The National Archives today released the complete 1940 census, a trove of personal information sure to delight genealogists, historians, and anyone interested in learning more about their roots. With 3.9 million images scanned from more than 4,000 rolls of microfilm, the database represents the largest collection of digital information ever released by the Archives, and the first time a census has been released online. It consists of individual responses to the survey that have been kept private for seven decades. “Many of us will be discovering relatives and older family members that we didn’t know we had, picking up threads of information that we thought were lost, and opening a window into the past that until now has been obscured,” David S. Ferriero, the nation’s archivist, said at a morning ceremony in which he performed the first search.