The Sheedy Disparometer is a clinical instrument designed to measure fixation disparity and hence determine forced vergence--fixation disparity curves. The target design includes a central 1.5 degrees fusion-free area. The effect of introducing a central fixation target to the instrument is investigated along with the influence of ambient illumination level on fixation disparity. All curve parameters, i.e. curve type, slope, X- and Y-intercepts were found to significantly alter with the introduction of a foveal lock. Illumination changes did not cause significant differences in any parameter.

A new fixation disparity measuring device, the Wesson Fixation Disparity Card, is compared with the modified Sheedy Disparometer. Twenty-eight subjects were divided into normal and abnormal categories of binocular status determined by classical clinical techniques rather than on patients' subjective symptomatology. Fixation disparity curves were generated on all subjects using both instruments. There were significant differences between the two instruments. Neither instrument was able to distinguish between normal and abnormal subjects but, over a five day period, each instrument produced repeatable results. The validity of fixation disparity measurements using either instrument on naive observers, especially children, is questioned.

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Factors affecting the size of Panum's fusional area are reviewed. One of these factors, prolonged fixation disparity, is investigated. The results show that this does not appear to significantly alter the size of Panum's area within the limitations of the tests used in detecting subjects with prolonged fixation disparity.

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