▁specific ▁condition ▁is ▁met : ▁▁ ▁the ▁bin oc ular ▁hor opter ▁is ▁the ▁loc us ▁of ▁is o - dis par ity ▁points ▁in ▁space ; ▁ ▁the ▁o cul om ot or ▁hor opter ▁is ▁the ▁loc us ▁of ▁is o - ver gence ▁points ▁in ▁space . ▁ ▁As ▁other ▁quantities ▁that ▁describe ▁the ▁functional ▁principles ▁of ▁the ▁visual ▁system , ▁it ▁is ▁possible ▁ ▁to ▁provide ▁a ▁theoretical ▁description ▁of ▁the ▁phenomen on . ▁ ▁The ▁measurement ▁with ▁ps y cho - phys ical ▁experiments ▁usually ▁provide ▁an ▁empir ical ▁definition ▁that ▁slightly ▁devi ates ▁from ▁the ▁theoretical ▁one . ▁The ▁underlying ▁theory ▁is ▁that ▁this ▁deviation ▁represents ▁an ▁adaptation ▁of ▁the ▁visual ▁system ▁to ▁the ▁regular ities ▁that ▁can ▁be ▁encountered ▁in ▁natural ▁environments . ▁ ▁History ▁of ▁the ▁term ▁ ▁The ▁hor opter ▁as ▁a ▁special ▁set ▁of ▁points ▁of ▁single ▁vision ▁was ▁first ▁mentioned ▁in ▁the ▁ele vent h ▁century ▁by ▁I bn ▁al - H ay th am , ▁known ▁to ▁the ▁west ▁as ▁" Al h az en ". ▁He ▁built ▁on ▁the ▁bin oc ular ▁vision ▁work ▁of ▁P to le my ▁and ▁discovered ▁that ▁objects ▁lying ▁on ▁a ▁horizontal ▁line ▁passing ▁through ▁the ▁fix ation ▁point ▁resulted ▁in ▁single ▁images , ▁while ▁objects ▁a ▁reasonable ▁distance ▁from ▁this ▁line ▁resulted ▁in ▁double ▁images . ▁Thus ▁Al h az en ▁noticed ▁the ▁importance ▁of ▁some ▁points ▁in ▁the ▁visual ▁field ▁but ▁did ▁not ▁work ▁out ▁the ▁exact ▁shape ▁of ▁the ▁hor opter ▁and