（Optics.) That lens or combination of lenses in a microscope or telescope which brings the image of an object to a focus in order to be viewed through the eye-piece.
Though acromatic lenses had been applied to telescopes by Dollond near the middle of the eighteenth century, yet in 1821, according to Biot, opticians regarded the construction of a good achromatic microscope as an impossibility.
In 1827 Professor Amici of Modena exhibited in England and Paris a horizontal microscope whose object-glass, of large aperture, was composed of three superimposed lenses.
A microscope constructed by Chevalier, on Amici's plan, was awarded a silver medal.
The theory of the subject was about this time investigated by Sir John Herschel, Professor Airy, and others, and, acting on their theoretical views, Mr. Joseph Jackson Lister succeeded in effecting one of the greatest improvements in the manufacture of achromatic object-glasses by uniting a plano-conv