Complex sentence: He thought of using two lanterns to make the spirit of Samuel appear out of a mist in his representation of the Witch of Endor, which led to the idea of using the technique with landscapes.
Split into simpler sentences: He thought of using two lanterns to make the spirit of Samuel appear out of a mist in his representation of the Witch of Endor. While working out the desired effect, he got the idea of using the technique with landscapes.