THE LUMINESCENCE OF SIDOT BLENDE. 43 



sulphide screen by a block of magnesium carbonate, whose surface was of 

 approximately the same color and roughness as that of the fluorescent 

 screen, and by measuring the light received from this block at different 

 points throughout the spectrum. The results of such a series of measure- 

 ments is shown in the lower broken line in the figure. It will be noticed 

 that the correction is inappreciable for the longer wave-lengths, but be- 

 comes important in the violet. The upper broken line in Fig. 40 shows 

 the intensity of the light reaching the spectrophotometer from the zinc 

 sulphide screen, being the combined effect of luminescence and reflected 

 light; and the heavy curve, obtained by subtracting the ordinates of the 

 lower curve from those of the upper, shows the corrected luminescence 

 spectrum during excitation. 



The curves in Fig. 40, like those shown in the previous chapters, are 

 expressed in terms of the acetylene flame as a standard. In other words, 

 each ordinate represents the ratio of the intensity of the luminescence light, 

 of the particular wave-length considered, to the intensity of the light of the 

 same wave-length from the acetylene flame; the curves are not energy 

 curves. 1 



When the screen was excited by the arc as described above no trace 

 of the violet luminescence, which had been so prominent with Roentgen- 

 ray excitation, could be observed. It seemed not unlikely, however, that 

 the absence of the violet band was due to the fact that the ultra-violet rays 

 suitable for exciting it had been removed from the light from the arc by the 

 glass prism and lenses of the dispersing system. We therefore rearranged 

 the apparatus so as to use a quartz prism and quartz lenses, while a spark 

 between metal terminals was substituted for the arc. Since the excitation 

 in the region studied was solely by ultra-violet rays that were incapable 

 of passing through glass, it was a simple matter to test for errors due an 

 impure spectrum, or to any other source of stray light, by inserting a piece 

 of glass in the path of the exciting rays. This test showed that stray light 

 was in no case present in appreciable amount. 



This ultra-violet excitation developed strong fluorescence in the extreme 

 violet, similar in color to that produced by Roentgen rays. But the green 

 band that had been excited by the visible rays of the arc was relatively 

 very weak. In spite of its faintness, however, the presence of the green 

 band could be readily detected, for while the violet luminescence died out 

 almost immediately when excitation ceased, the green persisted as a slowly 

 decaying phosphorescence observable for several minutes. 



The similarity between the effects of ultra-violet light and those of 

 Roentgen rays as exciting sources is worthy of note. In each case the chief 

 luminescence is in the extreme violet, and is of short duration. But in 

 each case also this is accompanied by luminescence in the green, which is 

 relatively faint but of long duration. As is illustrated in numerous other 

 cases, the Roentgen rays, when comparable at all with rays of light, are 

 rather to be compared with ultra-violet light than with the rays of the 

 visible spectrum. 



The agreement between the luminescence spectrum excited by Roentgen 

 rays and that observed in the case of ultra-violet excitation is not exact. 



For a description of a method of reducing such curves giving the distribution of energy in fluorescence 

 spectra see Nichols and Merritt, Physical Review, xxx, p. 328; also Chapter XII of this memoir. 



