How to use the Brock String:
One end of the Brock string is held on the tip of the patient's nose, while the other is tied to a fixed point. The string is pulled taut. 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 the sensation of convergence.
The patient should alternate fixation on each of the three beads, one at a time.