When we look at a collection of crystals we see that they can be divided into groups according to their symmetry. Thus, one group the members of which form the cubical system, have a symmetry in three directions at right angles to each other ; that is to say we shall find similarly arranged planes at the extremity of three properly chosen axes. In other groups the symmetry is not so perfect. Thus, in the tetragonal system we find that the symmetry in the direction of the principal axis is different to the symmetry in the direction of the lateral axes.