In 1905 the author became interested in selenium cells because of the unusual challenge to understanding which they presented. One of the bestknown selenium cells of that time was the Giltay Cell—a thin soapstone plate around which two parallel wires were wound, with selenium melted and crystallized between the wire electrodes. There were other types, but all had the same remarkable capacity to increase in electrical conductivity when exposed to light. By comparison, a metal, such as a blackened platinum foil, decreased in conductivity when exposed to light. Because of the light sensitivity, selenium cells had proved useful in several kinds of machines and instruments; for example, in the Optiphone and phonopticon, which could enable the blind to read the printed page.