Resonance Spectra of Iodine Vapour. 473 



excited by the 5790 mercury line is much more intense than 

 the series excited by the 5770 line, a circumstance which we 

 may safely ascribe to a closer coincidence between the wave- 

 length of the less refrangible yellow line of mercury and one 

 of the absorption-lines ot the iodine vapour. 



The yellow light can be removed from the exciting 

 radiations by means of a saturated solution of neodymium 

 chloride, and we obtain in this case the series excited by the 

 green mercury line. 



Photographs of the spectra obtained in this w T ay are 

 reproduced on PI. IV. Fig. 1 is the composite of the three 

 resonance spectra excited by the total radiation of the 

 mercury arc, the green and the two yellow exciting lines 

 being indicated by arrows. The short-line spectrum which 

 runs along the centre is the neon spectrum, which, being- 

 very rich in orange and red lines, was used lor a comparison 

 spectrum in the present investigation. Fig. 2 shows the 

 same spectrum without the neon lines. The broadening of 

 the lines is due to overexposure, the fainter lines appearing 

 sharp. The series excited by the green mercury line 

 operating alone is shown in fig. 3. This is one of the finest 

 cases of a resonance spectrum made up of equidistant lines 

 that I have found up to the present time, eighteen members 

 having been photographed and measured. The gradual 

 decrease in the distance between the members of the series 

 as the red end of the spectrum is approached, is of course 

 due to the circumstance that the photograph was made with 

 a prism spectrograph. As a matter of fact the interval 

 between the lines increases slightly with an increase of wave- 

 length. The more or less regular alternation between very 

 strong lines and very weak lines, which is characteristic of 

 so many of the resonance spectra of sodium vapour, is very 

 conspicuous ; in fact, this spectrum reminds one forcibly of 

 the resonance spectrum of sodium excited by the cadmium line 

 4800, in which case w T e have the exciting line followed by a 

 strong line, then a faint line, then four strong lines, a faint 

 line, and four more strong lines. In the case of iodine we 

 have the exciting line followed by a strong line, then a faint 

 line, a strong line and a faint one, two strong lines, a faint 

 line, strong, faint, strong, two faint lines, strong, faint, 

 strong. 



Fig. 4 is a photograph of the resonance spectra excited by 

 the two yelloAv lines of mercury, the green line having been 

 removed by the eosine screen. We find here 15 pairs of 

 lines, reminding us of the repetition of the magnesium 

 triplet in the case of the resonance spectrum of sodium 



