functions, and the multiplication table for space groups, and the application of these methods to setting up symmetrized plane waves, and symmetrized linear combinations of atomic orbitals, for use with the OPW and APW methods of approximating to electronic wave functions in crystals.