Uncertainty in the measurement of fixation disparity due to zones of perceived nonius alignment.

Fixation disparity was measured by means of limits within which nonius alignment was perceived. Two pairs of limits were found, one starting from perceived alignment and the other starting from perceived misalignment. These limits outline zones of perceived alignment of nonius lines. The extent of the zone of first perceived misalignment was found to be greater than that of first perceived alignment. The zone width can be expressed either in terms of disparity at a particular prism vergence or as a range of prism vergence at a particular disparity. The latter expression is comparable to what has been called the range of zero-associated phoria.