Carl Kellner (1826–1855) was a German optician and the founder of the Optische Institut in Wetzlar, which later became the famous Leica Camera AG. Born in Hirzenhain, Hesse, Kellner was primarily self-taught in optics. He developed an interest in improving optical instruments, leading to his invention of new types of achromatic lenses that significantly enhanced the performance of microscopes and telescopes. 1849, Kellner established the Optische Institut in Wetzlar to produce his improved optical instruments. Although he died at 29, his work laid the foundation for future advancements in optical technology. After Kellner's death, the management of the Optische Institut was taken over by his widow and later by Friedrich Belthle, an employee and relative.Ernst Leitz I (1843–1920) entered the picture when he joined the company in 1864, bringing innovative ideas and a good understanding of large-scale production. Recognizing the potential of Kellner's foundation, Leitz took over the company in 1869, changing its name to Ernst Leitz Wetzlar. Under Leitz's leadership, the company expanded its product line beyond microscopes, eventually venturing into the production of cameras.Starting in 1851, Kellner began selling his microscopes, but it wasn't until the 65th instrument that the practice of engraving serial numbers was introduced; the oldest preserved microscope bearing the signature C. Kellner, Wetzlar, marked with serial number 86, is currently housed in the Deutsches Museum in Munich. The example offered here is the medium microscope from the three models offered by Kellner in 1854. It shows the exceptionally early serial number 108 and remains in excellent condition. This instrument, now a venerable 170-year-old, continues to function flawlessly, it is also complete with three eyepieces, two lenses and a fitted wooden case.