A photographic two‐crystal spectrometer method employing the penetrating W Kα1 line has been developed. In the present investigation this technique was used to test large crystals with areas up to 25×50 mm in reflection and volumes up to 25×3.5×5 mm in transmission; angular deviations in the lattice as small as 0.5″ were detected. The first crystal was a nearly perfect calcite 30×70×10 mm used in the second order and located 100 cm from the tungsten x‐ray tube and also 100 cm from the second crystal (one tested) which was usually set in the −2 order. The 200‐cm distance from the focal spot to the second crystal separated the α1α2 lines and allowed the α2 line to be blocked out when testing a calcite crystal. In most tests the crystals were of different d∞'s, and a small dispersion resulted, but essentially gave the rocking curve expected from the Darwin‐Prins single‐crystal diffraction pattern. A NaI detector and a scaler were used to locate the peak and half‐intensity points of the rocking curve. A photographic film was placed between the detector and test crystal, and an exposure of 5 to 20 min was made at each of the three positions. Structure in the image gave a measure of the imperfections and their location on the surface of the test crystal. Approximately 100 samples of calcite, quartz, silicon, and germanium crystals have been tested, of which only six or seven approximated perfection. The sensitivity and speed of the method appear superior to any thus far developed, and it provides not only an excellent test procedure for selecting crystals for precision x‐ray wavelength measurements and high resolving power studies, but also for further perfecting synthetic crystal ‐growing techniques.