The Adoration of the Shepherds

Here the Christ Child, the Light of the world (John 8:12), is the most important source of light, and all contrasts and reflections of light can be traced back to Him. The face of Mary, gazing directly out of the picture, is illuminated most powerfully by that light, which is reflected somewhat more weakly on the faces of St. Joseph and the shepherds.
Mengs gives the figure of St. Joseph special prominence. The only standing figure in the painting, St. Joseph forms a triangle with the kneeling Virgin who holds the Infant Jesus in her arms, perhaps alluding to the Holy Family as the earthly trinity. St. Joseph is depicted as a protective presence, as well as rapt in contemplation of the events taking place. The artist’s alignment of St. Joseph with the palm tree in the background landscape is also significant. In one of the most famous sermons ever preached on the saint, St. Francis de Sales offered an exposition of the “exact resemblance and analogy” between St. Joseph’s virtues and the palm tree’s attributes, as the latter were understood since Classical Antiquity, specifically virginity, humility, constancy, courage, and perseverance. Francis explains that this parallel is indicated by Psalm 92:12, “The just is like a palm tree,” the entrance antiphon for Mass on the Solemnity of St. Joseph, the occasion for this sermon.
(Fr. Joseph F. Chorpenning, O.S.F.S.)