The following components of the Universal Demountable Cell are sold separately: Demountable (circular) cell windows, either 2 or 4 mm thick, are available in the following materials: NaCl, KBr, CaF2 BaF2, KRS-5, AgBr, and ZnSe Semi-Demountable (rectangular) cell windows, either 2 or 4 mm thick, are available in the following materials: NaCl, KBr, CaF2 BaF2, KRS-5, AgBr, and ZnSe Demountable (circular) Spacers, either lead or plastic, in a variety of thicknesses. Semi-Demountable (rectangular) Spacers, either lead or plastic, in a variety of thicknesses.