Documentary grandmaster Frederick Wiseman delves into one of the world’s greatest museums, a London institution housing seven centuries’ worth of art and several individuals responsible for keeping it freely available for public observation. To report his aesthetic and administrative findings, he’ll just need 181 minutes of your time. For context, that’s nearly 40 minutes longer than the new Hobbit movie, safely described as more of a romp than this, although Wiseman might be said to share Peter Jackson’s eye for severed heads, which the National Gallery’s paintings contain in surprising abundance. (Presumably none of them belong to Orcs.) Loping along, lacking an explicitly obvious structure, Wiseman’s film at best approximates the experience of drifting through the museum and crashing the occasional tour group when a guide’s lecture gets good, only eventually to feel the urge to move along and cover more ground while time allows. It also reveals the behind-the-scenes procedures of curatorial discretion, including fretting over marketing strategies and the ever-encroaching lowest common denominator of public taste. Wiseman always has been good at hanging around during meetings, gathering subtle behavioral information in addition to the copious discussions people have in order to make their institutions work. In National Gallery, these parts could be shorter, whereas the staring-at-painting parts might benefit from being lengthened to allow for more sustained contemplation. By the time we’re back outside, in the bustle of Trafalgar Square, it feels good to get a breath of fresh air.