1. Beran, M. J., Ph.D. Dissertation, Harvard University (1955).
2. Rifai, M. N. E., Ph.D. Dissertation, University of California (Berkeley, 1956).
3. de Josselin de Jong, G., Trans. Amer. Geophys. Un., 39, 1160 (1958).
4. Christiansen, C., Ann. Physik und Chemie, 23, 298 (1884).
5. Summer, W., “Photosensitors”, 300 (Chapman and Hall, London, 1957).