an epithet expressing want of colour. The word is Greek, being compounded of α privative, and χρωμα, colour. This term was first introduced into astronomy by De la Lande.

ACHROMATIC Telescopes, are telescopes contrived to remedy the aberrations in colours. They were invent- Achromatic ed by Mr John Dollond, optician, and have been since improved by his son and others. See ABERRATION.—A more particular account of the invention and construction of these instruments will be found under OPTICS.