[编者按]应许多读者的要求．本刊从此期开始将恢复并陆续推出“讲座”这一栏目。力求针对读者普遍关心的技术问进（新的基础理论、新方法、难点问题等）。以“实用”为原则，以“深入浅出”的叙述手法，向广大读者尤其是在职技术人员据供一个系统的了解和学习的园地。实践证明 [Editor’s note] Should be requested by many readers. This period from the beginning will resume and gradually launched “seminar” this section. Strive for the readers generally concerned about the technical progress (new basic theory, new methods, difficult issues, etc.). To “practical” principle, “easy to understand” approach to narrative, to readers, especially serving technical staff according to a systematic understanding and learning field. Practice proved