How do the hidden windows at San Carlo alle Quattro Fontane affect the appearance of its dome?

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Rome: Piazza Navona — Francesco Borromini Piazza Navona, Rome, with the church of Sant'Agnese in Agone, designed by Francesco Borromini, and (foreground) the Fountain of the Moor, originally designed by Giacomo della Porta and revised by Gian Lorenzo Bernini.

At the church of San Carlo alle Quattro Fontane, the hidden windows play a crucial role in creating a specific visual illusion. By concealing the light sources within the zone below the dome, Francesco Borromini ensures that the dome appears to be floating in mid-air, resembling a hallucinatory vision. This effect is further intensified by the use of calculated lighting and geometric coffers that decrease in size, which together trick the eye into perceiving additional height and an ethereal, suspended quality.12