688 Prof. R. W. Wood on Resonance Spectra of 



we may say that the iodine vapour acts something like a 

 specro scope of high resolving power, an extremely narrow 

 group of absorption-lines giving rise to similar groups on a 

 much larger scale disposed at regular intervals along the 

 spectrum. 



We are now confronted with a great difficulty, for the 

 groups are by no means strictly identical with each other or 

 with the groups of absorption-lines, and we must ascertain if 

 possible which lines in the resonance group are associated 

 with, or excited by, the lines in the absorption group. 



I have made a beginning towards the solution of this 

 problem by removing certain frequencies from the green 

 mercury line by passing the light fro in the arc through 

 bromine vapour before it entered the iodine tube. Some of 

 the bromine lines coincide almost exactly with the iodine 

 lines, and these therefore cannot respond to the excitation, 

 since the necessary frequencies have been removed by the 

 bromine filter. To apply a selective ray-filter within the 

 region of the spectrum occupied by a single emission-line 

 seemed at first a rather large undertaking, but the plan was 

 found perfectly feasible, and very satisfactory photographs 

 were obtained. The circumstance that the satellite lines 

 (if one may so term them) which accompany the resonance 

 doublets are somewhat differently disposed according as we 

 use the green line of the Cooper-Hewitt lamp or the quartz 

 lamp, furnishes us with another method of analysis. Up 

 to the present time I have photographed only the satellite 

 lines grouped around four of the resonance lines ; and the 

 circumstance that the general appearance of the groups of 

 lines reminds one of a highly magnified image of the group 

 of absorption-lines falling within the green mercury line, 

 made me think at first that each line in a resonance group 

 resulted from the stimulation of a definite absorption-line. 

 I no longer believe that this is the case, however, and until a 

 further study of the subject is made it will be difficult to say 

 just how the groups arise. 



A study of the green mercury line in comparison with the 

 absorption spectrum of iodine in the same region, with the 

 mercury lamp running at different temperatures, throws 

 some light on the subject however, and has most clearly 

 indicated the lines along which the work must be carried 

 in the future. 



We are dealing with a group of seven iodine absorption- 

 lines, which for convenience we may designate as two doublets 

 and a triplet, and number 1, 2, 3, 4, 5, 6, and 7, beginning 

 at the short wave-length end (first doublet). If the mercury 



