Iodine by Multiplex Excitation. 685 



in this case in making the calculations, as it fits the series 

 better. The 5397 line is not a doublet, neither can I find 

 any indication that the 5460*7 line (the primary line) is 

 double, though it is accompanied by faint companions like 

 all the other doublets, as we shall see presently. I took 

 great precautions to prevent any diffused light from the 

 mercury arc from getting into the spectroscope, but it is 

 just possible that this line is really emitted by the iodine 

 vapour as a doublet, the gap between the components being- 

 filled up by stray light from the arc, which would be the 

 case if the doublet were symmetrical about the green mercury 

 line. 



As is apparent from the table, the frequency differences 

 increase a trifle in passing from one end of the spectrum to 

 the other. 



The photographs of the entire resonance spectrum taken 

 with the concave grating were made with a very fine slit, and 

 were not fully exposed even with a 24 hour exposure. The 

 lines were beautifully sharp, and served admirably for 

 measurement, but they are scarcely strong enough to bear 

 reproduction. A portion of the spectrum, how T ever, was 

 taken on an isochromatic plate which is much more sensitive 

 in the yellow, and this plate is represented on Plate XVII., F. 

 The lines are rather broad on this plate, and the fainter com- 

 panions of the doublets come out as well : these will be 

 discussed presently. Spectrum Gr was made of the vapour 

 excited by the light of the quartz mercury arc, while F was 

 made with the glass Cooper-Hewitt lamp. The resonance 

 spectra will be found to be quite different in some respects. 

 On Plate XVI. will be found the resonance spectra photo- 

 graphed w T ith the large prism spectrograph. All of the 

 spectra, with the exception of A, are reproduced as negatives, 

 as the lines came out better. On this plate A and B are the 

 spectra excited by the Cooper-Hewitt lamp, C by the 

 Westinghouse quartz lamp, D the resonance spectrum ex- 

 cited by the green line acting alone (quartz lamp), and E 

 the spectrum excited by the two yellow lines acting alone. 

 Spectrum D is the series of doublets which we have just 

 discussed, though they are not resolved on the plates which 

 are reproduced. 



We will now consider the series excited by the two yellow 

 lines. The more refrangible of the two yellow lines excites a 

 series of doublets when we employ the Cooper-Hewitt arc. 

 These can be seen in spectrum B. If the quartz arc is used 

 (spectrum C) we find triplets in place of the doublets, the 

 middle component being very strong,- and the two outer ones 



