▁used ▁sing len ess ▁of ▁vision ▁as ▁a ▁criter ion . ▁▁ ▁The ▁term ▁hor opter ▁was ▁introduced ▁by ▁Francis cus ▁Agu ilon ius ▁in ▁the ▁second ▁of ▁his ▁six ▁books ▁in ▁opt ics ▁in ▁ 1 6 1 3 . ▁In ▁ 1 8 1 8 , ▁Ger hard ▁Vi eth ▁argued ▁from ▁E uclidean ▁geometry ▁that ▁the ▁hor opter ▁must ▁be ▁a ▁circle ▁passing ▁through ▁the ▁fix ation - point ▁and ▁the ▁nod al ▁point ▁of ▁the ▁two ▁eyes . ▁A ▁few ▁years ▁later ▁Johannes ▁Müller ▁made ▁a ▁similar ▁conclusion ▁for ▁the ▁horizontal ▁plane ▁containing ▁the ▁fix ation ▁point , ▁although ▁he ▁did ▁expect ▁the ▁hor opter ▁to ▁be ▁a ▁surface ▁in ▁space ▁( i . e ., ▁not ▁restricted ▁to ▁the ▁horizontal ▁plane ). ▁The ▁theoretical / ge omet rical ▁hor opter ▁in ▁the ▁horizontal ▁plane ▁became ▁known ▁as ▁the ▁Vi eth - M üller ▁circle . ▁However , ▁see ▁the ▁next ▁section ▁The oret ical ▁hor opter ▁for ▁the ▁claim ▁that ▁this ▁has ▁been ▁the ▁case ▁of ▁a ▁mistaken ▁identity ▁for ▁about ▁ 2 0 0 ▁years . ▁ ▁In ▁ 1 8 3 8 , ▁Charles ▁Whe at stone ▁invent ed ▁the ▁s tere os cope , ▁allowing ▁him ▁to ▁explore ▁the ▁empir ical ▁hor opter . ▁▁ ▁He ▁found ▁that ▁there ▁were ▁many ▁points ▁in ▁space ▁that ▁yield ed ▁single ▁vision ; ▁this ▁is ▁very ▁different ▁from ▁the ▁theoretical ▁hor opter , ▁and ▁subsequent ▁authors ▁have ▁similarly ▁found ▁that ▁the ▁empir ical ▁hor opter ▁devi ates ▁from