Abstract
When the rays emerging from a Jamin interferometer are reflected back through the instrument, fringes similar in appearance and behaviour to three-beam fringes are obtained. These fringes can be used to measure small angular displacements of one of the beam-dividing plates, with an accuracy of the order of 0.01". A modified setup is also described with which angular displacements of a comparatively light, auxiliary mirror can be measured with the same degree of accuracy.