IT'S not often that cannons get used for target practice. These images show some of the first machines built to emit X-rays, now given a taste of their own medicine. The stark, ghostly quality of X-ray scans have transfixed us for over a hundred years. When you shine an ordinary beam of light at an opaque object, it casts a shadow. But if the light's frequency is sufficiently high, the beam can pass through tissue as though it were transparent.
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