To know something like the back of your hand is a timeless concept, one taken yet further by Wilhelm Konrad Roentgen.
While working on a series of experiments with a Crookes tube, he noticed that a bit of barium platinocyanide emitted a fluorescent glow. He then laid a photographic plate behind his wife’s hand (note the wedding rings), and made the first X-ray photo.
Before that, physicians were unable to look inside a person’s body without making an incision.
Roentgen was the recipient of the first Nobel Prize for Physics in 1901.