A century ago, the living body, like most of the material world, was opaque. Then Wilhelm Roentgen captured and X-ray image of his wife's finger--her wedding ring "floating" around a white bone--and our range of vision changed forever. By the 1920s, X-ray technology was common-place: all army recruits had lined up for chest pictures during WWI, and children were examining the bones of their feet in shoe store fluoroscopes, spectacularly unaware of...