— dioptral , adj.
/duy op"teuhr/ , n.
1. Optics. a unit of measure of the refractive power of a lens, having the dimension of the reciprocal of length and a unit equal to the reciprocal of one meter. Abbr.: D
2. an instrument, invented by Hipparchus, to measure the apparent diameter of the sun or moon or to estimate the size or elevation of distant objects.
Also, esp. Brit., dioptre .
[ 1585-95; dioptra di- DI- 3 + op- (for ópsesthai to see) + -tra n. suffix of means ]