It is a matter of common knowledge that the field of gravitation influences the propagation of light, but the question to which extent electromagnetic fields do something similar is not completely answered. The birefringence caused by electric and magnetic fields is subject of several articles in Quantumelectrodynamics, but the absolute refractive index is discussed rather rarely. Some hypothetical considerations about the feasibility of an experiment to analyze the refractive index in electromagnetic fields were given in a former article by the same author. A first experimental test is reported now. Perhaps an evidence for an influence of an electric field on the refractive index was found, but the result is not reliable enough to claim already, that the influence was seen - but it should encourage to perform further measurements with different methods of higher precision.