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What Hall and Dollond did was to make the outer or crown lens of the objective as before, and place behind it a plano-concave lens of dense flint glass.

Two lenses only—a plano-convex and a plano-concave—were needed for the composition of each, and this simple principle is that still employed in the construction of opera-glasses.

Lens, 184. concave, 184. convex, 184. meniscus, 184. plano-concave, 184. plano-convex, 184.

E is a double concave, being, in all respects, the reverse of D. C is a plano-concave, flat on one side, and concave on the other.

A. Thoma's Model, plano-concave, with handle adapted for clamping immediately to the knife carrier of the microtome, in box.