http://poincare.matf.bg.ac.rs/~rvikt..._Cosmology.pdf
Relativity, Gravitation, and Cosmology, T. Cheng

p.49: This implies that the speed of light as measured by the remote observer is reduced by gravity as

c(r) = (1 + phi(r)/c^2)c (3.39)

Namely, the speed of light will be seen by an observer (with his coordinate clock) to vary from position to position as the gravitational potential varies from position to position.

p.93: Namely, the retardation of a light signal is twice as large as that given in (3.39)

c(r) = (1 + 2phi(r)/c^2)c (6.28)
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Equation (3.39) gives the variation of the speed of light with the gravitational potential predicted by Newton's emission theory of light.

Equation (6.28) gives the variation of the speed of light with the gravitational potential predicted by Einstein's general relativity. The factor 2 on the potential term is the essence of general relativity. This factor makes the general relativity's prediction incompatible with the frequency variation f(r)=(1+phi(r)/c^2)f confirmed by the Pound-Rebka experiment.

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