254 PROCEEDINGS OF THE AMERICAN ACADEMY, 



wave-length 4670, which is the shortest thus far detected in the mag- 

 netic 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 indistin- 

 guishable for a considerable distance below this point. The broad 

 flutings seen in the spectrum stimulated with white light are doubt- 

 less to be referred in some way to the circumstance that the lines of 

 the different series get into and out of step periodically : they may 

 thus be considered analogous to the bands seen when two diffraction 

 gratings of slightly different spacing are superpoaed. 



Barium Excitation. 



The fluorescence excited by the barium arc appears to be due chiefly 

 to the line 4934, which coincides with one of the extra lines in the 

 magnetic spectrum. Line 4932 of the second magnetic spectrum is 

 also very near it, and we find that the fluorescence spectrum contains 

 lines which coincide with the magnetic lines of the second series, as 

 well as lines which coincide with some of the extra magnetic lines. 

 The barium arc contains a good many other fainter lines which may 

 give rise to some of the fluorescent lines. It will be necessary to re- 

 peat the experiment with the 4934 line isolated. 



Sodium Excitation. 



As I showed in the earlier paper referred to, if we stimulate the 

 vapor with intense sodium light, we obtain a yellow fluorescence 

 which the spectroscope shows to be made up of two lines in the position 

 of the D lines. We have here a re-emission of light of the same wave- 

 length as the exciting light, and nothing else. This I have called 

 resonance radiation, as we may find that it is different from fluores- 

 cence, though the two are doubtless intimately related. As there are 

 several pairs of lines in the ultra-violet which belong to the same 

 series as the D lines, it seemed of great importance to determine 

 whether these appeared in the spectrum of the fluorescence excited by 



