Note 11 — ON THE MOLECULAR TACTICS OF A CRYSTAL By LORD KELVIN, P.R.S.

‘A twin-crystal is composed of two crystals joined together in such a manner that one would come into the position of the other by revolving through two right angles round an axis which is perpendicular to a plane which either is, or may be, a face of either crystal. The axis will be called the twin-axis, and the plane to which it is perpendicular the twin-plane.’ Miller’s Treatise on Crystallography, p. 103. In the text the word ‘twin-plane,’ quoted from the writings of Stokes and Rayleigh, is used to signify the plane common to the two crystals in each of the cases referred to: and not the plane perpendicular to this plane, in which one part of the crystal must be rotated to bring it into coincidence with the other, and which is the twin-plane as defined by Miller.