This is an instrument for studying birefringent objects such as crystals and minerals using polarised light. The eyepiece on the left in the photograph contains a Nicol prism that passes only light with one plane of polarisation. The object would be mounted on the ring holder at the centre, and this can be tilted through measured angles. The incident light should come through another Nicol prism on the right, but this has been replaced by a sheet of polaroid in a comparatively crude modern holder.

Inscriptions / Translations:  Mon J. DUBOSCQ, Ph PELLIN, PARIS