Rhombohedral calcite crystal, 'Iceland Spar' prism with brass eye piece magnifier (removed from instrument), unsigned, European, 1880-1920. Used to demonstrate the double images produced by the mineral calcite. Rhobomhedral crystal of 'Iceland Spar' with magnifier 1880-1920
Calcite prism in brass tube plus black paper diaphragm Calcite prism in brass tube plus black paper diaphragm