D O C . 6 X - R AY R E F R A C T I V E I N D I C E S 5 3
Published in Deutsche Physikalische Gesellschaft. Verhandlungen 20 (1918): 86–87. Received
21 March 1918, published 30 June 1918.
Alban Köhler (1874–1947), a professor and radiologist at St. Joseph Hospital, Wiesbaden,
observed narrow, light borders within the shadows of human limbs on positive X-ray photographs
(Köhler 1916). In his preliminary considerations he excluded the effect of secondary X-rays and pho-
tochemical causes (e.g., solarization) but not X-ray diffraction. He consulted with Max von Laue
(1879–1960), Professor of Physics at the University of Frankfurt, but left the problem unsolved.
Bernhard Walter (1861–1950), Professor at the State Laboratory of Physics, Hamburg, was of the
opinion (Walter, B. 1917) that the stripes were due to an optical illusion, an explanation that, in addi-
tion to diffraction, had already been proposed by Cornelis H. Wind (1867–1911), Professor of Math-
ematical Physics and Theoretical Mechanics at the University of Utrecht (Haga and Wind 1899) to
interpret lines found on photographs produced by passing X-rays through a slit. Walter added that the
illusion itself was discovered by Ernst Mach in the case of visible light (Mach 1865, 1866a, 1866b
and 1868). In order to eliminate the stripe, Walter proposed Wind’s method (Wind 18991900) of
shifting a piece of black paper of the same shape as that of the limb’s shadow toward the stripe: the
stripe disappears before the paper completely covers it. Köhler again discussed Walter’s arguments
with Laue (Köhler 1918), who apparently encouraged Köhler to send a copy of his first paper to Ein-
stein. (Reprints of Köhler 1916 and Walter, B. 1917 are in Einstein’s reprint collection.)
According to the “classical” theory of dispersion (“Drude-Lorentz theory”):
where n is the refractive index of the medium traversed by light of frequency ν, N is the number of
electrons per unit volume of the medium, e is their charge, me their mass, and νe their eigenfrequency
(see, e.g., Lorentz 1909, p. 151).
Köhler observed the stripe within the shadows.
Since the discovery of X-rays until 1912, when Max von Laue incontestably proved the exist-
ence of X-ray diffraction (Laue 1912), stripes and lines were often observed on X-ray photographs.
It was debated whether these could be attributed to diffraction, optical illusion, photochemical pro-
cesses or to their combination (see, e.g., Fomm 1896, Haga and Wind 1899, Wind 18991900, Walter,
B. 1902). After Arthur H. Compton’s experiments on the reflection of X-rays on crystals (Compton
1917a, 1917b), Einstein further explored their total reflection and refraction with this paper (see
Schön 1929). In his letter to Heinrich Zangger of 22 April 1918 (Vol. 8, Doc. 514), Einstein was con-
fident that with total reflection he had succeeded in explaining Köhler’s stripes “with high probabil-
ity” (“mit grosser Wahrscheinlichkeit”).
Upon reading the paper, Edgar Meyer (1879–1960), Professor of Experimental Physics at the Uni-
versity of Zurich, attributed the stripes to Mach’s optical illusion and proposed to Einstein Wind’s test
(see note 1) without mentioning Mach’s or Wind’s name (see Edgar Meyer to Einstein, 12 September
1918 [Vol. 8, Doc. 614]).
The total reflection of X-rays was eventually demonstrated in 1922 by Compton (1892–1962),
Professor of Physics and Head of Department of Physics at Washington University, who also mea-
sured the refractive indices of several substances (Compton 1923). (A reprint of Compton’s paper is
in Einstein’s reprint collection.)
n2 1 –
n2 2 +
3me( νe ν) –