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(Received July 24, 1914.)

Two papers by published at the end of 1913, caused me to write a supplement in this proceedings at the beginning of 1914.

has provided and executed an interference experiment, aimed at deciding the alternative "aether or relativity principle?"; I conclude that the "Sagnac effect" does not decide this alternative.

According to reports later given to me, I am able to add the following:

1. A similar interference experiment was already described by in 1904. The (not executed) experiment of 1904  and the  experiment of 1913 have the fundamental idea in common, to split a light ray in a rotating circle, and to circulate one ray counter-clockwise, and the other one clockwise. The difference consists mainly in the fact, that uses a circle rotating relative to Earth, while  wanted to use a circle at rest relative to Earth, thus he wanted to use the self-rotation of Earth.

2. In the year 1911, concluded that this expected  effect cannot bring a decision between aether theory and relativity principle.