Strahlensysteme mit gemeinsamem sphärischen Bild

In this paper we investigate a class Σ* of congruences which have common spherical image and which are mainly derived by suitable linear transformations from a given congruence S. A necessary and sufficient condition is given so that a congruence S* of Σ* has a common middle envelope with S. Further we investigate the correspondence between special ruled surfaces of a congruence S* and special curves on the middle envelope of the congruence S.