Hartnack joined the firm of his uncle, Georges Oberhaeuser (1798-1868), in Paris in 1857, and assumed full control of the firm in 1860. He moved to Potsdam, Germany, in 1870, and the Parisian branch of the business was eventually taken over by Nachet et Fils. Hartnack is credited with the first use of water-immersion lenses in the commercial production of microscopes and the adoption of the substage condenser in his later instruments.

Moody Medical Library collection includes:

1.049

Similar in design to Oberhaeuser instruments with horseshoe foot, the microscope is supported by a round pillar attached to square stage. The body-tube moves by rack and pinion and is connected to the limb by a solid brass arm. The substage Abbe condenser swings on an axis. No carrying case and accessories. About 1875. Signed on the tube: Dr. E. Hartnack Potsdam.

References:

Bradbury, The Evolution of the Microscope, pp. 224, 234-35.
Turner, The Great Age of the Microscope, p. 210