colors) serving as the reference line for saturation, while a plane passing through the axis and the extreme spectral red forms the reference plane for hue. (See Fig 1.)
Fig. 1. The Dimensions of Psychological Color Solid
C. Boundaries.—It is certain that the bounding surfaces of such a color solid will have an irregular contour. The axial dimension will be about thirty times the radial one at the greatest value of the latter. Both in the directions of high and low brilliance, there will be convergence towards a vertex. If the edges of planes perpendicular to the axis are regarded as determined by the saturations obtainable by spectral stimuli, these edges will be maximally near the axis at the yellow, receding on either side to find a constant value for the intermediaries of red and blue.
