|P. P. Ewald was responsible for
interpreting Laue's results in terms of reciprocal lattices. He
devised a simple geometric construction that demonstrates the
relationship in quite elegant but simple terms.
He separated the origins of direct and reciprocal space placing the crystal at the center of a sphere of radius 1/l. The origin of reciprocal space is placed on the surface of the sphere where S0 (the incident beam) exits the sphere.