Complex sentence: In his continuation of the Hals project at Haarlem, he brought in the chemist Gosen Van der Sleen to aid in his analysis--the first known case in the Netherlands of a chemist advising on a painting restoration project.
Split into simpler sentences: In his continuation of the Hals project at Haarlem, he brought in the chemist Gosen Van der Sleen to aid in his analysis. This was the first known case in the Netherlands of a chemist advising on a painting restoration project.