The plates are reproductions of some of the original photographs 

 of the spectra described in the paper. It has been found quite impossible 

 to reproduce them so as to give all the details of the original negatives, 

 and, consequently, much of the sharper and finer differences do not 

 appear. 



The figures at the top of each plate give the wave lengths of the 

 spectral bands photographed ; the red end of the spectrum being on 

 the left. 



Plate I, No. 1. Absorption spectra of equivalent strengths of 

 solutions of the nitrate and chloride of Didymium. The strongest 

 solutions in a cell of 5 mm. thickness, and the diluted solution in 

 a tube of 3599 mm. length ; consequently the ratio of the dilution 

 is 1 : 719-8. 



The chief differences in the strong solution of Didymium nitrate in 

 the 5 mm. cell from the other three photographs are fairly well seen. 



Plate I, No. 2. Absorption spectra of equivalent strengths of 

 solutions of the nitrate and chloride of Didymium. The strongest 

 solutions in a cell of 5 mm. thickness, and the diluted solution in 

 a tube of 1224 mm. length; consequently the ratio of the dilution 

 is 1 : 244-8. 



The chief differences in the strong solution of Didymium nitrate 

 can also be seen here, but they are not so well marked as in Plate I. 



The photographs taken in tubes of 612 mm. length, and the cell of 

 5 mm. width, i.e. using solutions whose ratio was 1 : 122 4, shew similar 

 differences. 



