278 JONES AND ANDERSON— ABSORPTION SPECTRA OF [April 25. 



exposures on a panchromatic plate for the red end of the spectrum, 

 using the same set of solutions. 



The scale accompanying the spectrograms was made by photo- 

 graphing an ordinary paper scale. Several photographs were taken, 

 the distance between the paper scale and the lens of the camera 

 being varied slightly from exposure to exposure. The resulting 

 negative which fitted the majority of spectrograms best was selected 

 and used throughout. 



Neodymium Chloride in Water — Beer's Law. 

 (See plate i.) 



The concentrations of the solutions of neodymium chloride were 

 so chosen and the depths of cell so selected that the total amount of 

 coloring matter in the path of the beam of light was kept constant. 

 From Beer's Law the absorption shown by the several solutions, 

 under these conditions, should be the same. The concentrations of 

 the solutions used in making the negative for a, plate i, beginning 

 with the one whose spectrum is adjacent to the numbered scale, 

 were 3.40, 3.02, 2.72, 2.38, 2.17, 1.90 and 1.70; the corresponding 

 depths of cell being 12, 13.5, 15, 17, 19, 21.5 and 24 mm. For b, 

 plate I, the concentrations were 3.40, 2.55, 1.70, 1.13, 0.80, 0.57 and 

 0.43 ; the corresponding depths of absorbing layer being 3, 4, 6, 9, 

 13, 18 and 24 mm. 



The most concentrated solutions appeared brownish yellow in 

 their bottles, from which the color changed on dilution to a yellowish 

 pink, the color being extremely faint in the most dilute solutions. 



The exposures to the light of the Nernst lamp and spark were, 

 respectively, i minute and 2 minutes ; the slit having a width of o.oi 

 cm. The exposures and slit width were not varied in the work 

 recorded in the present chapter, the object being to make the spectro- 

 grams as nearly comparable as possible. 



Both a and b of plate i show the presence of some general 

 absorption in the ultra-violet, which decreases quite rapidly with 

 dilution. The absorption bands also narrow somewhat with de- 

 crease in concentration, especially from 3.4 normal to about 1.7 

 normal. For concentrations less than about 1.5 normal Beer's Law 

 seems to hold very accurately indeed, with the exception of the 



