Our trial cases are supplied with broad-rimmed, plastic mountings, diameter 1.5" / 38 mm (lens diam. 0.8" / 21 mm ). The lenses are grounded with the greatest precision (pitch polished like high quality objectives). They are plano-convex or -concave. The lenses can be positioned close together, so that their refraction is equal to the sum of single lenses. The reduced aperture ensures sharp image formation and light weight.