This textbook, which encompasses the entire field of physics in two handy volumes, has emerged from lectures I delivered at the University of Göttingen. The foundation consists of my lectures on experimental physics; however, I have also integrated much that formed the subject of more specialized presentations or seminar exercises. My intention was to provide readers with as clear and complete an insight as possible into the facts and ideas that constitute the foundation of our modern physics. In light of this broader aim, I wished for the book to be easy to read; therefore, I have used mathematical developments sparingly and, where unavoidable, kept them within elementary limits. As invaluable as mathematics is in establishing and formulating physics.