German, c.1910, the glass Apertometer engraved ,Abbe's Apertometer CARL ZEISS JENA' and to the maroon leather covered case 'APERTOMETER NACH ABBE CARL ZEISS JENA', blue velvet lined case with cased Apertometer system objective, adjustable covers and glass Apertometer, case 11cm. Note: An apertometer is used to measure Numerical Apertiure often abbreviated as NA or n.a. indicating the resolving capability of an objective lens. NA, in essence, quantifies a microscope objective's capacity to collect light and distinguish intricate specimen features at a set object distance. Presently, employing a Bertrand lens or a Phase telescope proves to be more efficient and straightforward compared to mounting an objective onto a drawtube