Rules · Domes · Ornament · Optics
Divide the inside of a dome into coffers that shrink in step with the meridians as they rise, so that their perspective deepens the dome and draws the eye to the light of the lantern.
If a dome is divided by equal meridian ribs, the coffers between them narrow as they approach the top; if their height is reduced in proportion, every coffer keeps the same shape at a smaller size. The effect exaggerates the dome’s apparent height, as a perspective drawing of a receding ceiling would. The Pantheon has five rows of diminishing coffers; Bernini at Sant’Andrea made them hexagonal and gilded, rising to a lantern full of light and putti.
Front a basilican church with two storeys of the order: the lower as wide as nave and chapels, the upper as wide as the nave alone, joined by scrolls and crowned by a pediment.
Il Gesù (reconstructed): lower ≈ 34 m wide, upper ≈ 20 m; scrolls fill the difference
Mark the main door by setting a segmental pediment inside a larger triangular one, or by breaking the pediment open to hold a cartouche or statue.
To combine the unity of a central plan with the direction a church needs, plan the space as an oval, set across the axis to widen the view of the altar, or along it to lead the eye towards it.
Light the climax of a Baroque interior (the altar or the dome) from windows the visitor cannot see, so that it seems to glow of itself.
To set a round dome on a square bay, imagine a sphere passing through the four corners of the square; cut it by the four vertical planes of the sides and by the horizontal plane of their crowns. The four spherical triangles left in the corners are the pendentives; the circle at their top carries the dome.
sphere radius R = S⁄√2; arches = sphere ∩ side planes (radius S⁄2); ring at h₀ + S⁄2 with radius S⁄2
Edge the beds before the portico with box, clip box into figures of animals set facing one another, and in the garden beyond cut it into many shapes, even into the letters of the owner’s name.
Cite this page Chris Paton, “Coffers that diminish towards the lantern”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/diminishing-coffers. Text, drawings and models licensed under CC BY-NC 4.0.