The two-dimensional color system in the textbook on optics, written by F. Aguilonius (1613) and illustrated by PP Rubens,(1) is equivalent to a three-dimensional color solid, which can also be constructed by applying the trichromatic theory of color vision on a color circle. (C) 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S17-S19, 2001.