IN a lecture delivered to the Physical and Chemical Society of University College, Nottingham, on February 4, Prof. E. Schrödinger directed attention to the difficulties and contradictions which arise from attempts to unite quantum theory with geometry and with the theory of relativity. Although the first researches on wave mechanics used the relativity ideas, they are really in flat contradiction to them. The theory of relativity supposes that rods and clocks can be used to measure exact lengths and times, and that in some way velocities can also be measured accurately. Unfortunately, Heisenberg's uncertainty principle, which appears to be inherent in quantum mechanics, lays down that simultaneous accurate measurements of position and velocity are impossible. Moreover, the regulation of a clock is not possible to more than a limited degree of accuracy, except when the clock is infinitely heavy. Similar considerations forbid us to apply our ideas of Euclidean geometry, based upon ideal rigid measuring rods, to small physical regions. Physicists have at least as much confidence in the special theory of relativity as in quantum theory; the problem of devising a unified theory appears to be still unsolved.