orthos

Noun

ortho (countable and uncountable, plural orthos)

1. (countable) An isomer of a benzene derivative having two substituents adjacent on the ring.
   • When the cinnamic acid had been nitrated the ortho was separated from the para, by suspending in alcohol and leading hydrochloric acid gas into the mixture : […].1887 December 21, Thomas M. Morgan, “On Some Derivatives of Ortho-Nitro-Cinnamic Acid”, in Chemical News^([269])
2. (countable) A certain type of flat eyepiece.
   • Orthos are excellent eyepieces, still preferred by some backyard astronomers for medium- and high-power applications, especially planetary observing.1998, Terence Dickinson, NightWatch: A Practical Guide to Viewing the Universe, page 76
3. (countable, dated) An orthochromatic plate.
   • Wherever the exposure is likely to be cut short— and this includes all high speed work and most portraiture — the ortho is not so suitable as the ordinary plate.1908 June, Charles F. Rice, “On the Use of Orthochromatic Plates”, in Photographic Times, page 174