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Fredrik Bull was born in Kristiania (the present-day Oslo) to Dr. Ole Bornemann Bull (1842–1916) and his first wife Marie Cathrine Lund (1843–1884). Dr. Ole Bull was a renowned eye doctor. He collaborated with Gerhard Armauer Hansen who discovered "Mycobacterium leprae", the causative agent of leprosy, while investigating the effects of leprosy on the eyes. He is also known for having developed a method to determine the degree of sensation of color.