One of the difficulties met by beginners in crystallography is the determination of the class to which a given crystal belongs. With a view to lessening this difficulty, the writer about two years ago prepared separate keys for the determination of the different classes belonging to each crystal system, and later a more comprehensive single key, including all the crystal classes. As this key has proved very satisfactory in use in the writer’s classes, it is presented here in the hope that it may be of service to others. Various forms of the key have been worked out from time to time in the effort to find the simplest and most satisfactory arrangement, the two tables given here being those which have seemed the best in practice.
The first of these tables makes use of a center of symmetry as a basis for its main divisions, adopting the simpler . . .