[27] L. P. Hugston, Twistor and Particles. Lecture notes in physics, Bd. 97, Springer, Berlin, Göttingen, Heidelberg~ New York 1979, S.102 ###>[27] L. P. Hugston, Twistor and Particles. Lecture notes in physics, Bd. 97, Springer, Berlin, Göttingen, Heidelberg, New York 1979, S.102.