A<unk>  in use. A<unk>  (named after Frederick W.<unk>  is an instrument used in vision therapy. It consists of a white string of approximately 10 feet in length with three small wooden beads of different colors. The<unk>  is commonly employed during treatment of convergence insufficiency and other anomalies of binocular vision. It is used to develop skills of convergence as well as to disrupt suppression of one of the eyes. During therapy, the one end of the<unk>  is held on the tip of the nose while the other is tied to a fixed point. The three beads are spaced out at various distances. The patient is asked to focus on one of the beads, while noting the visual input of each eye and sensation of convergence. The patient can use variable techniques to make easier or more difficult by bringing the beads closer\further to the nose and by employing lenses and prisms. Although this procedure is usually associated with binocular vision and anti-suppression therapy, it can also be a valuable procedure for developing accurate fixation skills under binocular conditions. Three (to five) colored beads are spaced on a length of string at least 20 inch long. The patient is instructed to alternate fixation and focus from one bead to the next while maintaining awareness of physiological diplopia. __TOC__
Brock string
Magic string 
Soemmering ring
Cabot rings
Surfer's knots
Wale mark
Perryville Tree engravings
Cramp-ring