The capital of an Ionic column features a disk known as an oculus at the center of each of its spiral scrolls.

• 包括圆形或椭圆形的小窗（如牛眼窗）、罗马万神庙穹窿顶上的圆孔，以及爱奥尼亚式柱头上涡卷形中心的圆凹等。