A mechanical ophthalmic device that is used during an ophthalmic examination; typically to determine a patient's prescription for glasses. It consists of two large, disk-like housings, joined together, that each contain a built-in paternoster works consisting of a large number of different lenses of differing strengths used to measure the refraction of the eye; it is a mechanized set of trial lenses. It is placed in front of the patient's eyes and viewing through the phoropter the patient is asked to read a Snellen eye chart placed at a preset distance. The optometrist changes the lenses and other settings, while asking the patient for subjective feedback on which settings give best vision.