A white string used to demonstrate physiological diplopia. One end of the string is placed against the bridge of the nose and the other end against a distant object (e.g. a doorknob). The subject should see two strings intersecting wherever the horizontal components of the visual axes meet. Red and green filters, one before each eye, enhance or facilitate the observation of the two strings. Several beads, each of a different colour, are usually threaded on the string so that they can be moved at will. One bead may be used for fixation while the other/s appear double; in crossed diplopia for the one closer to the eyes than the fixation bead, and in uncrossed diplopia for the one further away than the fixation bead. Brock's string is commonly used in visual training. The observation of physiological diplopia with Brock's string is often referred to as Brock's string test. Syn. bead on string.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann