Fundamental improvements in the microscope were made by Abbe, who was a mathematician. In 1878 he introduced the oil immersion lens; in 1886 he made an apochromatic objective corrected for three colors in which the secondary spectrum was not noticeable, while he is also remembered for the sub-stage condenser which bears his name. A translation of the above article is in J. roy. micr. Soc., 1887, 20-34.