我们发现曲面在一点的Dupin指标线与水浸该曲面在同一点的水边线是同一个图象,这是一个很直观而又有趣味的现象,本文想说明这个现象与微分几何中的两个基本形式Ⅰ与Ⅱ的内在联系。我们将曲面浸入水中时,把水面与曲面的交线叫做水边线,让曲面徐徐下浸,当水边线逼近曲面的某一点时,就可以直接看出来,水边线的图象与该点的Dupin指标是同一个相似图象,为了简单起见,先观察几个特殊曲面。 We find that the Dupin line at a point of the surface is the same image as the water line at the same point where the surface is immersed. This is an intuitive and interesting phenomenon. This article attempts to illustrate the relationship between this phenomenon and two of the differential geometry The Internal Relations between Basic Forms Ⅰ and Ⅱ. When we submerge the surface into water, we call the intersection of the water surface and the surface a water edge, letting the surface submerge slowly. When the water edge approximates a point on the surface, we can directly see that the image of the water edge is different from the Dupin Indicators are the same similar image, for the sake of simplicity, first observe a few special surfaces.