air visible. This method was found to be also important for the high-speed cinematography. Motion pictures have also been used to study phenomena that occur so fast that they cannot be recorded on normal cameras. An immense amount of ingenuity has been applied to the solution of many problems in this field. Schlieren optical system The principal of Toepler's Schlieren photography, known as the Foucault knife-edge test, was originally developed as a sensitive test for lenses, mirrors, and other optical components by Léon Foucault in 1859. Toepler was the first to modify the principle for real time observation of liquid or gaseous flow and sound waves, where it continues to be widely used. Trivia Toepler's son Maximilien Toepler was also a physicist, and worked independently in the same field. Toepler was also famous for inventing the Toepler Pump as seen in the bottom right hand picture. References External links Toeplers's Method for calculating Square Roots on mechanical calculators 1836 births 1912 deaths 19th-century German physicists Academic staff of Riga Technical University