The Abbe Apertometer, manufactured by the legendary Carl Zeiss firm in Jena, Germany, is a precision optical instrument designed to measure the numerical and angular apertures of microscope objectives and condenser lenses. It was developed in association with Ernst Abbe, the brilliant physicist and optical theorist who transformed Carl Zeiss from a small workshop into one of the world’s premier optical manufacturers in the latter half of the 19th century.
To use the instrument, the back focal plane of the objective being measured is viewed through an auxiliary lens while the apertometer is positioned beneath the microscope stage. The device shows the position of an indicator just intersecting two opposite edges of the conoscopic field of view, allowing the user to read the numerical aperture directly from the finely engraved scale. This measurement was critical for microscopists who needed to understand the resolving power and light-gathering capability of their lenses.
The “1.620” marking visible on this example’s scale represents the maximum refractive index range the instrument was calibrated to measure — a testament to the precision engineering for which Carl Zeiss Jena became world famous. The elegant semicircular brass and glass construction, bearing the proud inscription “Carl Zeiss — Abbe’s Apertometer — Jena,” makes this not only a functional scientific instrument but a beautiful artifact of the golden age of German optical craftsmanship.
