By Rachel K. Sobel

The strange glow emanating from Wilhelm Roentgen's laboratory in 1895 started out merely as part of an experiment to explore the laws of physics. But the invisible stream of particles producing this luminescence turned out to do a lot more. Roentgen found that the rays (which he labeled "x," since their properties were unknown) left impressions on photographic plates, making the seemingly solid and impenetrable–human skin and tissue–translucent and transparent.

Within weeks, Roentgen's discovery had forever changed the understanding of human biology. "No one has ever seen the same way as they did before 1895," says Bettyann Holtzmann Kevles, a history lecturer at Yale University. As described in her book Naked to the Bone: Medical Imaging in the Twentieth Century, it was as if "the mind was walking in among the tissues themselves." In an era of bodily embarrassment, X-rays invited ordinary citizens to boldly reveal their inner selves. The cultural impact was stunning. Soon, X-ray machines for public entertainment appeared everywhere, although scientists later deemed them dangerous. In Chicago, people lined up to get a glimpse of the bones in their own hands at a coin-operated X-ray machine. At an opera house in Lawrence, Kan., couples could have X-ray snapshots taken as love tokens. And in New York, the wealthy visited X-ray studios to get their inner portraits done.

Writers and artists embraced the unfettered spirit of the X-ray. In his 1897 novel, The Invisible Man, H. G. Wells built upon the already startling notion of seeing beneath the skin. His protagonist discovered rays that were similar to "Roentgen rays" but instead made the entire body invisible. A few years later, a group of painters in Italy who called themselves futurists adopted X-ray imagery in their early-20th-century manifesto. Asked their spokesman Umberto Boccioni, "Who can still believe in the opacity of bodies . . . ?"

By then, X-rays had also become a medical necessity, proving critical for surgeons trying to set fractures and identify bullets or other foreign material in the body. "The X-ray served as a kind of beginning of a whole new way of delivering healthcare," says Joel Howell, professor of history and in­ ternal medicine at the University of Michigan. In a departure from the patient's reliance on the family physician, "it helped build a profound and abiding trust in the idea that science and technology will lead to better healthcare," Howell adds.

By the end of the 20th century, thanks to X-ray's daughter technologies–CAT, MRI, and PET– biological images took on sharp clarity, revealing not only bones but also muscle, tendons, blood vessels, and cartilage. Neurologists as well as scholars in philosophy, literature, and anthropology have taken to studying snapshots of the brain: What portions of the brain are affected by stroke? Where does our concept of self come from? How does love arise from our neural circuitry? And do novels activate the same synapses as daydreaming does? As scientists continue to pry deeply into the human body, they may one day illuminate–in the spirit of Roentgen–many more secrets of human nature.