Any two rectilinear light vibrations A and B whose angle is given by u always have a time-average direction. They are thus able to induce an anisotropy in a silver-silver chloride photographic emulsion H. Here we show that when they are incoherent, their linear combination may be retrieved by observation of H between crossed polarizers after exposure. In the case they are coherent, H exhibits an interferogram whose black fringes characterize the value of angle u.