“Von Betzold (in 1873) and Brucke (in 1878) independently discovered that variations in light level change the sensation of color. Practically, an increase in luminance causes (1) a yellowing of red and of yellow-green and (2) a movement toward blue or violet and blue-green. There seem to be places in the spectrum that are invariable: the yellow of 571 nm and the blue of 476 nm, toward which other colors tend to move with increasing luminance. Situated between these two is a third invariant color (a bluish-green of 508 nm). Colors near to it approach its color more closely with increasing luminance (see Walraven 1961). As a concrete example, two colors, one matching 525 nm and the other 660 nm at 1000 Td, appear to have matches respectively with the color of 545 and 636 nm at 100 Td. The change is thus far from negligible. Experimentally investigating the slippage of color with luminance has shown that the colors that remain stable are in the yellow 571, the green 508, and the blue 476 nm.”