18 



STUDY OF ABSORPTION SPECTRA 



Owing to the large number of possible sources of error, it was regarded 

 as desirable to repeat the work bearing on the point under discussion. 

 This was done three times; and that the results obtained are very con- 

 cordant can be seen by comparing table 4 with table 3. The slit-width 

 used was 0.2 mm. The depths of layers were the same as in table 3, 

 and the concentration of the new solution was six-tenths normal. 



The results seem to justify the conclusion that the intensities of the 

 bands for both the dilute and concentrated solutions are practically the 

 same, and that Beer's law holds fairly well for solutions of neodymium 

 chloride. The bands in the case of the more dilute solution are broader 

 and only very slightly more intense than for the more concentrated. 



EFFECT OF SLIT-WIDTH ON THE ABSORPTION OF LIGHT BY WATER. 



It will be remembered that increasing the width of the slit greatly 

 changed the transmission of salts of neodymium. Since we were mak- 

 ing an extensive study of the water-bands, it was desired to know what 

 changes would take place in the transmission of water with increasing 

 slit-widths. The Ifj, water-band compared with the neodymium bands 

 is relatively broad, and the 1.25yu water-band is very broad, it being 

 difficult to find accurately a maximum of absorption. To study the 

 Ifj, water-band a depth of 20 mm. was employed, while 10 mm. sufficed 

 to study the 1.25^1 band of water. 



TABLE 5. Effect of slit-width. 



When the bands are as wide as the Ifj, and 1.25ju water-bands, it 

 will be seen from table 5 that slit-widths as great as 0.3 mm. can be 

 employed without causing any appreciable difference in the intensity 

 of the absorption. The slit should, however, be kept as narrow as 

 possible, and as we could easily make fairly accurate measurements in 

 this region with narrow slits, we determined to use in all of the work 

 described below a slit-width of 0.2 mm. 



