Lister, in a paper contributed to the proceedings of the Royal Society the same year, pointed out how the aberrations of one doublet could be neutralised by a second. He further demonstrated that the flint lens should be a plano-concave joined by a permanent cement to the convex crown-glass. The first condition, he states, “obviates the risk of error in centring the two curves, and the second diminishes by one half the loss of light from reflection, which is very great at the numerous surfaces of every combination.” These two conditions then--that the flint lens shall be plano-concave, and that it shall be joined by some cement (Canada balsam) to the convex--may be taken as the basis for the microscopic objective, provided they can be reconciled with the correction of spherical and chromatic aberration of a large pencil. Andrew Ross was not slow to perceive the value of Lister’s suggestions and in 1831 he had constructed an object-glass on the lines laid down by Lister, Fig. 112; _a a′_ representing the anterior pair, _m_ the middle, and _p_ the posterior, the three sets combined forming the achromatic objective, consisting of three pairs of lenses, a double-convex crown-glass, and a plano-concave of flint. [Illustration: Fig. 111.--Lister’s double-convex crown and plano-concave flint cemented combination.] [Illustration: Fig. 112.--Andrew Ross’s 1/4-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 1/2-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. [Illustration: 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 1-1/2-in.; at _E_, 1-in. and 2/3-in.; and at _F_, the 1/2-in., 4/10-in., 1/4-in. and 1/25-in. objectives. Combination _D_ was for many years known as the Norfolk Objective.]