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§ 72. The mentioned physicists have compared their observations, not with that formula, but with another in which the last term is missing; a satisfying agreement occurred at this place. Namely, if we put

$W''={\frac {V}{n}}\pm {\mathfrak {p}}\epsilon$,

thus the coefficient ε can be derived from the experiments. Now, while Michelson and Morley found in this manner

$\epsilon =0,434$,

"with a possible error of $\pm 0,02$", $1-{\frac {1}{n^{2}}}$ for D-light has the value 0,438.

By our theory it should be

$\epsilon =1-{\frac {1}{n^{2}}}-{\frac {1}{n}}T{\frac {dn}{dT}}$

or, if we consider n as a function of wave length λ in air,

$\epsilon =1-{\frac {1}{n^{2}}}-{\frac {1}{n}}\lambda {\frac {dn}{d\lambda }}$

For the Fraunhofer line D, this becomes

0,451.

Thus formula (90) somewhat further deviates from the observations than the simpler equation

$W''={\frac {V}{n}}\pm {\mathfrak {p}}\left(1-{\frac {1}{n^{2}}}\right)$;

(91)

however, the observation were possibly not as exact for allowing us to put weight to this condition.

If it should be achieved, which namely appears to be difficult but not impossible, to experimentally distinguish between the equations (90) and (91), and if the first one should be justified, then we would have observed the Doppler variation of the oscillation period for an artificially generated velocity. It is only by consideration of this variation, that we have derived equation (90).

§ 73. It is hardly necessary to recall at this place the importance of the role, which is played by formula (84) in the theory of aberration and the related