Refraction Calibration

I planted both feet against the smooth floor grain to establish an upright vertical axis. A few paces ahead, midday light passed through a heavy glass tumbler, projecting concentric caustic bands across the surface. I stepped into the illuminated zone and extended my white staff into the brightest focal ring. The beam split along the rod's cylindrical curve, scattering narrow bands of cyan and white across the floorboards. Rotating my wrist eight degrees widened the dispersion pattern predictably. I adjusted my stance, holding the staff steady to study how the refraction shifted as the light moved.