The Bezold-Brücke phenomenon for purple colors

A bipartite stimulus 2 degrees in diameter was used to study the Bezold-Brücke phenomenon for purple colors. A mixture of 460 nm and 667 nm in one half at a given luminance was matched with a mixture of the same two wavelengths at a luminance level 10 times higher. The hue of the 667-nm stimulus was not affected by the change in luminance. There is one mixture of red and blue which remains unchanged in hue as the luminance increases. Reddish purples shift toward red and bluish purples toward blue. These findings are consistent with a zone theory of color vision such as proposed by Adams.