In Rome's Trastevere, this extraordinarily compressed and distilled funerary chapel offers visions of the world of the spirit through bravura performance in our material sphere.

Santa Maria in Trastevere is a typical Roman basilica. Nothing could be more reassuring and comprehensible than its simple planning and construction But an arch off the north aisle frames quite a different sort of architecture: the funerary Capella Avila, designed by Antonio Gherardi in the 1680s.

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This must be one of the most frenzied 18 metres cubed of space to be found anywhere. A toplit, monochromatic double cube has been articulated to the point of its own dissolution. Four walls and a roof have become a Greek cross supporting a dome. This itself opens onto a vision of four angels carrying a little sunlit tempietto.

The altar wall houses another little building. Above, a bright upper chapel is glimpsed through a miniature balconied window. Below, a colonnade recedes in false perspective to the sarcophagus of the donor, bathed in light. But how much of this are we actually meant to believe? Why create a self-evidently false perspective? Why make a balcony only a midget could appear at? Are we really meant to believe that it's angels performing structural gymnastics up there?

Paradoxically, the pleasure we derive from an illusion is increased by our knowledge that it is an illusion--by the very fact that it doesn't work. This chapel invites us to marvel at the improbability of the world it implies, and to reflect upon its impossibility.

This is a tomb. The architecture of the chapel is a gateway from one world to another, from the irreducible world of statics and ergonomics and lighting studies to one in which no such rules need apply. This tiny building is the representation of another more heavenly one, never to be realized this side of the grave.

We may be living in an age lacking in such heavenly models, yet the Capella Avila still delights, not because it is fired with faith, but because it is so knowing about our lack of it. We peer through it into another world, while we applaud its bravura performance in this one. Real and illusion, it allows us both to believe and not to believe.

EDWARD HOLLIS Photographs by the author