The introduction of the equivalence principle in 1907 marked both the beginning of a long effort to formulate a theory of gravity and a shift in emphasis away from the special theory. Einstein’s June 1911 paper, “On the influence of gravitation on the propagation of light,”30 contained another treatment of mass-energy in relation to gravity. This time he envisioned two optical instruments, I1 on the z-axis at a distance l beneath device I2.