Einstein offered two equations showing how the speed of photons moving
radially with respect to a spherical mass varies with the
gravitational potential V:

(A) c' = c(1 + V/c^2)

(B) c' = c(1 + 2V/c^2)

(see http://www.mathpages.com/rr/s6-01/6-01.htm )

Equation A is consistent with the Pound-Rebka experiment and also
(through the application of Einstein's equivalence principle) with the
equation c'=c+v given by Newton's emission theory of light (v is the
speed of the light source relative to the observer). It is not clear
what equation B is consistent with.

Pentcho Valev
pvalev@yahoo.com