Secchi diskSecchi disk
a circular, white or colored disk lowered into a body of water to estimate the clarity of the water by measuring the depth at which it disappears
Origin of Secchi diskafter P. A. Secchi (1818-78), Italian astronomer
A disk, divided into black and white quarters, used to gauge water clarity by measuring the depth at which it is no longer visible from the surface.
Origin of Secchi diskAfter Pietro Angelo Secchi, (1818–1878), Italian astronomer hired to test such a disk.
Secchi + disk