  The shifted qualia scenario that is usually discussed  concerns—unsurprisingly—color vision. There is a substantial amount of variation between people classified as having “normal” color vision by standard tests, due to differences in the lens of the eye and photoreceptor pigments, among other factors. In one experiment, introduced into the philosophical literature by Hardin (1993, 79–80), Hurvich et al. (1968) found that the apparent location of unique green for spectral lights among 50 subjects varied from 490 to  520nm.[26]  This is a large range: 15nm either side of unique green looks distinctly bluish or yellowish. For example, when Smith looks at the third patch in figure 7 (in isolation), his experience might have the quale that Jones’s experience has when she looks at the fourth patch in Figure 7, keeping the viewing circumstances fixed. And when Smith looks at the fourth patch, his experience might have the quale that Jones’s experience has when she looks at the fifth patch. Thus—at least over a small range of hues—Jones’s and Smith’s color qualia are “shifted” with respect to each other. Accordingly, this sort of scenario can be described as a case of “shifted spectra” (Block 1999). (NB: here we are assuming that the Frege-Schlick view mentioned in subsection 2.3.2 is incorrect.)