Not long ago, a woman living alone in the west of England was disturbed by the sound of a thief in her home. She screamed, and the thief ran away. The incident was reported to the police. They found an article of clothing in the kitchen of the woman's home and sent it to Forensic (法医学的)Science Laboratory in London. Scientists at the laboratory, on examining the clothing, found tiny shreds of a certain unusal metal which then had only one use. It was used in the manufacture of jet-engine rotors. That put the police in mind of one particular man. He worked as rotor grinder in a factory near the woman's home. He was the man all right. And that is what is meant by trace evidence. There is nothing new about trace evidence, as any reader of detective stories knows. That, after all, was what Sherlock Holmes was looking for through his magnifying glass. What is new is the way forensic scientists find it, and use it, to help track down criminals. A thief who broke into a church in the north of England was caught because traces of wax, found on his clothes, had come, not from household candles, but the sort of candles used only in churches. A man who kicked a girl in the face was found to have in the toe of his boot traces of white powder. Although scientists from the laboratory occassionally visit the scence of a crime, the evidence as a rule comes to them, sometimes in the form of stained or torn clothing, or sometimes in the form of weapons used to commit a crime. One of the points they look for is "fiber transference". "That means the transfer of fibers from one's clothes to another one's clothes," one of the laboratory's senior staff said. "That would help us to establish the degree of contact between two people. We have to go over every inch of a piece of clothing, picking up tiny shreds of fibers to see if they match what we're looking for."