Schlieren and shadowgraph photography make the invisible visible: density gradients in air become luminous silver shock lines and shadowed dark bands against a mid-gray field. First devised by August Toepler in the 1860s and refined for high-speed flow through the twentieth century, it is the canonical grammar of the wind tunnel — strictly achromatic, high-contrast at the shock front, smooth gray everywhere else. This system borrows that discipline: cool grotesque type, precise thin rules, and airflow etched into light.