I 



Iodine by Multiplex Excitation. 689 



arc is running- at a low temperature, as is the case in the 

 Cooper-Hewitt lamp, the bright central component of the 

 green line almost exactly bisects the second doublet (lines 3 

 and 4). It is obvious that, under these circumstances, one of 

 the absorption-lines is excited by a frequency slightly greater 

 than its own, and the other by a frequency slightly less. The 

 light which is re-emitted without change of wave-length 

 (resonance radiation) gives us a single line so far as 1 have 

 been able to find with the comparatively low dispersion 

 brought to bear upon the resonance spectra. The 40-foot 

 spectrograph would probably show it double of course. The 

 lines of the resonance spectrum, however, are double, and 

 the components of each doublet are separated by a distance 

 about 30 times as great as the distance between the absorption- 

 lines. This magnification of the doublet occurs only in the 

 case of the light which is emitted with altered wave-length, 

 and not at the primary line. As a working hypothesis we 

 may ascribe it to excitation by a frequency intermediate 

 between the frequencies of the absorption-lines ; and we 

 may test our hypothesis by slightly altering the frequency 

 of the exciting line, so as to make it approach one or the 

 other of the two absorption- lines. It seems by no means 

 impossible to accomplish this, since the distance between 

 the absorption-lines is only j-Jq of the distance between the 

 D lines of sodium; and a very slight alteration in the pressure 

 of the mercury vapour would give the required shift of the 

 emission-line. If the iodine vapour is excited by the quartz 

 lamp we still get sharply defined lines in the resonance 

 spectrum, notwithstanding the circumstance that, in this case, 

 the green mercury line has broadened out into a continuous 

 band of light, which completely covers the group of absorption- 

 lines with the exception of the point midway between lines 3 

 audi, where the green line is weakened by reversal. 



This makes it seem doubtful if the hypothesis just suggested 

 is correct. It will be necessary to excite the iodine vapour 

 with the mercury lamp running under several different con- 

 ditions as to current and temperature before any very definite 

 statement can be made. That this method of investigation 

 is sure to be a very fruitful one is made clear by spectrum N, 

 Plate XVII., on which I have shown the appearance of the 

 green mercury line with the lamp at different temperatures, 

 and the iodine absorption-lines in coincidence with it. This 

 is a drawing. Spectrum 0, Plate XVII., is a photograph 

 of the green emission-line of the quartz arc, when first ignited, 

 taken in coincidence with the same line from the lamp at 

 high temperature, the light being passed through iodine 



