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 The magnitude $$U$$ is, what we usually call the velocity of the light ray.

If we now pass to the corresponding motion in the progressing body, then the considered line remains (§ 60, b) a light ray, and we obtain for the determination of the deviations of equilibrium, in the different points of it, expressions as

or, by (34) and (85),

where $$\mathfrak{p}_{s}$$ is the component of $$\mathfrak{p}$$ in the direction of the light ray, while the new constant $$B''$$ has the value

The expression (86) goes over into

and here, $$U'$$ is the velocity of the light ray in the moving body, when we put

From that we conclude

a formula, whose shape agrees with (82), in which $$U$$ and $$U'$$ are now related to light rays of the same direction.

§ 71. Formula (84) has found a nice confirmation by the experiment, that were first executed by and later