The clergy of San Luigi considered Caravaggio's roistering and unruly lifestyle scandalous enough, but his realistic treatment of sacred subjects -- seen in three paintings in the last chapel -- was too much for them. They rejected outright his first version of the altarpiece, and they weren't especially happy with the other two works. Thanks to the intercession of Caravaggio's patron, the influential Cardinal Francesco del Monte, they were persuaded to keep them -- a lucky thing, since they are now thought to be among the artist's finest paintings. Have a few one-euro coins handy for the light machine.