  We consider the magneto-electric optical properties of the quantum vacuum and
show that all the different phenomenona are related by Lorentz invariance. As a
model calculation we show how crossed fields properties can be calculated
starting from single field properties by using Lorentz transformations. Using
this method we have studied for the first time the case of a crossed static
magnetic field and electric field applied with one of these two fields parallel
to the direction of light propagation. We also show that parallel field
properties can be found using general symmetry properties.
