6 pages (pp. 136-141) IN: Deutschen Physikalischen Gesellschaft, Jahrg. 8, No. 7. Modern wrappers. Whole number offered entire. Braunschweig, 1906.
Planck's third major interest, relativity, developed in the winter of 1905-06. It is characteristic of him that, in 1907, he connected the "principle of relativity" with his quantum of action h.