一、原理在一方形木块上面画一个圆,再作互相垂直的两直径MM’、NN’。将圆分为四个象限。在第四象限内作一半径OA(∠N’OA约为40°),并在它上面的A、B两处各正插上一枚大头针。然后浸入盛水盆中,使水面恰好落在MM’上,如图1所示,这时在第二象限内用一只眼睛来观察水中的两枚大头针(实为其象)。我们发现在第二象限C处可插上另一枚大头针,使眼睛看过去时,C将水中的大头针挡 First, the principle of drawing a circle above a square block, and then make two mutually perpendicular diameter MM ’, NN’. Divide the circle into four quadrants. Make a radius OA (∠N’OA about 40 °) in the fourth quadrant, and insert a pin on each of A and B above it. Then immerse it in the basin to make the water fall exactly on the MM ’, as shown in Figure 1, with one eye in the second quadrant to see the two pins in the water. We found that in the second quadrant C can be inserted in another pin, so that when the eyes look past, C water pin