Fig. 112.—Andrew Ross’s ¼-inch Objective.
Lister proposed other combinations, and himself made an object-glass consisting of a meniscus pair with a triple middle, and a back plano-convex doublet. This had a working distance of ·11 and proved to be so great a success that other opticians—Hugh Powell, 1834; James Smith, 1839—made objectives after the same formula.
The publication of Lister’s data proved of value in another direction: it stimulated opticians to apply themselves to the further improvement of the achromatic objective. Andrew Ross was one of the more earnest workers in giving effect to Lister’s principles and a short time afterwards found that a triple combination, with the lenses separated by short intervals, gave better results. In the accompanying diagram the changes made in the combination of the objective from 1831, and extending over a period of about twenty years from this date, are shown.
Each objective, from the ½-inch to the 1⁄12-inch, is seen to be built up of at least six or eight different fronts, the back combinations being a triplet formed of two double-convex lenses of crown glass with an intermediary double concave lens of flint-glass.
Fig. 113.—Combinations of Early Dry Objectives.
A, Double-convex lens; B, Plano-concave; C, Bi-convex and plano-concave united; shown in their various combinations, as at D, form the 3-in., 2-in. or 1½-in.; at E, 1-in. and 2⁄3-in.; and at F, the ½-in., 4⁄10-in., ¼-in. and 1⁄25-in. objectives.
Combination D was for many years known as the Norfolk Objective.
