and the Rotatory Dispersion Formula. ±'2o 



and then pushing it up across the soot-film. Intensities 

 could be recorded approximately by varying the length of 

 the lines. The sodium lines were also recorded on the plate 

 in the same manner to serve as reference marks. An iron 

 arc spectrum was then photographed together with the sodium 

 lines, and by means of the two plates the wave-lengths of the 

 lines of the "red rotation spectrum could be determined. The 



Fig-. 6. 



I 



smoked plate was fixed by flowing it with a very dilute 

 solution of collodion in ether. A print from the plate 

 mounted " in register/' with a photograph of the absorption 

 spectrum, is shown in fig. 6, Plate V. Owing to the 

 inequality of the absorption in the different parts of the 

 spectrum above the D lines, it was necessary to take photo- 

 graphs, at three different vapour densities, to represent the 

 entire set of lines. The prints have been carefully registered 

 in mounting. The small percentage of lines which exercise 

 rotatory power is very marked in this case. The position 

 of the D lines is indicated on the left-hand end of the 

 plate, the red being to the right. The wave-lengths of the 

 lines in the red rotation spectrum are given in the following 

 table : — 



