“Portrait of an Old Man” by the Dutch master Rembrandt Harmenszoon van Rijnis a master class unto itself. The oil painting, which graces the walls of the Harvard Art Museums, reveals much about both technique and emotion. Curators say the piece, completed in 1632, showcases “the dramatic lighting and painstaking description and level of detail and texture that inform his early work.” This summer the evocative portrait will hang alongside one of Harvard’s newest Rembrandt holdings, “Four Studies of Male Heads,” an early drawing in brown ink that connects visitors even more closely with Rembrandt’s hand — a hand that inspired countless others.