He is best known for writing two editions of an illustrated history of Norway, the Illustreret Norges historie. He moved to Kristiania in 1878, and worked at the National Archives there until 1885. He then began writing what would become the first edition of his Historie, which was published in five volumes from 1885 to 1895.
He was at the same time developing a 12-volume edition of the work, finishing in 1898.
In between working on this major work which received fundings from Stortinget, he also wrote a number of other, smaller studies, biographies, and reviews. He was characterized by Terje Bratberg as "one of the most prolific historians Norway has produced".