Interferometry, a technique widely used in physics to study radiation fields, has been applied to the investigation of dowsing. Pairs of horizontal linear components, typically some metres in length and a metre or so apart, and parallel in a vertical plane, produce patterns of lines that can be mapped by dowsing rods. The lines are parallel to each other and to the components, and are equally spaced. The spacing of the lines depends on the length, separation and composition of the components. The existence of the patterns shows that the dowsing field is wave radiation. Since there is no detectable difference between patterns produced by pairs of components made of good electrical conductors such as copper, and those produced by good insulators such as uPVC, it is concluded that the radiation field is not electromagnetic. Furthermore, aluminium and tin behave differently from copper and steel. The spacing of the lines has been found to depend on time of year. It increases rapidly in late April and falls rapidly in late November; in between these times it appears to be relatively stable except for an isolated maximum in early March. It shows a high degree of repeatability from year to year.