When a bi-prism is used to split a field into two parts a dark line is seen at the dihedral edge. This can be demonstrated to be due to an imperfect edge. This can be avoided by using a mirror to split the field. A different kind of artifact can arise when the source is made conjugate to the pupil instead of the retina. Other sources of trouble are axial chromatic aberration, lateral chromatic dispersion and diffraction. These variables can be controlled to produce an acceptable bipartite pattern for beterochromatic luminance matches and testing the sensitivity to differences in chromaticity.