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. Doc. 231. 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. Rupp lists maximum coherence lengths of 15.2 cm for Hβ and 62 cm for Hg (p. 14). The proposal of an experiment in Einstein 1926p (Doc. 223), the proofs of which Einstein had enclosed in Doc. 231. 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.