The spots of a Laue crystal-photograph may be regarded as due to the reflection of X-radiation by the planes of the crystal and the interpretation of these photographs involves the determination of the index of the plane by which any given spot is produced. This 'interpretation can be readily accomplished by two distinct though related methods. We may, in the first place, utilize the stereographic projection and determine the indices of the planes by means of intersecting zone- circles. This method, the earliest in point of time, was explained by W. L. Bragg in 1913. In recent years, however, it has become usual to employ for this purpoee the gnomonic projection which affords a solution in some respects impler and more direct than the stereographic method used by Bragg, and detailed illustrations of its application in this field have been given by R. W. G. Wyckoff, and E. Schiebold.