Answer: dioptres

AnswerCrossword Clue
dioptresinstrument, invented by Hipparchus, to measure the apparent diameter of the sun or moon or to estimate the size or elevation of distant objects
dioptresunit 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
dioptresDIOPTRE, a measure of refractive power, also DIOPTER