Disc with mineral sections illustrating optic-axis figures

Mahogany disc (diameter: 7.5 cm) rotatable between thin sections of tourmaline (mounted in ivory eyepieces); 10 crystal sections are mounted to show optic-axis figures: topaz, aragonite, gypsum, sulphate of barytes, carbonate of lead, fluor spar, quartz, borax, nitre, calc spar.
The instrument (total length 18 cm) rests in its plush-lined cardboard case (8.2 x 19.3 cm) with a label reading "Toermalijn | platen | (opt. assenbeelden)" i.e. "tourmaline sections (optic axis figures)".
Unsigned, second half 19th century.

Collection: Museum for the History of Sciences, Universiteit Gent, Belgium (inventory number: MW 03/5117)