('Figuras do Espanto' p. 70):
"To demonstrate the effect of drawing in perspective, Filippo Brunelleschi painted the Baptistery of San Giovanni from about six feet inside the center door of Santa Maria del Fiore. The painting included everything that could be seen from his positioned location. Instead of painting the sky, he affixed a plate of polished silver. He carved a peephole in the painted panel at the perspective vanishing point.
Standing at the same location where he painted the view, he demonstrated the accuracy of perspective drawing by holding the painted panel facing away, with the panel held up to his eye to peer through the hole. With his other hand he held a mirror at arms length in front of and facing the painting so that he was looking directly into the mirror at the reflection of the painting.
The view through the hole into the mirror revealed the painting, drawn in perfect perspective, in the place where the subject of the painting would be viewed. The polished silver plate reflected the actual sky complete with drifting clouds. The view was so realistic that the viewer could not tell the difference between the painted scene and the actual image of the building’s shape and proportion."