How do forensic scientists track down evidence? Ever since Sherlock Holmes and his magnifying glass, we have been fascinated by detectives looking for clues. ANGELA SINGER begins a series on the work of forensic scientists at the Forensic Science Service in Huntingdon. Next week: DNA, documents and drugs. Pictures: HELEN DRAKE SUE LAVER put on a pair of large pink knickers and asked a colleague to rip them off her. It was important the knickers were torn, in order to see where they would tear in a sexual attack.