16 



STUDY OF ABSORPTION SPECTRA 



the bands the transmission is greatest with the widest slit. As we 

 leave the bands the transmission is least with the widest slit. This is 

 to be expected, since the spectrum lines have their centers displaced 

 slightly towards the red with increasing slit-width. It will be readily 

 seen that the neodymium bands are exceedingly sharp and narrow. 

 Doubling the slit-width will, for some wave-lengths of light, change 

 the transmission 15 per cent or more. It is clear that the centers of 

 the bands do not become more intense with increasing slit-widths. 



TABLE 3. Water absorption. 



Indeed, just the opposite effect is noted. Table 2 also shows that, 

 aside from considerations of the purity of the spectrum, the slits of 

 the spectroscope we used should actually be of almost infinitesimal 

 width for lines and bands as sharp and narrow as those of neodymium. 

 If the slit was narrower than 0.2 mm., the resulting deflections would 

 be so small that the error in reading the scale would be considerable. 

 The effect of the width of the slit on the absorption bands of water 

 was also studied, and will be discussed later. 



WATER ABSORPTION. 



If we consider the water absorption we find that the neodymium 

 bands increase in intensity only slightly with dilution. Take, for 

 example, the band represented by head-reading 489; 2 mm. of solution 

 have less than 1 per cent absorption at this point, due to the water 

 present. It was found from the specific gravity and concentration of 

 the solution that 95.6 per cent by volume was water. Therefore, one 

 cell was filled with 2.87 mm. of water and the second with 0.95 mm. 

 of water. Each cell, then, had just as much water in it as it had when 

 filled with 3 and 1 mm., respectively, of the solution. In this way the 

 absolute transmission for a layer of water equal to that in the 2 mm. 

 of solution is determined. 



Considering now the more dilute solution, the cell which contained 

 24 mm. of solution actually contained 23.83 mm. of water, the percent- 

 age of water present being 99.3. The second cell contained 7.94 mm. 

 of water. By the differential method we thus find the transmission 

 for a layer of water equal to that contained in 16 mm. of the more 

 dilute solution. The particular region which we are considering is in 



