A microscope was developed for the in situ, quant. mapping of solute concn. profiles around growing crystals. By this optical instrument, which is based on a combination of the Schlieren technique and the dark field method, images of diffusion fields are readily obtained. The position-dependent intensity of these images is proportional to the local x or y gradients of solute concn. After calibration, video recording, digitizing and numerical integration of the image, quant. and detailed concn. maps were obtained. The viability of the method was demonstrated by the in situ measurement of AgNO₃ diffusion fields around Ag dendrites growing from aq. soln. by electrolysis.