2. A preliminary review of the structural and kinematic aspects of double galaxies leads to the recognition of the following types of significant features. We find in the first place that there exist double galaxies of considerable permanence whose components cannot escape from one another. On the other hand, two galaxies may temporarily appear to form a double system, although their relative velocities are high enough to lead to an ultimate and progressively increasing separation. Permanent multiple galaxies are dynamic units analogous to physical double or multiple stars.
The components of permanent multiple galaxies may be clearly separated, or they may be coexisting. Revealing information on two or more types of galaxies coexisting within the same volume of space has recently been obtained through the application of the method of analytical composite photography , . The structural features revealed by this method have in fact led to the recognition not only of the universality of the phenomenon of coexistence of many different structural types of galaxies, but of the coexistence of different dynamic formations within gaseous nebulae of the type of the Crab Nebula  as well.
In many cases the components of permanent groups of multiple galaxies are clearly interconnected by luminous and probably also by dark intergalactic formations. Such connections were of course recognized very early and are clearly in evidence in many of the systems photographed originally by PEASE . That there are many spectacular luminous intergalactic formations which link very widely separated galaxies and groups of galaxies was in recent years shown by ZWICKY'S systematic discoveries and investigations , .
The multiple galaxies which are of a temporary nature and which cannot be considered as stationary dynamic units may be of the following characteristic types. In the first place the galaxies may pass around each other, the encounter starting from a state of complete separation and finishing again in a state of complete separation. There is a change of velocity and direction of motion of each of the galaxies with respect to a universal inertial system of reference , but only insignificant internal rearrangement takes place within the two galaxies. Secondly the encounter may take place at very close range with induced tidal effects, internal rearrangement of the structure of the galaxies. This case involves an appreciable loss of the total translational energy of the galaxies involved, with transfer of the lost energy to internal energy of these galaxies. Finally, the encounter may be so close or even head-on that either considerable disruption of both systems results, or perhaps even total mutual capture.
The general systematics of the morphological features of double and multiple galaxies will of course need many refinements through the consideration of the physical character of the individual galaxies which constitute the groups in question. Obviously, the results of close encounters will be quite different, depending on whether the galaxies involved contain much dust and free interstellar gas or if they are composed essentially only of stars. In the first case the disruption not only will be of a quite different overall geometrical character than in the second, but it may also lead to the generation of radio waves, cosmic rays and other radiations which will not occur during the collision of two purely stellar systems which do not contain any finite dispersed matter. Unfortunately, our observational data, at the present time, are entirely insufficient to allow many detailed statements about the physical conditions within interacting groups of galaxies. Most of the results which we present here are therefore merely indicative. They are not only incomplete, but in many cases quite uncertain and should be considered merely as starting points for a future thoroughly comprehensive program of investigation.