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The owl was Billy’s optometer in his office in Ilium.

An optometer is an instrument for measuring refractive errors in eyes— in order that corrective lenses may be prescribed.

Opsiometer, op-si-om′e-tėr, n. an optometer.

Test with optometer, and in reporting use the number on the stem preceded by F for far-sightedness, and N for near-sightedness.

In using the optometer, find the place where vision of the dots is the easiest and most distinct.