Aberration, Continued

Dutifully to follow the storyline taken in the standard textbooks: “An entirely different piece of experimental evidence shows that Fresnel’s equation must be very nearly correct. In 1871 Airy remeasured the angle of aberration of light using a telescope filled with water”, and “it will be seen that if the velocity of the light with respect to the solar system be made less by entering the water, one would expect the angle of aberration to be increased… Actually the most careful measurements gave the same angle of aberration for a telescope filled with water as for one filled with air.”(27)

It was, as said, feasible to explain this strange phenomenon with Fresnel’s dragging coefficient, but “a different explanation is now accepted, based on the theory of relativity”.(28) Or to quote van der Waals: “It is possible generally to prove how Fresnel’s theory entails that not a single optical observation will enable us to decide whether the direction in which one sees a star has been changed by aberration. By means of aberration we can hence not decide whether the Earth is moving or rather the star: only that one of the two must be moving with respect to the other can be established. Fresnel’s theory is hence a step in the direction of the theory of relativity.”(29)

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