c. 1600, in ancient Greek music, in reference to one of the three standard tetrachords, from French diatonique, from Latin diatonicus, from Greek diatonikos, from diatonos "extending; pertaining to the diatonic scale," from dia "through, across" (see dia-) + teinein "to stretch," from PIE root *ten- "to stretch." In modern music, "using tones, intervals, and harmonies of the standard major and minor scales without chromatic alteration," 1690s. Related: Diatonically.
"measurement of the range of vision; measurement of the visual powers in general," 1886, from optometer (1738), name given to an instrument for testing vision, from opto- "sight," from Greek optos "seen, visible" (from PIE root *okw- "to see") + -metry "a measuring of." Probably influenced by French optométrie.
When I made the foregoing Experiments, I designed to repeat them with more Care and Exactness, and to make some new ones of the same Sort, by means of an Instrument I had contrived for that Purpose; which from its Use in measuring the Limits of distinct Vision, and in determining with great Exactness the Strength and Weakness of Sight, may be called an Optometer. [Dr. William Porterfield, "An Essay Concerning the Motions of our Eyes, Part II," in Medical Essays and Observations, Vol. IV, Edinburgh, 1738]
late 14c., optik, "of or pertaining to the eye as the organ of vision," from Old French optique, obtique (c. 1300) and directly from Medieval Latin opticus "of sight or seeing," from Greek optikos "of or having to do with sight," from optos "seen, visible," related to ōps "eye," from PIE root *okw- "to see." Meaning "relating to or pertaining to vision or sight" is from 1590s. Optics "eyes" is from 1640s; "formerly the learned and elegant term; afterwards pedantic, and now usually humorous" [OED].