I was at the Mathematical School, where the Maſter taught his Pupils after a Method ſcarce imaginable to us in Europe. The Propoſition and Demonſtration were fairly written on a thin Wafer, with Ink compoſed of a Cephalick Tincture. This the Student was to ſwallow upon a faſting Stomach, and for three days following eat nothing but Bread and Water. As the Wafer digeſted, the Tincture mounted to his Brain, bearing the Propoſition along with it.
Source: wiktionary