rosin does hardly show any colour change in rotation with polarisation and retardation plate. This is not a surprise as rosin is amorphous. Colour differences are probably caused by light dispersion due to different focus per layer in the z-stack the images are composed of. Rosin dispersed in water, sample was diluted before mounting. Sample courtesy Hugo Geerdinck, Coldenhoven papier, Eerbeek [micrographs and z-stacks BvV].