▁above ▁or ▁below ▁the ▁Vi eth - M üller ▁circle ▁have ▁to ▁be ▁taken ▁into ▁account , ▁as ▁was ▁calculated ▁by ▁Hel m hol tz . ▁In ▁this ▁case ▁the ▁hor opter ▁becomes ▁a ▁single - loop ▁spir al ▁passing ▁through ▁the ▁fix ation ▁point ▁and ▁conver ging ▁toward ▁the ▁vertical ▁hor opter ▁at ▁the ▁top ▁and ▁bottom ▁extrem ities ▁and ▁passing ▁through ▁the ▁nod al ▁point ▁of ▁the ▁two ▁eyes . ▁This ▁form ▁was ▁predicted ▁by ▁Hel m hol tz ▁and ▁subsequently ▁confirmed ▁by ▁Sol om ons . ▁In ▁the ▁general ▁case ▁that ▁includes ▁the ▁fact ▁that ▁the ▁eyes ▁cycl or ot ate ▁when ▁view ing ▁above ▁or ▁below ▁the ▁primary ▁hor opter ▁circle , ▁the ▁theoretical ▁hor opter ▁components ▁of ▁the ▁circle ▁and ▁straight ▁line ▁rotate ▁vert ically ▁around ▁the ▁axis ▁of ▁the ▁nod al ▁points ▁of ▁the ▁eyes . ▁ ▁Emp ir ical ▁Bin oc ular ▁Hor opter ▁▁ ▁As ▁Whe at stone ▁( 1 8 3 8 ) ▁observed , ▁the ▁empir ical ▁hor opter , ▁defined ▁by ▁sing len ess ▁of ▁vision , ▁is ▁much ▁larger ▁than ▁the ▁theoretical ▁hor opter . ▁This ▁was ▁studied ▁by ▁P . ▁L . ▁Pan um ▁in ▁ 1 8 5 8 . ▁He ▁proposed ▁that ▁any ▁point ▁in ▁one ▁ret ina ▁might ▁yield ▁sing len ess ▁of ▁vision ▁with ▁a ▁circular ▁region ▁cent red ▁on ▁the ▁corresponding ▁point ▁in ▁the ▁other ▁ret ina . ▁This ▁has ▁become ▁known ▁as ▁Pan um ' s ▁fus ional ▁area , ▁or ▁just ▁Pan um