3 9 6 D O C U M E N T 2 3 3 M A R C H 1 9 2 6 ALS. [20 386]. The verso of p. 3 has calculations in Einstein’s hand (see Doc. 234). There are perfo- rations for a loose-leaf binder at the left margin of the document. [1]Doc. 231. [2]Rupp used a Michelson interferometer with slits in front of the mirrors at the end of each arm. Here and in what follows, page numbers refer to Rupp 1926a. [3]Rupp lists maximum coherence lengths of 15.2 cm for Hβ and 62 cm for Hg (p. 14). [4]The proposal of an experiment in Einstein 1926p (Doc. 223), the proofs of which Einstein had enclosed in Doc. 231. [5]A Kerr cell uses the electro-optic Kerr effect (a change in refractive index of a substance under the influence of an electric field) to act as a very fast shutter.