From Today In Science History:
On this day in 1846, ten years after his voyage on the Beagle, Darwin began his study of barnacles, which was to appear in four volumes on living and fossil Cirripedes (barnacles).
For his observations, he had a single lens microscope made to his own design. Intended to be more practical, it did not fine focusing and had a larger stage than the Beagle microscope to take shallow dishes for aqueous dissections. image