Iodine by Multiplex Excitation. 691 



feature brought out is that the strong doublets are shifted 

 towards the violet in the case of the excitation by the high 

 temperature arc. It is not an actual shift, but merely a 

 change in the distribution of intensity among the lines 

 forming the groups. In addition to this, certain lines appear 

 in one case which are absent in the other. This is especially 

 noticeable in the group at 5725, in which the second lines 

 from the left are clearly displaced with reference to each 

 other. I am of the opinion that the shift in the region of 

 maximum intensity towards the violet in each group results 

 from the widening of the green line in the same direction ; 

 but it seems impossible to assign the lines in the groups to 

 definite absorption-lines until photographs have been secured 

 with the exciting line under a larger number of different 

 conditions. Very probably the excitation of a single line 

 gives rise to resonance lines accompanied by fainter com- 

 panions. The primary line at 5461 is accompanied by these 

 companions but is itself single, as I have already pointed out. 

 In addition to varying the temperature of the lamp, we may 

 pass its light through bromine or some other vapour showing- 

 fine lines in its absorption spectrum, and so modify ihe 

 appearance of the emission-lines. This has already been 

 accomplished. The light from the quartz arc, rendered con- 

 vergent by a large condenser, was passed through a large 

 spherical flask containing a small amount of bromine in vacuo. 

 The air was exhausted by freezing the bromine to the wall 

 with solid C0 2 and ether, and exhausting rapidly with a 

 Gaede pump. The effect of the bromine filtration was to 

 remove the right-hand component of the triplet which lies 

 to the left of the strong doublets at 5525 and 5660. (See 

 small inserts below spectrum Gr.) This same triplet is found 

 to the left of the primary line in the resonance spectrum 

 (5461), and the bromine filtration weakened the right-hand 

 member in this case also. 



I have photographed the bromine absorption spectrum in 

 coincidence with that of iodine with the 40-foot spectrograph. 

 The two are shown by spectrum V, Plate XVIII., taken with 

 sunlight, and by spectrum R, taken with the green line of 

 the quartz arc. It seems probable that the line removed 

 from each resonance group by the bromine filtration of the 

 exciting light results from one of the absorption-lines 2, 6, 

 or 7, since these three are in coincidence with bromine 

 absorption-lines. We find, however, that the two outer 

 components of the triplet appear in the resonance groups 

 excited by the Cooper-Hewitt lamp, which has practically no 

 energy at the point occupied by the iodine absorption-line 2, 



