Copied from al illustration in the Handbook of Rifle Shooting, originally published in 1869. It is based on an object height of 6 feet, and is incremented from 50 to 800 yards. To find the yardage, hold the brass range finder in one hand, place the knotted end of the hemp string between your teeth, pull the string tight and move the slide until your object touches both the top and bottom of the window; then read the yardage from the scale stamped on the brass range finder. Replacement string length is 25” from the range finder to the knot at the end of the string. Size is 3” X 1 ¼”, just right to fit into shooting bag or pocket.