halides increases strongly upon decrease of the ratio S/D (where S is the space between adjacent halogen ions in a [110] direction of the normal lattice and D the diameter of the halogen atom). In going from KBr to NaBr, S/D diminishes by a factor of approximately two, while the energy needed to form an F center becomes 600 times greater. In the cesium halides, as one goes from CsCl to Csl, the corresponding S/D ratio decreases by a factor of 2.5. Consequently, one might