﻿518 Prof. E. W. TVood on Fluorescence and 



perfect agreement in position between the fluorescent lines in 

 both cases, not only in the blue, but also in the yellow-green 

 region. The lines in the spectrum excited by line 468, or at 

 least as many as could be identified, have been marked. The 

 identification was of course made by comparison with the 

 spectrum excited by helium. 



Lithium Excitation. 



An arc was caused to play between a carbon rod and a 

 large carbon block on which the lithium salt was placed. The 

 image of the red flame was projected upon the window of the 

 retort and excited a bright fluorescence. Two of the lithium 

 lines were operative, — one at 4601, the shortest monochro- 

 matic stimulation thus far found, which gives the series in the 

 violet (see chart, PL XV.), and a large number of lines in the 

 yellow-green ; and another at 4971, which gives a beautiful 

 series in the green, coinciding exactly with the second mag- 

 netic series. The 4971 stimulation should be especially in- 

 teresting, since there are several lines in the magnetic spectrum 

 very close to it. The line is unfortunately not very bright, 

 and the fluorescence lines were so feeble that they could only 

 be measured with difficulty. 



The lines in the yellow-green region are also of considerable 

 interest, since they result from a single monochromatic stimu- 

 lation applied practically at the extreme lower end of the 

 fluorescence spectrum. In fact, this line is considerably below 

 the limit of the fluorescence spectrum as usually seen with 

 white-light stimulation : this limit is not far from wave-len gth 

 4670, which is the shortest thus far detected in the magnetic 

 spectrum photographed with the grating, though faint lines 

 are visible even below 4600 on negatives made with the 

 prism spectrograph. There seems to be evidence of a number 

 of series in the yellow-green region, the spacing, however, 

 decreasing with increasing wave-length, just the opposite of 

 the state of things which holds in the blue-green region. 

 It is more likely, however, that the apparent decreasing of 

 the spacing as the yellow end of the spectrum is approached 

 is due to other series similar to those which are found in 

 the green and blue, the nearness of the lines resulting from 

 the large number of superposed series. With white-light 

 stimulation no trace of the lines can be seen above wave- 

 length 555, and they are so faint as to be almost indis- 

 tinguishable for a considerable distance below this point. 

 The broad flutings seen in the spectrum stimulated with 

 white light are doubtless to be referred in some way to the 



