The following proposition are proved:Let P_0 be a point on the contour of a smooth surface withnegative gaussian curvature S,the asymptotic direction of P_0 on S is the critical direction for the visi-ble contour of S in the neighborhood of P_0.

• 证明了下面两个命题:(1)设P_0为负高斯曲率曲面S的实廓线上的一点。 在P_0的邻域内,S上P_0的渐近方向是P_0的可见轮廓的临界投影方向。