In 1856 James Clerk Maxwell15 described a dark spot in the blue region of a prismatic spectrum. The spot moved with his eye but disappeared upon looking elsewhere in the spectrum. He concluded that the spot was entoptic, being produced by a localized reduction of retinal illuminance that resulted from absorption of blue light by the yellow macular pigment. In challenging Maxwell's explanation Gullstrand,9 in 1905, offered a purely optical basis for the phenomenon, and Walls and Mathews,23 in 1952, hypothesized that color receptors within the fovea were responsible for the entoptic image. Recent evidence on ocular pigments led Brindley2 * in 1960 to favor Maxwell's theory. In the present paper we shall use the eponym "Maxwell's spot" and the abbreviation "MS" without implication regarding its cause.
Most modern methods for eliciting MS employ a purple dichromic filter which transmits light from only the red and the