1. Institute of Engineering Mechanics, University of Tsukuba, 1-1-1, Tenodai, Sakura, Niihari, Ibaraki-Ken 305, Japan
2. Department of Physics, Keio University, 3-14-1, Hiyoshi, Kohokuku, Yokohama-shi 223, Japan
3. Central Research Laboratory, Hitachi Ltd., 1-280, Higashi Koigakubo, Kokubunji-shi, Tokyo 185, Japan
4. Fuchu Works, Toshiba Corp., 1, Toshiba-cho, Fuchu-shi, Tokyo 183, Japan
5. INTEC Inc., 3-37-18, Hatagaya, Shibuya-Ku, Tokyo 151, Japan
6. FUJITSU FANUC Ltd., 3-5-1, Asahigaoka, Hino-shi, Tokyo 191, Japan