The Secchi Disk is used to measure how far an observer can see into the water. It is simply lowered into the lake using a measured length of waterproof line to determine the depth at which it disappears from view. This depth is called the "Secchi Depth," and is a measure of the turbidity , or cloudiness, of the water.

Related web links:
History of the Secchi Disk
The Great North America Secchi Dip-In