analyzer, it will be dark again (§ 60, b), and we may therefore conclude:
Which direction Earth's motion may have, whether from the polarizer to the analyzer, or vice versa, light will always be erased at the presupposed position of the analyzer, as long as nothing is changed as regards the relative period of oscillation and the direction of the rays in relation to the apparatus.
Obviously, these conditions would have been met by the experiments, when the sun would have emitted white light. The relative oscillation period would thus have been as it is required by Doppler's law, and namely at each position of the apparatus. As for the direction of the beams in relation to the glass columns, it has probably not been exactly the same with respect to the various readings; however, this has not caused an error, since an influence of a small directional change of the incident light would hardly have been overlooked by the observer.
§ 95. The phenomenon that was expected by Fizeau and what he really believed to have observed, would have to occur even when using homogeneous light. Thus, here we come to a contradiction, that I can not solve. A source of error, of which one could say that it would have caused the differences in the analyzer locations, I could not discover. The activated circular-polarizing substances were probably a little too thick to allow a prominent influence of Earth's motion considered in § 87. Nor is it possible to think of an effect of terrestrial magnetism. The only thing might yet be, that the two mirrors located east and west of the apparatus, have not always received light of the same nature. Namely, to reflect rays of the sun, sometimes by one, and sometimes by the other mirror, the heliostat had to have different positions; between the angles at which he threw back light in both cases, there was a difference depending on the position of the sun, and we know that light reflected by a metal surface, has not the same composition in all directions of incidence.
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