The baptism of Christ. ca. 1440-50. National Gallery, London.
During his teens [ca. 1439], Francesca studied his craft in Florence while working under Dominico Venezian on a series of murals for the hospital Santo Maria Nuova. It was also during this early part of his career that he began exploring the relationship between mathematics and art. One treatise authored by him, On perspective for painting, is the first to deal with the mathematics of perspective [creating a three dimensional effect in two dimensional works].
The baptism of Christ, originally part of a triptych, displays Francesca's utilization of the golden mean in the composition of the work. The golden mean, or phi, describes a ratio found in nature and it is expressed as a is to a+b as b is to a.
The figure of Christ, John's hand and the dove form a vertical axis which divides the painting in half. The tree on the left then creates a vertical axis which divides the left half by the golden mean.