The usual book on the theory of spectral line formation begins with an in-depth dis cussion of radiation transfer, including the elegant methods of obtaining analytical solutions for special cases, and of the physics of line broadening. Neither of those features will be found in this book. It is assumed that the reader is already familiar with the essentials of transport theory and of line broadening and is ready to investi gate some of the particular applications of the theory to the flow of line photons through the outer layers of a star, or other tenuous media. The main thrust of this book is toward the compilation and presentation of a vast quantity of computational material available to the author in the form of computer output. The material presented represents a highly filtered sample of the published work in this subject plus an extensive set of previously unpublished results. To present large quantities of computer output in an intelligible and efficient way is a difficult task, for which I have found no really satisfactory solution. Chapters III and IV, in particular, contain almost exclusively this type of presentation. The reader may find these chapters somewhat tedious because of the level of condensation of the material. I have tried to reach a reasonable balance between over condensation and excessive detail, which in the long run may be irrelevant."

The widespread tendency in solar physics to divide the solar atmosphere into separate layers and to distinguish phenomena of solar activity from phenomena of the quiet Sun emphasizes the wide ranging diversity of physical conditions and events occurring in the solar atmosphere. This diversity spans the range from a neutral, essentially quiescent atmosphere to a highly ionized, violently convective atmosphere; from a domain in which magnetic field effects are unimportant to a domain in which the magnetic pressure exceeds the gas pressure, and from a domain in which the particle motions are Maxwellian to a domain in which an appreciable fraction of the particles is accelerated to relativistic energies. It is now widely recognized that the chromosphere and corona have a common origin in the mechanical energy flux generated in the hydrogen convection zone lying beneath the photosphere. Furthermore, magnetic field phenomena appear to be as vital to the structure of th~ quiet Sun as to the active Sun. For these reasons it appears desirable to present a unified treatment of the entire solar atmosphere, both active and quiet, in a single volume. On the other hand, such a treatise must be very long if it is to avoid being superficial, and it is very difficult for a single author to write authoritatively on such a wide range of topics.

The devotion of an IA U symposium entirely to the topic of chromo spheric fine structure at a time when models of the spherically symmetric chromosphere are still evolving constitutes a valid recognition of the growing feeling among solar astron omers that the chromosphere cannot be understood independently of its discrete structural features. Network structure, which seemingly borders the photospheric supergranule cells, persists intact throughout the chromosphere and most of the chromosphere-corona transition region. The network is the locus of the bright coarse mottles, and the spicule bushes and is the terminus for one end of the quiet chromo spheric fibrils as well. Additionally, it is the locus of most of the magnetic flux of the quiet chromosphere. It is not surprising, therefore, that current studies of the chromosphere tend to center around efforts to better describe the network phenom ena and to ascertain the physical properties of the network features. Clearly, the supergranule cells and associated network structures constitute a fundamental and singularly important feature of solar structure in the boundary layers. Just as it is now clear that much of the chromo spheric fine structure is associated with the network bordering supergranule cells, it seems equally clear that structural features are almost universally associated with both fluid flow and magnetic geometry. Indeed, many observers claim that the brightness features faithfully map the mag netic lines offorce while still others claim that associated with each class of brightness feature there is a more or less unique fluid flow."

The usual book on the theory of spectral line formation begins with an in-depth dis cussion of radiation transfer, including the elegant methods of obtaining analytical solutions for special cases, and of the physics of line broadening. Neither of those features will be found in this book. It is assumed that the reader is already familiar with the essentials of transport theory and of line broadening and is ready to investi gate some of the particular applications of the theory to the flow of line photons through the outer layers of a star, or other tenuous media. The main thrust of this book is toward the compilation and presentation of a vast quantity of computational material available to the author in the form of computer output. The material presented represents a highly filtered sample of the published work in this subject plus an extensive set of previously unpublished results. To present large quantities of computer output in an intelligible and efficient way is a difficult task, for which I have found no really satisfactory solution. Chapters III and IV, in particular, contain almost exclusively this type of presentation. The reader may find these chapters somewhat tedious because of the level of condensation of the material. I have tried to reach a reasonable balance between over condensation and excessive detail, which in the long run may be irrelevant."