1. Department of Scientific Instrumentation and Control, Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan; Department of Mechanical Engineering, Osaka University, Toyonaka, Osaka 560, Japan
2. Health Science Research Laboratory, Matsushita Electric Works, Ltd., Kadoma, Osaka 571, Japan
3. Department of Scientific Instrumentation and Control, Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan