A method of determining the constants of propagation for ultrasonic waves in gases, by use of the Pierce interferometer, was devised by Hubbard, and application of it extended to a limit of two megacycles by his various students. Herein is described a new interferometer which carried these investigations to the four‐megacycle region. Alignment of the piston and crystal to the order of one light fringe was attained and maintained by employing Newton and Haidinger optical fringe systems. Velocities were measured to an accuracy of one‐tenth of one percent, and absorption and reflection coefficients to fifty percent in air and CO2. The limit of accuracy in both cases was determined by the measurement of length, as measured to one micron with a micrometer screw. Preliminary measurements on H2 gave evidence of molecular dispersion between four and eight megacycles.