The Cappella dei Principi leads into the Sagrestia Nuova, the New Sacristy, built, with interruptions, by Michelangelo between 1520 and 1534, and designed to offset Brunelleschi's Old Sacristy. The term "sacristy" is misleading since this is in fact a funerary chapel for the Medici.
The chapel was Michelangelo's first work as an architect and where he simultaneously applied his talents as painter and sculptor. This can be seen in the articulation of the internal walls, the three-dimensional treatment given to the architectural elements, the niches and pediments, arches and gables, both projecting and inset. The interior with its predominant "colors" of dark gray and brilliant white was evenly lit by the windows in the dome.
Besides acting as architect, Michelangelo was also commissioned to sculpt the tombs for members of the Medici family. He completed only two of the tombs, however, those of Giuliano, Duke of Nemours, and Lorenzo, Duke of Urbino (Lorenzo the Magnificent, his brother Giuliano, murdered in 1478, and Duke Alessandro, murdered in 1537, are all also entombed in the chapel but without monuments). Neither Giuliano, with his military commander's baton, nor Lorenzo with the grotesque helmet on his head (maybe a sign of his feeble-mindedness) are depicted as actual likenesses. Michelangelo, when reproached with this fact, responded that in a thousand years it would not matter to anyone what the two deceased really looked like. He deliberately wanted to transcend pure portraiture and create timeless figures, hence the simple names for them of "la vigilanza" (Vigilance) and "il pensiero" (Thought).
Giuliano de Medici, seated and robed as a Roman general, his baton in his hand, looks with a watchful eye over towards the Virgin and St Cosmas and St Damian, a group of saints much venerated by the Medicis, placed on the tomb of Lorenzo the Magnificent. Below Giuliano, on the slanting sarcophagus lid, lie the figure of Night, with crescent moon and stars in her hair, and poppy, owl and mask, and the figure of Day, unfinished, with an inscrutable rough-cut stone gaze into the void. Both figures are modeled on Classical lines. Night is reminiscent of a Leda from a Roman sarcophagus and Day is similar to the Belvedere torso. Michelangelo was thus quite consciously coming to terms with Classical sculpture while giving it a new Christian philosophical dimension.
The seated figure in the niche in the opposite wall represents Lorenzo de Medici. His head rests on his left hand, a pose indicative of pensiveness. Below the figure, on the sarcophagus lid, are the allegorical representations of Dusk, or Evening (left), and Dawn, or Morning (right). The male figure of Dusk embodies mental fatigue, the inert bulk of the sleeping body, contrasting with the female figure that symbolizes awakening, the body and spirit slowly regaining in strength. This contrasting pair also illustrates the struggle within Lorenzo, who was mentally deranged when he died. Lorenzo, like his opposite counterpart, also looks to the Virgin in the hope of redemption.
The Virgin, in her turn, is looking at the altar on the wall facing her, thus referring to Christ's sacrificial death, and the consequent Resurrection and eternal life. In this way all the figures communicate with one another, their interlocking gaze covering the room, an original concept of Michelangelo's.
Unfortunately the chapel reveals little of Michelangelo's grand design for the work as a whole. Giuliano de Medici's tomb, for example, should have had the two river gods of the Arno and the Tiber at its base, glorifying Tuscany and Latium as the two provinces ruled by Giuliano. In the niches to the left and right of the Duke of Nemours there should have been allegorical representations of Heaven and Earth in stone. The ceiling above should have held the display of trophies that can now be seen in the way through to the New Sacristy. And above it all, in the semicircle of the wall articulation, there should have been a fresco of the Resurrection, drawing the eye heavenwards via painted cassettes in the light area of the dome. With this chapel Michelangelo sought to bring architecture, sculpture and painting together to create a philosophical and artistic image that would reflect the path of life from the material world (river gods, sarcophagi), via humanity (day and night = life and death, Giuliano's statue) to the life eternal and the resurrection fresco.