In 1895, Wilhelm Roentgen, a professor of Physics in Worzburg, Bavaria, was the first to find a way to peer inside the body without surgery. On the evening of November 8, he was experimenting with the conduction of electricity through low-pressure gases using an induction coil and a partially-evacuated glass tube when he accidentally discovered a mysterious ray capable of lighting up a fluorescent screen a few meters away. When he passed his hand between the ray and the screen, he glimpsed a shadow of his own bones. Further experimentation showed that the screen could be replaced by a photographic plate—and the x-ray was born. Roentgen would later earn the first Nobel Prize in physics for his discovery.

Since then, x-rays have revolutionized medical diagnosis and made a huge impact on astronomy, chemistry, and other branches of science. They’ve allowed us to peer inside our own DNA, as well as into distant galaxies. In 2009, the x-ray was named the most important modern scientific discovery by nearly 50,000 people in a Science Museum of London poll; even penicillin came in second.

1. THE X MEANS UNKNOWN.

Roentgen named his discovery X-strahlen—strahlen being German for “beam” or “ray,” and “x” being used in mathematics to indicate an unknown quantity. Even though we now know much more about how x-rays work, their name has retained a sense of their original mystery. The rays have also been called “Roentgen's rays,” and the images they produce are sometimes known as “roentgenograms.”

2. ONE OF THE EARLIEST X-RAYS WAS OF THE DISCOVERER’S WIFE’S HAND.

Like many scientists, Roentgen started out by experimenting on his wife. One of his first x-rays—if not the first—was of his wife Anna Bertha's hand with her wedding ring on her finger (above). She was reportedly unimpressed by the image; by some accounts, she exclaimed “I have seen my death!” after looking at it for the first time. (You can see other very early x-rays courtesy of the British Library here.)

3. THEY WERE ALMOST IMMEDIATELY PUT TO USE.

Within weeks of Roentgen’s announcement, European surgeons were using x-rays to find bullets and other foreign substances in human bodies. One of the earliest diagnoses was by a British doctor who found a needle embedded in a woman's hand. By the following year, an x-ray department had been set up at the Glasgow Royal Infirmary, and x-rays were being used clinically in the US to diagnose bone fractures and gunshot wounds.

Not all uses were medically necessary, however—the daughter of one early adopter later reported that “at one of my birthday parties we had fancy rings for the children to wear and showed them their skeletal hands to loud shrieks of excitement: knowing what we do today, of course, he wouldn’t have done it.”

4. PEOPLE USED TO THINK THEY WERE HARMLESS.

In the early days, people thought x-rays passed through the body as harmlessly as normal light. It wasn’t until Thomas Edison’s assistant Clarence Dally, who had worked extensively with X-rays, died of skin cancer in 1904 that people started taking the health concerns about the new technology seriously.

Partly as a result of the perceived harmlessness—but mostly because of the novelty factor—there was a late-19th-century and early 20th-century vogue for x-ray machines, which started to appear at carnivals and as a curiosity in theatrical shows. The word “x-ray” was even added as a promotional gimmick to products like headache tablets and stove polish—part of a brief “x-ray mania” that saw the rays frequently mentioned in advertising, songs, and cartoons.

During the 1930s, '40s, and early '50s, x-ray machines were also a not-infrequent feature of American shoe stores, which used them to ensure a better fit. You can see a demonstration of the concept in this clip from the 1920s silent film, General Personal Hygiene:

5. THEY REVOLUTIONIZED THE TREATMENT OF TUBERCULOSIS.

Until the advent of antibiotics for tuberculosis in the mid-20th century, rest in a sanatorium was generally considered the only cure for TB. Early detection was thought to be key for the cure to work, but the traditional method of diagnosing was to listen to chest sounds, which could sometimes be difficult to diagnose accurately. X-rays finally allowed doctors to see the characteristic shadows and spots on the lungs caused by masses of the M. tuberculosis bacteria, and mass radiography began to be used in armies, factories, and mines, with many lives saved as a result.

6. THEY CAN KILL CANCER.

Early experimenters with x-rays noticed that the rays had a tendency to burn skin, a tendency made worse by the fact that older machines exposed people to much higher doses of radiation than today. But while overexposure to the rays can cause cancer, they can also cure it. Even back in Roentgen's day, doctors were using x-rays to burn off moles. Besides being used for diagnosis, today narrowly focused beams of x-rays are used in some forms of cancer radiotherapy to destroy tumor tissues.

7. THEY ALLOWED US TO FIND THE STRUCTURE OF DNA.

Hulton Archive/Getty Images

Our understanding of the double-helix shape of DNA was provided in part by x-ray crystallography—a technique in which x-rays bounce off the three-dimensional pattern of atoms within a crystal lattice to form a shadow image of its structure. In the early 1950s, a British researcher name Rosalind Franklin took the x-ray photos that first showed DNA’s structure, but died before she could share the Nobel Prize with the men more generally given credit for discovering the shape of the “secret of life”—James Watson and Francis Crick.

8. THEY’VE HELPED US SEE INTO SPACE

More than a dozen telescopes that detect x-rays have been launched into space, which have allowed us to make discoveries far beyond our own solar system. In 1999, NASA deployed their Chandra X-ray Observatory aboard the Space Shuttle Columbia, which has since discovered black holes, advanced our understanding of dark matter, and looked at the huge black hole at the center of the Milky Way, among other achievements.

9. THEY’VE CHANGED OUR UNDERSTANDING OF ART AND ARTIFACTS.

X-rays have allowed scientists and art historians to see “underpaintings”—the rough sketches painters once used to guide their work—among other secrets. Seeing these underpaintings can help art historians gain a better understanding of the way artists once worked. X-rays can also show how paintings have been changed and restored over time, sometimes allowing for more authentic restorations.

X-rays have also been to study priceless artifacts—like Egyptian mummies—without damaging them. And they’ve revealed surprises, like the human corpse inside one Chinese statue. They’ve even been used to peer inside opaque amber to view otherwise invisible fossils of ancient animals, insects, and plants.