As I mentioned, we say the "Capitoline Museums" because the complex includes several collections: one of the most valuable and ancient is the Capitoline Art Gallery that was founded in the mid-18th century and offers a collection of paintings of different ages. It has many great works from the Venetian masters of the sixteenth century, including Titian and Lorenzo Lotto, and especially Italian and European masterpieces from the 17th century: Velázquez, Van Dyck, Guido Reni, Domenichino, and an immense painting by Guercino that was originally destined for St. Peter's Basilica. But the museum's pièce de résistance are two masterpieces by Caravaggio that I'll tell you about right now.
The first is called The Fortune Teller and is perhaps the first work where Michelangelo Merisi, also known as Caravaggio, portrayed two figures interacting with each other. It shows a scene of everyday life that was typical of the streets of central Rome: a young and smiling gypsy captures the attention of a naive young man of a good family, and while pretending to read his hand, she skillfully steals a ring from his finger. The model depicted in the picture was a good friend of Caravaggio's, as well as a painter that was well known above all in his native Sicily.
The second painting is titled St. John the Baptist and is probably the most joyful image of all of Caravaggio's paintings. The boy's position is almost the same as the "ignudi", or naked, men painted by Michelangelo on the vault of the Sistine Chapel, but without the robust muscles that characterized the athletes portrayed by Buonarroti. The boy is usually identified as a young Saint John the Baptist; however, the animal accompanying the Baptist is usually a lamb, and not a ram as is depicted here. So it has been suggested that the portrait is instead of Isaac, who is understandably happy to have just escaped being sacrificed and is embracing the animal that is about to die in his place!
FUN FACT: you might not believe it, but after Caravaggio died his artistic fame and popularity sharply declined until the 1900s, when he was again recognized as a great master of painting. Even today, not only painters but also photographers and movie directors recognize the influence Caravaggio has had on their work. Even the great Martin Scorsese said he had been inspired by Caravaggio in his movies Mean Streets and The Last Temptation of Christ.