Single activated phosphors absorb very little Λ 2537 radiation and, consequently, do not fluoresce appreciably when so excited. Under cathode ray excitation the emission is strong and is made up of two bands, one peaking at 5600 Å, the other at 6200 Å, the longer of the two becoming more prominent at higher concentrations of . Complete solid solution of larger amounts of is made possible by firing the phosphors in atmospheres of steam and of hydrogen. Single activated phosphors produce blue or ultraviolet fluorescence, depending upon the conditions of preparation.