Let's take a closer look at Brunelleschi's perspective experiment.
You can better understand how he was placed by considering the above aerial photo of the center of Florence, with the Basilica of Santa Maria
del Fiore (also called the "Duomo") to the right; the Baptistery is to
the left. The artist stood in the doorway of the Duomo facing the
Baptistery. The basilica was not completed at the time.
In fact, when people speak of his experiment they usually think of this one, owing to the eyewitness account of his biographer, Antonia Mannetti, of which I have prepared a translation below. But there was another perspective panel of the Vecchio Palace seen from its northwest corner.
"Brunelleschi first conducted his perspective procedure on a small panel about half a braccio [note: a “braccio” is a Renaissance measure of half the length of a man’s arm, or 12 inches or 30 centimers] on which he put a picture of the outside of San Giovanni in Florence. To paint it, he apparently stood some three braccia [six feet - 1.82 meters] inside the central doorway of Santa Maria del Fiore; it was done so delicately and carefully in the precise colors of white and black marble, better than any miniaturist could have done.
In the foreground he painted the part of the square that the eye takes in…he also put polished silver on the panel so that the actual air and atmosphere were reflected in it, and therefore actual clouds could be seen in the silver as the wind blew them across the sky.
Antonio Mannetti, painted by Paolo Uccello
With this kind of painting the painter has to determine
prior what the single viewpoint will be from which his painting will be looked
at; he must also take into account the length and width of the sides and the
To ensure accuracy in looking at it (since any viewpoint other than that single one would change the shapes the eye sees) he made a hole in the painted panel on the Church of San Giovanni which is just at the level where the eye falls for any person standing in the central doorway of Santa Maria del Fiore as he did to paint the Baptistery.
The hole was as small as a lentil on the painted side and on the back it widened like a cone, like a woman’s straw hat, to about the circumference of a ducat coin, or a bit more.
He asked that anyone wanting to look place his eye on the
back where the hole was biggest, bringing the hole up to his eye with one hand
and holding a flat mirror in the other hand, such that the painting would be
reflected in it [while standing where the artist had stood].
Filippo Brunelleschi by Masaccio
The hand holding the mirror was to move it back and forth until it was proportionately at the distance to the panel that [Brunelleschi had placed himself to paint the Church] to see the illusion of the Church of San Giovanni.With the items described above of the polished silver, the public square, the viewpoint and so on, the spectator felt he was looking at the real scene when he looked at the painting. I have had [the painted panel] in my own hands and have seen it many times in my days, and so I can bear witness to this.”