On the picture: A detail of the ceiling, covering the south aisle of the prayer hall, where the mihrab is. Prism-shaped triangles connect the circular dome with the walls of the square mihrab area.
The painted decoration is called ‘kalem işi’ (= work of the pen/paintbrush), because it is inspired on the geometric or stylised ornamentation used by Ottoman calligraphers to illuminate books. These (recently refreshed) murals are a 19th century attempt to imitate the medieval Ottoman decoration.

Correspondent: J.M.Criel, Antwerpen.
Source: ‘Vakıf Abideler ve eski Eserler (volume III)’ - Vakıflar Genel Müdürlüğü, Ankara 1983