Dr. Robert Wichard Pohl was born in Hamburg, where he also began his studies in physics, which he continued in Berlin. There, he later wrote his habilitation thesis "The Physics of X-rays," which he published as a book in 1912. His work emerged from the lectures and presentations of his own teaching activities as an assistant at the University of Berlin and aims to explain the current state of knowledge about X-rays at that time by presenting essential research findings. With this work, as well as his later professorship in Göttingen, Dr. Pohl made a significant contribution to the dissemination of physical knowledge. This is a carefully revised reprint of the 1912 edition.