INFLUENCE OF RED AND INFRA-RED RAYS UPON SIDOT BLENDE. 83 



The effect of the longer waves during excitation is unfortunately as hard 

 to reconcile with this explanation of the phenomena as with that first 

 suggested. 



VARIATIONS OF THE EFFECT WITH THE LENGTH OF THE LONGER WAVES. 



In the case of several phosphorescent substances, including Sidot blende, 

 the effect of rays of different wave-length in suppressing phosphorescence 

 has been studied photographically by Dahms. 1 Our own results, obtained 

 by an entirely different method, in general confirm his conclusions in a very 

 satisfactory way. 



The arrangement of apparatus was similar to that used in our first experi- 

 ments on the decay of phosphorescence. 2 A spark was used for excitation 

 and a shutter was so arranged that the screen was exposed to the infra-red 

 rays and the phosphorescent light allowed to fall on the slit of the spectro- 

 photometer at the same instant that the excitation was brought to a close 

 by short-circuiting the spark. A Nernst glower was used as a source of 

 infra-red rays. This was mounted in the place of the slit of a large mirror 

 spectrometer having a quartz prism. 3 The Sidot-blende screen was covered 

 with black paper except for a narrow rectangular region having about the 

 same width as the Nernst glower. The adjustment of the spectrometer 

 having been determined by observations in the visible spectrum, wave- 

 lengths in the infra-red were computed from the angle of deviation. The 

 effect of different rays from the Nernst glower was measured by the differ- 

 ence between the times required for the phosphorescence to fall from its 

 initial intensity to a definite final intensity with and without exposure to 

 the rays to be tested. Each point of the curves shown in Fig. 76 is deter- 

 mined from the average of ten observations with infra-red and ten without, 

 the observations being made alternately. The difference between the two, 

 expressed as a fraction of the normal time of decay, has been plotted for 

 the different wave-lengths used, which ranged from 0.6 /x to 2.3 /x. The 

 observations refer to the region of maximum intensity in the phosphores- 

 cence spectrum (0.512 xx). 



Referring to Fig. 76, it will be seen that the effect of the longer waves is 

 observable to some extent in the visible region. A maximum is reached at 

 about 0.9 /jl, followed by a minimum at about 1.0 /x and another maximum 

 at 1.3 xx. From 1.2 /x on the observations were repeated under slightly 

 different conditions the following day (see broken line). In this case the 

 chief maximum appears to lie at 1.37 /x. The results are probably in all 

 cases uncertain to the extent of 2 or 3 per cent, and errors are especially 

 likely to be serious in regions where the effect is small. For this reason we 

 can not feel certain of the third maximum at 2 . 1 8 xx, although the probability 

 is that it really exists. As far as any important effect is concerned, how- 

 ever, our results confirm the conclusion of Dahms that the action does not 

 extend beyond 1.5 xx. 



It can scarcely be doubted that absorption of the active rays is necessary 

 before they can produce any effect upon phosphorescence. It seems 



'Dahms; Annalen der Physik, 13, p. 425. 

 : See Chapter III of this memoir. 



3 Tests (by direct eye observation) with a rock-salt prism showed that no effect was observable for wave- 

 lengths longer than those transmitted by quartz. 



