D O C U M E N T 1 5 6 N O V E M B E R 1 9 2 3 1 5 1
156. From Peter Pringsheim
Berlin W, 63 Lützow St., 17 November 1923
Dear Professor Einstein,
I received your postcard from Leyden, likewise—with some delay—the two let-
ters still dated from Berlin, which were not entirely correctly addressed. I had set
about implementing the experimental arrangement for the “fundamental experi-
ment” after our first conversation exactly as you sketch it in your letter. However,
because I recalled an earlier conversation with
about related questions,
for safety’s sake I wrote to Göttingen at the same time and have meanwhile re-
ceived the reply that he has a graduate student sitting on the same topic, working
already for one and a half years essentially according to just our method as well.
The research is now as good as finished and should be going into press in the com-
ing weeks, initially on Hg resonance
measurements with Na vapor are
supposed to follow. As Franck does not like to write and suspects that you are still
here, he asked me to report to you about what in his view are now already secure
results. Well, the effect is not present, not even if one brings the reflecting mirror
to a raised temperature—an eventually present broadening of the line is surely be-
low the measurement precision. However, one does immediately get a noticeable
line broadening, as would be expected, when one induces the narrow resonance line
of cold Hg vapor to absorb and reemit in Hg vapor at high
This is
now supposed to be expanded in order to determine what happens when the vapor
luminescence of the hot vapor is augmented in the second tube until regular (selec-
tive) reflection occurs. Here, in my view, another complication arises, though, that
according to Wood’s measurements this regularly reflected line always manifests a
one-sided broadening toward short waves, corresponding to the course of the
anomalous dispersion in the vicinity of the absorption
This regular reflec-
tion is known to be interpretable as nothing but a continuation of the resonance ra-
diation, in that it already completely vanishes at a specific vapor density and only
begins to appear at a noticeably higher vapor density. The non-presence of this ef-
fect for reflections off normal mirrors is hardly going to surprise you anymore, after
what you have written in your last postcard, of course.
Now that this problem has been sorted out in this way for me, I want to try
whether I can do something with the astonishingly high polarization of the Hg-
vapor resonance in field-free space. The idea, which is apparently not unappealing
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