If an arc or circle of light is observed in the back focal plane of the objective, change to a slightly larger field stop. The image of the lamp condenser partially fills the viewfield, obscuring details of the specimen. The slide may be too thin or the condenser may be adjusted too high. Measure the thickness of the microscope slide and check to make certain the oil contact is not broken. If difficulty is encountered keeping the bottom of the slide oiled, try using a thicker slide.
Colors appear in the background, or the background is unevenly lit with a gray cast.
This problem often occurs with whole mounts that are very thick or when too many light-scattering contaminating artifacts are captured in the mounting medium. Close down the field diaphragm and rotate the specimen slide to locate the best orientation, then determine whether the problem has been corrected. The specimen may have to be re-mounted in a thinner mount or the mounting medium may need to be filtered. Carefully clean the entire microscope slide and coverslip with a soft lens tissue or cotton swab. Convection currents are being created due to slow evaporation at the edges of the coverslip.