An oculus, plural oculi, from Latin oculus: eye, denotes a circular opening in the centre of a dome or in a wall. Originating in Antiquity, it is a feature of Byzantine and Neoclassical architecture. It is also known as an œil de boeuf from the French, or simply a "bull's-eye".
Classical origins 
The oculus was first used by the Romans, one of the finest examples being that in the dome of the Pantheon. Open to the weather, it allows rain to enter and fall to the floor, where it is carried away through drains. Though the opening looks small, it actually has a diameter of 27 ft (8.2 m) allowing it to light the building just as the sun lights the earth. The rain also keeps the building cool during the hot summer months.
Byzantine architecture 
The oculus was widely used in the architecture of the Byzantine Empire. It was applied to buildings in Syria in the 5th and 6th centuries and again in the 10th century. In Constantinople's Myrelaion Church (c. 920), there are two oculi above the stringcourse on both lateral facades.
Neoclassical revival 
Since the revival of dome construction beginning in the Italian Renaissance, open oculi have been replaced by light-transmitting cupolas and other round windows, openings, and skylights. They can be seen in the pediments of Palladio's Villa Rotonda, though not in the dome. Use of oculus windows became more popular in Baroque architecture. Widely used by Neo-Palladian architects including Colen Campbell, one can be seen in the dome of Thomas Jefferson's Rotunda at the University of Virginia.
Other uses 
Oculus, Latin for eye, is also used in medicine and optics.
In archaeology, oculus is the name given to a motif found in western European prehistoric art. It consists of a pair of circular or spiral marks, often interpreted as eyes, and appears on pottery, statues and megaliths. The oculus motif may represent the watchful gaze of a god or goddess and was especially common during the Neolithic period.
- "Oculus window". Royal Institute of British Architects. Retrieved 24 January 2013.
- "Oculus of the Pantheon", About.com. Retrieved 24 January 2013.
- Slobodan Ćurčić; Mark Joseph Johnson; Robert G. Ousterhout; Amy Papalexandrou (1 January 2012). Approaches to Byzantine Architecture and Its Decoration: Studies in Honor of Slobodan Ćurčić. Ashgate Publishing, Ltd. pp. 148–. ISBN 978-1-4094-2740-7. Retrieved 24 January 2013.
- "The Cathedral of Santa Maria del Fiore, Florence". Retrieved 24 January 2013.
- "Daylighting in two centroidal spaces at the University of Virginia: Case Study", The Rotunda and Caplin Pavilion: The University of Virginia. Retrieved 24 January 2013.
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