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the standpoint of the previously resting system (aether "at rest", while the disc is rotating).

Fig. 3 serves to illustrate this: The polygon shall be (in the stationary process) a closed square, $O123O$ for light ray $R$ and $O321O$ for light ray $L$; $OO'$ be the arc ${\tfrac {v}{c}}\cdot U$ followed by point $O$ during the normal light circulation $T={\tfrac {U}{c}}$, when the apparatus is rotating as above (arrow direction $L*$). In this view, a real contraction occurs for light path $R$, and for light path $L$ a real elongation, since the light paths are in the same approximation as above $O1'2'3'O'$ and $O1''2'3''O'$. If one computes the phase displacement, then the same amount is given as above (formula 6).

Furthermore, the same is given in a familiar way from the standpoint of the anti-aether theory (relativity principle)^[1], one only has to replace the word aether by the word "one inertial system".

Thus this effect must be demanded also from the standpoint of the relativity principle, namely the cause for its realization is, that the apparatus is rotating relative to an "inertial system", and that (seen from an inertial system) an objective contraction or elongation of first order arise in the total light paths.

The same conclusions then naturally also hold (from the standpoint of relativity) for any inertial system.^[2]

8. Consequently, also assertion c) is correct: The effect doesn't prove the aether.

The rotating relative aether wind of § 4 shall not be used at all from the anti-aether standpoint; the rotating system is, in the sense of the terminology of relativity, no

1. It is to be considered, that the relevant translatory velocity of the mirrors lies in the mirror plane. A potential influence of a possible temporal duration of the reflection process itself, is of course neglected (as it was also done by Sagnac). My conclusions are also based on the assumption, that the often disputed process of rotation of a rigid system as such, constitutes no problem for the relativity principle.
2. For example, also for that one (changing continuously), in which point $O$ is present at every moment (in the direction of the circle tangent, the velocity relative to the "rest"-inertial system is $v$ as given above).