#18: X-rays discovered in cathode ray experiments
Something was glowing where it should not have been. That was the puzzle Wilhelm Röntgen noticed in 1895 while experimenting with cathode rays. A nearby screen reacted even though the tube was covered and visible light was blocked. He realized that an unknown kind of ray was passing through materials, and he called it X-rays, with the X standing for the unknown. One of the early images, showing bones inside a hand, must have seemed almost unbelievable at the time.
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