Receptor potentials have been recorded intracellularly from single retinular cells in the anterior and dorsal quadrants of the compound eye of the crayfish Procambarus (Fig. 1) stimulated with equal quantum flashes of linearly polarized monochromatic light. Comparisons between two orthogonal stimulus e-vectors respectively parallel and perpendicular to the microvilli of each receptor cell's rhabdomere were made sequentially in about one minute's running time at 20 nm intervals between 400 and 740 nm (Fig. 2)