If the vacuum velocity v of light is a function of the gravitation field only (Einstein-effect), then v is independent from the light frequence ν. This statement results from the Maxwell equations only and is independent from the special form of gravitation field theory. The frequence independence of the velocity of light is in a good arrangement with the astrophysical facts. – However, in non-linear generalizations of Maxwell's electrodynamics a hypothetical dependence of the light velocity from the intensity of the radiation field may result (Freundlich-effect). This hypothetical effect may give a dependence of v from the frequence. – With this excess of the light deflection an excess in the red-shift of the solar spectral-lines should be associated. In the visual domain of the solar spectrum the real value of the red-shift could be more than 20% greater than the Einstein-value.