This Encyclopedia aims, basically, at summanzmg the wealth of well established facts and outlining the relevant theories in the different branches of physics. With this as goal, the writers were asked to present their specific field in such a way that access is possible to any scientist without special a priori information in that field; the basic concepts of physics are assumed to be known to the reader. The survey given in each paper was also to be long lasting, so that even a few years after publication, each volume would be useful, for example as an introduction for newcomers or as a source of information for workers in a neighbouring field. In the field of geophysics, dealt with in Vols. 47--49 of the Encyclopedia, this task is difficult to achieve because during the last decades there has been a much faster development of basic information and theory than during the decades before. When I came to contribute to this work the famous Julius Bartels, then editor of the geophysical part, told me that Vol. 49 should certainly take into account the results of the "International Geophysical Year" 1957/58 (I. G. Y. ), and that we had better wait until these were accessible than produce a kind of information which might be obsolete in a short time."

In 1986, an International Symposium on Monitoring of Gaseous Pollut- ants by Tunable Diode Lasers was organized by the Fraunhofer-Institut fuer Physikalische Messtechnik in Freiburg, FRG, and held at this institution on November 13-14, 1986. The papers presented at that occasion extensively covered the applications of tunable diode laser spectroscopy (TDLS) to the analysis of trace and exhaust gases together with the latest improvements brought to the tech- niques used. It was generally consented by the participants that meetings of this kind are of major importance and should take place on a regular basis. The organizers were thus encouraged to plan a Second International Sympo- sium on Monitoring of Gaseous Pollutants by Tunable Diode Lasers which was held two years later on October 17-18, 1988, again in Freiburg. In addi- tion to the main subjects treated in the first Symposium, the Scientific Programme this time presented a session on the development of lead chalco- genide lasers. Once more, only a limited attention was given to applica- tions in high resolution molecular spectroscopy. In order to complete the update, sessions on components and techniques were included in the Pro- gramme. As for the previous Symposium, some space was also given to alter- native optical techniques. This time, in particular contributions on laser photoacoustic detection were included.

By now it has been nearly twenty years since the pioneering studies at the MIT-Lincoln Laboratories, Lexington, USA, demonstrated the unique capabilities of lead salt tunable diode lasers (TDL) for infrared absorption spectroscopy. The progress in the use of TDL instrumentation for a wide variety of scientific applications was described by a great number of papers since, however, comparatively few meetings were specifically devoted to this subject. In 1980 the conference on "High Resolution Infrared Spectroscopy Applications and Developments" at the National Bureau of Standards in Gaithersburg, USA, reviewed the state of the art of tunable diode lasers together with Fourier Transform Spectroscopy and other laser spectroscopic techniques. Three years later in 1983 the SPIE Conference in San Diego, USA, dedicated one meeting to "Tunable Diode Laser Development and Spectroscopy" Applications. It appeared appropriate after a further interval of three years to organize another meeting about this quickly advancing field. In November of 1986 an International Symposium on "Monitoring of Gaseous Pollutants by Tunable Diode Lasers" was organized by and held at the Fraunhofer-Insti tut fuer Physikalische Messtechnik in Freiburg, FRG. The main emphasis of this conference was put on the applications of TDL techniques to the solution of problems of environmental relevance : The measurement of atmospheric trace gases and the monitoring of exhaust gases from automobile and power plant stack emitters. The state of diode laser development and application of TDL instruments to scientific molecular spectroscopy were not directly subjects of the meeting.