INFLUENCE OF RED AND INFRA-RED RAYS UPON SIDOT BLENDE. 75 



light and practically inseparable from it. By making observations only 

 at points lying between the bright lines of the mercury spectrum we had 



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 of infra-red were obtained by using 

 in one case one piece of black rubber and in the other case two pieces 

 between the arc lamp and the screen. By this procedure it is possible to 



Fig. 66. 



Effect of infra-red on fluorescence 

 of "Emanations-pulver," ex- 

 cited by Uviol lamp. 



