▁or ▁may ▁not , ▁be ▁the ▁fix ation ▁point ). ▁This ▁hor opter ▁geometry ▁of ▁a ▁circle ▁in ▁the ▁fix ation ▁plane ▁and ▁a ▁per pend icular ▁line ▁remains ▁approximately ▁fixed ▁relative ▁to ▁the ▁eye ▁cent ers ▁as ▁long ▁as ▁the ▁eyes ▁are ▁fix ating ▁somewhere ▁on ▁these ▁two ▁lines . ▁When ▁the ▁eyes ▁are ▁fix ated ▁anywhere ▁off ▁these ▁two ▁lines , ▁the ▁theoretical ▁hor opter ▁takes ▁the ▁form ▁of ▁a ▁tw isted ▁cub ic ▁passing ▁through ▁the ▁fix ation ▁point ▁and ▁asympt oting ▁to ▁the ▁two ▁lines ▁at ▁their ▁extrem es . ▁( Under ▁no ▁conditions ▁does ▁the ▁hor opter ▁become ▁either ▁a ▁cyl inder ▁through ▁the ▁Vi eth - M üller ▁circle ▁or ▁a ▁tor us ▁centered ▁on ▁the ▁nod al ▁points ▁of ▁the ▁two ▁eyes , ▁as ▁is ▁often ▁popular ly ▁assumed .) ▁If ▁the ▁eyes ▁are ▁fix ating ▁anywhere ▁at ▁infinity , ▁the ▁Vi eth - M üller ▁circle ▁has ▁infinite ▁radius ▁and ▁the ▁hor opter ▁becomes ▁the ▁two - dimensional ▁plane ▁through ▁the ▁two ▁straight ▁hor opter ▁lines . ▁ ▁In ▁detail , ▁the ▁identification ▁of ▁the ▁theoretical / ge omet rical ▁hor opter ▁with ▁the ▁Vi eth - M üller ▁circle ▁is ▁only ▁an ▁approximation . ▁It ▁was ▁pointed ▁out ▁in ▁Gul ick ▁and ▁Law son ▁( 1 9 7 6 ) ▁ ▁that ▁Müller ' s ▁an atom ical ▁approximation ▁that ▁the ▁nod al ▁point ▁and ▁eye ▁rotation ▁center ▁are ▁coinc ident ▁should ▁be ▁ref ined . ▁Unfortunately , ▁their ▁attempt ▁to ▁correct ▁this ▁assumption