A device is described which was developed for the rapid computation of rays in a medium in which the index of refraction or speed is a function of only one rectangular coordinate. It was used during the war for the computation of sound fields in water in the ray approximation, and hence is called the sonic ray plotter. It could also be applied to the ray approximation solution of the problem of the propagation of radar signals in a stratified atmosphere, since it is possible to transform the curvature of the earth into an additional refractive index that depends only on the height above a plane earth.