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Policeman undergoing forensic training. They are using a reagent to search for latent bloodstains in a staged crime scene. The reagent is able to reveal bloodstains even when the blood is diluted to one part in ten thousand, and detergents have been used to remove them. In the presence of blood the reagent emits a blue-white glow that can be seen in low light. Photographed in Fontainebleau, France. (KEYSTONE/SCIENCE PHOTO LIBRARY/MATTEIS/LOOK AT SCIENCES/SCIENCE PHOTO LIBRARY)