The second job that Uccello had at the Cathedral was to paint the clock. The clock has two faces, one that can be seen outside in the square and another which is on an inside wall of the cathedral. Uccello had to paint a clock face around the hand on the inside wall. (It's a 24-hour clock and it's only got one hand.) Uccello painted the round clockface into a square shape and put the head of a prophet in each corner. The prophets' faces are all very different but the thing that is the most clever about them is that Uccello painted bright light and dark shadows on their faces as if the light was coming from a real window which is high on the wall to the left side of the clock.