取一只装照相胶卷的黑色塑料空盒,去掉盖子在其底部中心位置钻一个直径约0.5毫米的小孔,再在塑料盒侧中段位置插入一根小钉,钉尖端深入塑料盒中心(见图1),用肉眼从小孔一端向盒内观察;可见到钉尖正立的影像。若把塑料盒颠倒过来,用眼从塑料盒口向小孔方向观察;则可见到在小孔明亮的背景上有钉尖倒 Take a photographic black plastic empty box, remove the cover and drill a hole with a diameter of about 0.5 mm at the center of the bottom, and insert a small nail into the middle of the plastic box. The tip of the nail is deep into the center of the plastic box (see Figure 1), with the naked eye from the end of the hole to the inside of the box; visible upright nail image. If you turn the plastic box upside down, look in the direction of the hole from the mouth of the plastic box with your eyes; you can see there is a sharp spike on the bright background of the small hole.