. Carnegie Institution of Washington publication. Fig. 65. Effect of infra-red on fluorescence. Sidot blende excited by " Uviol" mercury-vapor lamp. Curve /. Fluorescence spectrum. Curve //. Fluorescence spectrum when screen is exposed to infra-red during excitation. Curve /'. Fluorescence spectrum with plate glass between screen and mercury lamp. Curve II'. Same as /', except that screen is also exposed to infra-red. R. Reflection of exciting light from white surface. M M .48 .50 SZ _B4 .bfe/Z. expected to be untroubled by reflected light. But owing either to optical imperfections i
. Carnegie Institution of Washington publication. Fig. 65. Effect of infra-red on fluorescence. Sidot blende excited by " Uviol" mercury-vapor lamp. Curve /. Fluorescence spectrum. Curve //. Fluorescence spectrum when screen is exposed to infra-red during excitation. Curve /'. Fluorescence spectrum with plate glass between screen and mercury lamp. Curve II'. Same as /', except that screen is also exposed to infra-red. R. Reflection of exciting light from white surface. M M .48 .50 SZ _B4 .bfe/Z. expected to be untroubled by reflected light. But owing either to optical imperfections in the apparatus or to the existence of a faint continuous spectrum in the light from the lamp, there was 18 always enough reflected light in the field of the spectrophotometer to be an important and dis- turbing factor. To get some idea of the inten- sity and distribution of this reflected light we made the observations plotted as curve R in Fig. 65 with a screen of MgO on cardboard instead of the Sidot blende screen. To avoid confusion this curve is displaced downward in the plot. The intensity for points on curve R should be read from the right-hand side of the figure. The irregular distribution of the reflected light and the great uncertainty in its measure- ment make the experiments plotted in Fig. 65 of little quantitative value. Especially is this true for the violet end of the spectrum, where the reflected rays are of great intensity. We could not even feel sure that the longer waves produced any effect at all in this region. With a different zinc sulphide screen, the so-called "Emanations-pulver" referred to in Chapter IV, the conditions were somewhat more favorable. The curves in Fig. 66 show the ordinary fluorescence spectrum (curve /); the fluorescence spectrum with exposure to weak infra-red rays (curve 77); the fluorescence spectrum during exposure to strong infra-red rays (curve 77/); and the reflected light deter- mined as before. Different intensities
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