1. Investigators of extragalactic objects have occupied themselves successively with the study of individual galaxies, multiple galaxies, clusters of galaxies and, most recently, with intergalactic matter. While all of these studies, in the past, have gone their individual ways, it is now not only appropriate but necessary to consider the interrelations and the transitions between the various forms of the large scale aggregations of matter. These aggregations are in fact not at all as distinct units as are for instance the stars and atoms. The decision of whether or not a specific individual star, dust cloud or gas cloud belongs definitely to any given galaxy, group of galaxies or to an intergalactic formation is indeed at the present state of our knowledge an almost impossible one to make. In order to shed some light on this problem, we therefore propose in this article to outline directives for the future investigation of multiple galaxies. For this purpose we shall sketch some possible morphological and structural aspects of characteristic double and multiple galaxies. Referring to the history of the subject at hand, it should be mentioned that good photographs of double galaxies were probably first obtained by F. G. PEASE with the 60-inch reflector of the Mount Wilson Observatory. PEASE published his results in two classic papers [1] in 1917 and 1920.
KNUT LUNDMARK [2], during his early pioneering work on extragalactic nebulae was the first to point out the importance of the study of double galaxies for the evaluation of the absolute masses and of the luminosity function of stellar systems. Following him, E. HOLMBERG [3] continued and extended LUNDMARK'S work in several important papers. Generally speaking, however, the study of multiple galaxies has been sadly neglected for several decades and it is only recently being reactivated through the work of F. ZWICKY [4], [5] on double and multiple galaxies which are interconnected by faint luminous intergalactic formations. The main reason for the neglect of double galaxies as an object of study is to be found in the fact that all efforts of the large reflectors had to be concentrated on the observation of individual galaxies as well as of large clusters of galaxies. The latter, as is well known, served as stepping stones for the construction of the redshift-distance relation to remote regions of cosmic space.
A review of a large part of the knowledge available on multiple galaxies has recently been given by ZWICKY [5]. In the present short account we shall only present those aspects of multiple galaxies which have not been treated in the Ergebnisse, to which the readers are herewith referred for additional information. Also, it should be mentioned that clusters of galaxies are treated separately in the following article in this volume.