The resplendence of the Boston Public Library's McKim Building unfolds with each step up the grand staircase. Past the imposing stone lions, the golden-hued stairwell gives way to an airy gallery of murals, a millennium-spanning celebration of the muses. Nearby, a narrower stairway leads to the hushed third floor, where people wander the opulent gallery to untangle John Singer Sargent's "Triumph of Religion," a monumental cycle of murals the artist left unfinished despite nearly three decades of work.