Check the listing for details. Vision Train Diameter Rope for Eye Training Eye Yoga. Condition: New with tags. Listed at 4.11 USD. img{max-width:100%}Reasons for purchasingCondition: NewType: Brock StringBeads Material: PlasticSize:Beads Diameter: 2cmString Length: 204cmBrock String-used to fix convergence insufficiency and sports training.The basic purpose of Brock String is to develop good binocular functions; in other words, making both eyes work well together.This activity teaches convergence: the ability to point both eyes more accurately at a target.More Product DetailsSteps:1. Look at the red ball closest to your eye until you see the phenomenon as shown.2. Look at the yellow ball in the middle of the eye until you see the phenomenon as shown.3. Look at the green ball furthest from the eye until you see the phenomenon as shown. Description: Brock String - used to fix convergence insufficiency and sports training.