Detection of accommodation with an optometer

Principle

When looking through a slide with two very small holes located very close together (optometer aperture), in the absence of accommodation relative to their distance, objects close to the eye appear to be relatively well focussed except that there appear to be two of them. This makes it easy to demonstrate whether a person's eye is accommodating properly (single image) or not (double image). Focussing on the more distant needle makes the nearer needle appear twice. Focussing on the closer needle means the more distant needle appears to be doubled.

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