On the Clustering Tendencies among the Nebulae.

Part I gives an investigation of double and multiple nebulae contained in the General Catalogue. An analysis of the distribution of angular distances between nebulae resifits in a statistical separation of physical and optical companions (Fig. 2); double and multiple systems are revealed as they actually exist in the sky. The average distances between physically connected nebulae (Tables i and 2) appear to be considerably larger than those previously assumed. In Parts IT-TV possible interpretations of the clustering tendencies are discussed. Double and multiple systems, as well as clusters, may be explained as a result of cap- tures between nebulae, effected by tidal forces at close encounters. A summary of the main conclusions is given in Part V