After Lawrence Weschler wrote about David Hockney’s theory about how Old Master painters might have used optical devices as aids in their work, he got an avalanche of protests. “I write about all sorts of things–hell, I write about relations between Jews and Poles, for God’s sake – so I’m used to getting letters. But I’d never found myself on the receiving end of anything like this. It turns out that the question of technical assistance may be the Third Rail of popular art history. Most people, it seems, prefer to envision their artistic heroes as superhuman draftsmen, capable of rendering ravishingly accurate anatomies or landscapes or townscapes through sheer inborn or God-given talent.”