Iodine by Multiplex Excitation. 681 



The wave-lengths o£ the seven iodine absorption-lines are 

 as follows : — 



5460*966 

 •910 

 •873 

 •768 

 •716 

 •640 

 •579 



As will be seen presently, the resonance spectra ought to 

 be studied under equally high resolving power, but this is 

 almost out of the question, on account of the comparative 

 feebleness of the light. I have, however, obtained excellent 

 photographs with a concave grating of six feet radius of 

 curvature, and hope to do better in the autumn. 



The examination of the absorption spectrum under the 

 highest dispersion possible is of interest in connexion with 

 the finite width of the lines due to the velocities of the 

 molecules in the line of sight. They are at their best only 

 when the iodine vapour is in a very high vacuum : the 

 presence of air, even at rather low pressure, causes the lines 

 to lose in sharpness, and eventually obliterates the finer 

 ones. 



A portion of the absorption spectrum 8 Ang. units in 

 width is reproduced on PL XVIII. spectrum W, in coincidence 

 with the corresponding portion of the spectrum from Hassel- 

 berg's map, enlarged to the same scale. The three broad 

 lines on my photograph are sofar lines, which appear on 

 Hasselberg's map immediately above his spectrum. Hassel- 

 berg's map was prepared by hand from his table of measure- 

 ments made from photographs taken with a grating of 

 inferior power, the drawing being subsequently verified by 

 comparison with the photograph. A comparison of the two 

 shows that the lines in Hasselberg's map mean nothing at 

 all, except that they indicate roughly the position of some 

 of the groups of lines. A study of these two pictures cannot 

 but furnish food for thought to astronomers working at the 

 problem of the canals on Mars. 



The group of seven lines which fall within the green 

 mercury line are enclosed with a bracket, and when it is 

 remembered that the entire portion of the spectrum repro- 

 duced embraces a range of the spectrum not much greater 

 than the distance which separates the D lines of sodium, the 

 frightful complexity of these absorption spectra becomes 

 evident. 



