WEBSTER'S ACHROMATIC SUBSTAGE CONDENSER

c. 1865-

Webster's achromatic condenser was the commonest achromatic condenser for many years. It simply consists of a planocovex lens(A) positioned above an achromatic doublet(B) which in turn is above a ring of apertures(F) and also shown at the bottom of the illustration. These apertures include a crescent opening for oblique illumination, a large aperture, a darkground stop, and a smaller aperture with a third and still smaller aperture which swings over it. This ring then gave a choice of 3 different apertures as well as an oblique beam and a darkground stop. Alternative arrangements of the disk apertures were provided by various makers.