Farciot Edouart:
"By 1921, he was settled again in Hollywood. He joined Paramount Pictures, where he spent most of his career. At first he made "glass shots," paintings of glass extensions on optical glass which, when positioned in front of a camera, would make the set seem more than it was. From there he progressed to the complicated "blue-backing" process. This was essentially the matte-work system of the 1920s, an incredibly exacting method of placing actors in a studio against an artificial background and making it look real."

from: The Art Of The Cinematographer: A Survey And Interviews With Five Masters by Leonard Maltin. Dover Publications, 1978.