The term oculus is taken from the Latin which means an eye. It is a circular opening at the centre of a building or a dome and so is called the eye of a dome. It is a main feature of Neoclassical and Byzantine architecture. But the classical example of oculus can be seen in Roman architecture, in the dome of Pantheon. This eye is kept open and it allows rain water to fall inside to the floor from where it is carried away through drains. In French it means a 'bull's eye'.