seen from United States
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يعتمد مبدأ تشغيل جهاز الثماني على مضاعفة الزاوية المقاسة (بسبب انعكاس الضوء بواسطة المرآة). ومن الغريب أن عالمين اخترعوا جهاز الثماني في نفس الوقت عام 1731 : الأول عالم الرياضيات جون هادلي John Hadley والثاني صانع الزجاج توماس جودفري Thomas Godfrey.
