ABSTRACT Brunelleschi's lost panel painting of the Florence Baptistery, created in the early 15th century, is frequently cited as the first work to accurately use perspective. The system he used is unknown, and the only information about the painting mentions a demonstration by which the painting was viewed through a hole in the panel as a reflected image in a mirror. The author argues here that the image was created in a camera obscura using the panel and a mirror in the same relationship as used in the demonstration. The author also proposes that the process revealed perspective's basic “rule”: Vanishing points for parallel, horizontal lines exist at the eye level.