A new and simple instrument which can be employed automatically for the rapid and precise determination of x‐ray back‐reflection lattice spacings is described. The instrument is essentially a precision back‐reflection focusing camera containing a Geiger‐Mu¨ller counter, a pin‐hole collimator system, and a movable x‐ray disk baffle with a specially designed circular slit system. Once the instrument is calibrated, the lattice spacings are measured by the position of the slit system on disk which in itself is part of a simple micrometer measuring device. The instrument is capable of high precision and great rapidity in the measurement of lattice constants and their changes.