Concave spherical lens, 31 pairs, 1 of which is incomplete, forming a series, numbered -0. 12,0.25(0.25)3.50(0.50)7(1)12(2)20 Concave spherical lens
Convex spherical lens, 31 pairs, one of which is incomplete, forming a series, numbered +0. 12, 0.25(0.25)3.50(0.50)7(1)12(2)20 Convex spherical lens
