1. Department of Physics, Faculty of Science, P. O. Box 60, Kragujevac, Yugoslavia
2. International Anti-Stress Center, Smiljanićeva 11/III/7, 11000 Belgrade, Yugoslavia
3. Astronomical Observatory, Volgina 7, 11160 Belgrade, Yugoslavia
4. Faculty of Electrical Engineering, P. O. Box 35–54, 11000 Belgrade, Yugoslavia