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The pseudoscopic enhancement factor of the Pseudoscope is X 5.5, and was considered the practical limit for a device which, by laterally switching the visual inputs, makes unusual vergence demands upon the visual system in order to accommodate for near and far vision. Any viewing conditions which result in double-vision or diplopia, should, be avoided, and suggest that the subject being viewed is too close. In most cases, for either device, this will in practice be between 10 to l5ft (3.OOm - 4.5Om).

In this context the limits to which eye separation may be extended are reached when fusion of the two retinal images is impossible. Objects may be fused if the disparity of the images is within 30 seconds of arc. Outside 60 seconds of are there will be no fusion. These limits are known as Panums Fusional Area. Some findings, however, which use flash stimuli to avoid eye movement, suggest that P.F.A. may be more extensive than classical measurements indicate.