We have now toured the church’s nave and triumphal arch, side aisles and chapels, transept and apse. Before we look at the central altar, one more architectural feature rests above us, the dome and cupola. These work together as a unit, rising in the center of the Roman Cross design of the basilica, and directly above the altar.
They serve the dual purpose of elevating the space above the altar, and allowing additional light, through windows in the dome and more in the cupola. Furthermore, they symbolize the heavens above the altar.
For the benefit of extra light, some churches, such as San Luigi dei Francesi, included domes and cupola over major side chapels. In a city built more and more densely, any attempt to draw in, and direct light from above was a premium.
The size of the church did not necessary dictate having a dome. Even a relatively small church such as Santa Maria Maddelena could have an elaborate visual space.
In some churches, which crossing barrel vaults (i.e. that of the nave and of the transept) which lacked an actual dome, one was painted on the ceiling, giving the illusion of a dome, for those seated at the corrected location in the center of the nave. My photo does not capture this well, but the dark color where the dome should be is a painting on the ceiling.
Finally, some domes were recycled Roman bath structures. Santa Maria degli Angeli e dei Martiri was built-in the ruins of the Bathes of Diocletian. The dome of over the altar is the remains of one of the bathing spas. (My photo is actually of the dome over the entry way. The dome over the altar, which I did not take a photo of, is another section of the bathes and much larger.) Why try to build something more impressive that what the Romans did?