An optical instrument used to measure refractive error in the eye and determine the degree of nearsightedness, farsightedness, or astigmatism.
From Greek 'a-' (not) + 'metron' (measure) + '-meter' (measuring instrument). This ophthalmological device was developed in the late 19th century as eyeglass prescription technology advanced.
When you go to the eye doctor and stare into that weird machine that flips lenses while you read letters, you're looking at an ametrometer's descendant—they originally couldn't figure out prescriptions accurately, so they invented machines to do it precisely.
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