﻿512 Prof. R. W. Wood on Fluorescence and 



of energy among the individual lines depends upon the point 

 of excitation. 



Unfortunately there are very few arc lights bright enough 

 to excite fluorescence. It was found, however, that the silver 

 line 5207, which coincides with one of the fluorescent lines 

 of the cadmium 480 series, was bright enough for the purpose. 

 The silver was carefully freed from copper, as the neighbour- 

 ing copper lines are powerful exciters ; in their absence, no 

 prismatic separation was necessary, as the rest of the silver 

 lines were inoperative. A photograph of the fluorescence 

 spectrum obtained with silver stimulation is shown on 

 PI. XII., g. The series in this case has no gaps in it, the line 

 at 5170, which is absent with cadmium excitement, comino- 

 out strong (see chart as well) . The monochromatic illumi- 

 nator, with its slits reduced to the width of a hair, was 

 arranged to furnish light of wave-lengths corresponding to 

 other lines in the series, and photographs obtained which are 

 recorded on the chart. It will be seen that faint or missino* 

 lines occur in each case, but that their position varies with 

 the point of excitation. If we consider each line caused by 

 a single electron or vibrator, the phenomena suggest that 

 the vibrators are united in some way, perhaps in a closed 

 ring, and that when the system is set in vibration there are 

 nodal points, the position of which depends upon the point in 

 the chain where the periodic force is applied. Moreover, 

 as has already been pointed out, if the force is applied at 

 the "high frequency " portion of the chain, the regions 

 excited are those of highest and lowest frequency, the inter- 

 mediate portion appearing to be at rest. This is especially 

 noticeable in the case of the bismuth excitation ( PI. XII., e; 

 and chart, PL XV.). 



In addition to the lines enumerated above, there are a 

 number of others at the upper end. These do not appear to 

 be distributed with the same regularity, though some of them 

 may form an extension of the series, or more probably may 

 be the beginnings of other series. In general it has been 

 found that in the simple spectra the lines are regularly spaced 

 between the extreme violet end and a point at about A,= 5350. 

 Above this point the spacing is generally very irregular, and 

 it is difficult to unravel the spectrum. Of this more will be 

 ^aid later. 



We will next take the fluorescent spectrum excited by the 

 green cadmium line 5086. 



This spectrum is remarkable in that it is made up of eleven 

 pairs of lines regularly spaced (PL XII., j, and chart). The 

 other two cadmium lines appear on the plate, as the spectro- 

 graph was not shielded from the diffused light from the lamp. 



