The visibility disc according to Secchi is a simple, although not very accurate, means of determining the visibility depth. The visibility disc is lowered into the water with a wire. When the disc is no longer visible in the water, a reading of the depth is made by means of the marks on the rope. The disc is lowered another 0.5 meter and is pulled up slowly. The second reading is done when the disc becomes visible. By taking the arithmetic mean from both readings the visibility depth is determined.