different between dioptry vs dioptra

English

dioptry (plural dioptries)

(obsolete) Alternative form of diopter
- 1974, Lawrence Durrell, Monsieur, New York: Viking, 1975, “Macabru,” p. 176,^[2]
  They also shared a fearful myopia of identical dioptry, were apt to bump into inanimate objects, sit down on invisible chairs, or bounce off each other as they shambled about talking and gesticulating.

dioptra

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