German, c. 1910, signed 'Abbe's Apertometer, Carl Zeiss Jena', the semicircular ground glass fixed to a black lacquered brass frame with single swing arm reader, with cased Zeiss Apert. System objective, all in a black leather covered case with original hand-written letter which reads 'The instructions are filed in the twinlock loose-leaf book "a". The viewing objective is used in my improvised phase 'telescope' (signed) AB, this apertometer came from Dr D. S. Spence'.