An instrument was developed for the rapid and accurate measurement of small diameters. The operating procedure consists of placing the test fiber between two optically flat surfaces and, using the fiber as a fulcrum, adjusting the upper flat until the surfaces are parallel and utilizing the absence of interference fringes between the surfaces to indicate parallelism. Controlled tilting of the upper flat is obtained by adjusting, with a micrometer screw, a sliding wedge under a lever attached to the upper flat. Fiber diameter is a linear function of wedge position as indicated by micrometer screw setting.