In bid to fight art fraud, Sotheby's announced Monday that it had purchased a forensics firm whose founder once helped the auction house belatedly identify a $10 million painting as a fake. Sotheby's said that Orion Analytical, based in Williamstown, Massachusetts, will be folded into the company and its founder, the artist, conservator and forensic scientist James Martin, will lead a new scientific research department charged with making sure the works the auction house deals with are authentic.