In a continuous particle electrophoresis cell, the present invention comprises means for illuminating a section of the electrolyte curtain with a narrow blade of light rather than with a broad beam which would flood the area of interest. Such blade of light illuminates the bands of sample particles only where they intersect the plane of light. By viewing the electrophoresis cell at a suitable angle, against a dark background, the crosssectional pattern of the particle band distribution becomes fully visible. This provides a valuable, practical tool for gauging and optimizing instrument performance.