WOOD. — SPECTRA OF SODIUM VAPOR. 253 



tion spectrum. It is worthy of remark that one of the extra lines of 

 the magnetic spectrum lies very close to the exciting line, yet none of 

 these lines appeared in the fluorescence spectrum. The fluorescence 

 excited by lead was very feeble, and even with an eight-hour exposure 

 the lines were very faint. 



Helium Excitation. 



A large "end on " helium tube with a 3 mm. bore was constructed 

 for the investigation. This tube could be run continuously with an 

 induction coil yielding a heavy ten-inch spark. An exposure of about 

 twelve hours was given. Two of the helium lines are operative : line 

 5015 gives a well-marked series, the lines of which fall exactly midway 

 between the lines of the second and third magnetic series; line 4713 

 gives a good series in the blue and at least six distinct lines in the 

 yellow-green, the wave-lengths of which can be seen from the chart. 

 The exciting line in this case coincides with one of the lines in the 

 spectrum excited by zinc 468, and there is 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 spec- 

 trum 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 monochromatic stimulation thus far found, which gives 

 the series in the violet (see chart), 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 magnetic series. The 4971 

 stimulation should be especially interesting, 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 stimulation applied prac- 

 tically 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 



