Abstract The following paper contains an account of a series of experiments on the perception of direction of sound which were undertaken with the object of investigating the nature of the influence of phase differences between the vibrations at the two ears. Lord Rayleigh has shown that such differences help to determine the apparent direction of the sound, the sound appearing to be on the side at which the phase is the more advanced. Professor More arrived at a similar conclusion to Lord Rayleigh by experiments of a different character. The following paper also contains a theory of the influence of phase differences which appears to offer a possible explanation of the observed effects. Most of our experiments have been done with an apparatus similar in principle to Professor More’s, but permitting of a continuous variation of the difference of phase. The apparatus consisted of a brass tube, AB (fig. 1), about 250 cm. long and 2∙5 cm. in diameter, with a short T-piece soldered on to it at its middle point. This tube could slide freely in two slightly larger brass tubes, CD and EF, which were supported horizontally a definite distance apart. From the ends of CD and EF wide tubes were led to caps fitting on to the ears of the observer. The tubes were made up of lengths of glass tubing joined together by pieces of wide indiarubber tubing, and the two sides of the apparatus were made as symmetrical as possible. The ear caps consisted of wooden discs with annular soft pads round them which could be pressed against the head. The caps were supported on retort stands clamped to a table, and were adjusted as symmetrically as possible.