X-ray viewing machine. Historical artwork of the fluoroscope designed and built by the French inventor Eugene Ducretet (1844-1915). This device was used to examine postal packets and parcels using X-rays. It was built only a few months after Wilhelm Roentgen had discovered X-rays in 1895. The fluoroscope worked by using a screen coated with calcium fluoride to stop the X-rays and produce light instead. This production of light by atomic excitation is known as fluorescence. If an object being examined stopped the X-rays, then it would show up as a dark area on the glowing screen. Artwork from A Travers l'Electricite (G. Dary, Paris, 1900).