The conoscopic image shows interference patterns of anisotropic specimen. To get a conoscopic image in a polarising microscope one eyepiece has to be removed from the tube. An auxiliary telescope or a Bertrand lens can be insThe conoscopic image shows interference patterns of anisotropic specimen. To get a conoscopic image in a polarising microscope one eyepiece has to be removed from the tube. An auxiliary telescope or a Bertrand lens can be inserted instead of the eyepiece into the tube. When an anisotropic specimen is placed between the crossed polarisers isochromatic curves are visible in the back focal plane of the lens.erted instead of the eyepiece into the tube. When an anisotropic specimen is placed between the crossed polarisers isochromatic curves are visible in the back focal plane of the lens.