1. DE JAGER, C.: Lecture given at this Summer School (see pp. 101–124 of this volume).
2. BOYD, R. L. F.: 1965, ‘Techniques for the measurement of extraterrestrial soft X-radiation’, Space Sci. Rev. 4, 35.
3. MANDEL’SHTAM, S. L., TINDO, I. P., VORON’KO, YU. K., VASIL’EV, B. N., and SHURYGIN, A. I.: 1963, ‘Investigations of the sun’s X-radiation, II, Measurements with space satellites’, Planet. Space Sci. 11, 61.
4. These papers are available on request from the Secrétariat du Service d’Electronique Physique, C.EA., Boîte Postale No. 2, 91 Gif-sur-Yvette, France.
5. LYOT, B.: 1930, Compt. Rend. Acad. Sci. 191, 834.