Abbe apertometer (manufactured by Zeiss) is a device to measure the numerical and angular apertures of an objective or condenser lens. The back focal plane of the objective is viewed within an auxiliary lens.
The apertomter shows the position of an indicator just intersecting two opposite edges of the conoscopic field of view.
Here is a beginning discussion of Abbe’s apertometer by Carl Zeiss.
Here is another description from Google.