Roentgen (unit) this derivative quantity the roentgen equivalent physical (rep) to distinguish it from the ICR roentgen. The introduction of the roentgen measurement unit, which relied upon measuring the ionisation of air, replaced earlier less accurate practices that relied on timed exposure, film exposure, or fluorescence. This led the way to setting exposure limits,and the National Council on Radiation Protection and Measurements of the United States established the first formal dose limit in 1931 as 0.1 roentgen per day. The International X-ray and Radium Protection Committee, now known as the International Commission on Radiological Protection (ICRP) soon followed with a limit of
Wilhelm Filehne at the medical polyclinic. In 1876 he became an associate professor of pharmacology at Erlangen, and ten years later, was appointed a full professor of the same subject at the University of Breslau. In 1911 he was succeeded at Breslau by Julius Pohl. In addition to work in the field of pharmacology, he made contributions in his research of optical illusions, being known for his experimentation with a phenomenon known as a Zöllner illusion. The so-called "Filehne illusion" is the illusory motion of a stationary background when smooth pursuit eye movements are made across the stationary background. Wilhelm Filehne Wilhelm