Transparent specimens are usually examined by dark field, phase contrast and interference contrast light microscopy. In dark field, specimens are illuminated by oblique light beams that come from the periphery of the illuminating apparatus. Therefore, some transparent objects, e.g. unstained native bacteria, are barely visible and fine structures inside them are often not visible. In phase contrast, the discernment of fine detail can be reduced by halo artifacts. The intensity of contrast, i.e. the difference in brightness between the background and specimen, is constant and not variable; it is determined by the specification of the phase ring within the phase contrast lens and dependent on the specific phase differences between the specimen and its surrounding medium. Interference contrast images are free from halo artifacts, but their contrast may be lower than in corresponding phase contrast or dark field images, especially, when transparent specimens are examined in thinlayer preparations.