Giannozzo Manetti (1396–1459), Brunelleschi established the need tofix
the position of the viewing subject. On a small panel, he depicted the
temple of San Giovanni as seen from afixed position in a portal directly
across from it. He then placed a hole at position of the viewer, across from
the point of convergence, and had the viewer look through the hole
through a mirror placed opposite in order to see the scene in accurate
perspective.^4 This construction did not rely on geometry but on observa-
tion. Similarly, an anonymous contemporary biography of Alberti relates
that he constructed a sort of peep show in Rome before he went to
Florence. He displayed works in a closed box through a tiny hole, which
fixed the position of the gaze so that it would be correctly positioned to
experience the illusion of size enabled through perspectival construction.
Alberti mentions this demonstration inDe Pitturaas one of the miracles of
painting which he used to demonstrate in Rome.^5
InDe Pittura, Alberti bases such physical demonstrations on survey
geometry, providing practical tips for artists. He recognizes the picture
plane as a cross-section of the visual pyramid that emerges from the
imaginary rays between the seen object and the eye at the apex of the
pyramid.
The plane is measured by rays that serve the sight–called by them visual rays–
which carry the form of the thing seen to the sense. For these same rays extended
between the eye and the plane seen come together very quickly by their own force
and by a certain marvelous subtlety, penetrating the air and thin and clear objects
they strike against something dense and opaque, where they strike with a point and
adhere to the mark they make...We can imagine those rays to be like thefinest
hairs of the head, or like a bundle, tightly bound within the eye where the sense of
sight has its seat. The rays, gathered together within the eye, are like a stalk; the eye
is like a bud which extends its shoots rapidly and in a straight line to the plane
opposite.^6
Concerned only with painting, he avoids the questions of extra- and
intromission as well as whether the visual image forms on the lens or the
base of the eye. Rather, his emphasis is on the structure of the picture plane.
He describes thisfirst through the metaphor of the window:
When [painters]fill the circumscribed places with colors, they should seek only to
present the forms of things seen on this plane as if it were of transparent glass. Thus
the visual pyramid could pass through it, placed at a definite distance with definite(^4) Alberti, 1966 : 106n. 27; Damisch, 1995 : 115–116. (^5) Alberti, 1966 : 106n. 27, 51.
(^6) Alberti, 1966 :45–46.
302 Perspectives on Perspective