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SIMPSON, McCRA W, AND MARTON0.03% of the sample. At the magnification employed, any hole larger than 300 A in diameter would have been noticed. Above this size, we conclude that the 61ms are essentially continuous.It will be noted that improved aperturing of the beam and the deliberate addition of a large incoming beam contamination, while radically changing the elastic distribution, have only minor effects on the characteristic distribution. Moreover, it appears that at angles beyond 4&(10 ' radian the angular dependence of the elastically scattered electrons is less rapid than that of the characteristically scattered electrons.The data for aluminum presented in Fig. 2 were analyzed in an attempt to confirm the dependence of the value of the characteristic energy loss, E, on the scattering angle, 0, reported by Watanabe' and attributed by him and others to the dispersion of plasma waves. The expected dependence is of the form 8=AO'.In a small region between 6 and 12 milliradians, our data show a 0' dependence, although the constant disagrees by 50 j~f rom that of Watanabe. Outside this region, neither the constant nor the exponent agrees with his values.Boron nitride can be excited to luminesce by (1) alternating electric 6elds, (2) ultraviolet photons, or (3) cathode rays. The luminescence, in all cases, has a complex emission spectrum, extending from approximately 2950 A to 6500 A. The relative intensities of the bands in the fine structure are affected by current density with cathode-ray excitation, and by the energy (frequency) of the photons with ultraviolet excitation. The maximum of the photoluminescence emission intensity with temperature occurs at about 875'K, with the luminescence emission intensity falling to 50% of the maximum value at approximately 1375 K.Correspondence of known infrared absorption bands with energy differences in the fine structure in the luminescence suggest vibrational origins for the fine structure. The identities of the excited electronic states have not yet been ascertained, although these may be due to exciton states, impurity states, or surface states.

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The choice of halide coactivators in the synthesis of electroluminescent powder phosphors of the normalZnfalse(S:normalSefalse):normalCu system markedly affects the spectral distribution and efficiency of the phosphors. Iodide coactivated normalZnS:normalCu can be prepared as a blue‐emitting phosphor exhibiting no shift in spectral emission with field excitation from 20 to 50,000 cps. Several‐fold increase in radiant output over the chloride coactivated normalZnS:normalCu phosphor is obtained by using iodide coactivator. normalZnSe:normalCu phosphors, iodide coactivated, also show no shift in spectral distribution with frequency. However, the emission from normalZnfalse(S:normalSefalse):normalCu materials, prepared with any of the halide coactivators, shifts to lower peak wave length with increasing frequency of excitation. A red‐emitting electroluminescent phosphor can be synthesized from the false(normalZn:normalCdfalse):normalSe:normalCu system by the use of bromide or iodide as a coactivator.The role of the halide is extended from that of a randomly distributed donor to one of association with the activator, and consequences of such activator‐ coactivator association are discussed.