Complex sentence: The museum was founded in the late 19th century by Artur Hazelius, who also founded the open-air museum Skansen, long part of the museum, until the institutions were made independent of each other in 1963.
Split into simpler sentences: The museum was founded in the late 19th century by Artur Hazelius, who also founded the open-air museum Skansen. For long part of the museum, the institutions were made independent of each other in 1963.