64 ABSORPTION SPECTRA OP A NUMBER OF SALTS 



In studying the changes which might occur in any band, it is of course 

 necessary that all conditions be as nearly as possible the same. One of the 

 most important factors here is that of the width of the slits of the spectro- 

 scope. With those solutions whose absorption bands are broad and diffuse, 

 not having such well-defined edges as with the salts of neodymium and 

 praseodymium, this is not such a determining factor. Should the band be 

 very narrow say approaching that of the width of the slit necessary to be 

 used in order to secure reasonable deflections of the instrument it is seen 

 that any slight change in the slit will make a large difference in the amount 

 of light falling on the thermal-junction. 



Considering a concrete example, let us suppose that the slit-width is just 

 equal to that of the absorption band, under a given dispersion. If, now, the 

 band and the slit exactly coincide, it is evident that no light will be falling 

 upon the junction, this being indicated by zero deflection of the instrument. 

 If, on the other hand, the slit is slightly wider than the band, some light will 

 enter around the edges of the band ; and, though the narrow band may act- 

 ually have complete absorption at a given point, it would not be indicated by 

 the instrument, since some light is entering around the edges of the band. 



Denoting the deflection of the instrument for a cell of 2 mm. depth of a 

 solution of x concentration by A, and the same for 1 mm. of the same solution 

 by B, we get, by the differential method discussed above, the ratio A/B for 

 the intensity of the light transmitted by (2 1) or 1 mm. of the solution in 

 question. 



By a similar reasoning we get the ratio A'/B' for the value of the trans- 

 it 

 mission of a solution of concentration , using absorbing layers 21 mm. and 



1 mm., respectively. While such a method is theoretically and mathe- 

 matically correct for infinitely narrow slit-widths, and practically so for 

 bands which are wide in comparison with the necessary slit-widths, yet in 

 the case of very sharp, narrow neodymium bands it has been found not to 

 give comparable results. The reason for this is clearly seen in the light of 

 the facts discussed above. 



Let us consider the ratios A/B and A'/B'. In the first case we are deal- 

 ing with concentrated solutions, where the absorption bands are broad; 

 hence B is small, and, in case the slit- width is comparable with the width of 

 the absorption band, B will be very much smaller than B', since B' is only 



x 

 1 mm. of an concentration solution. In a word, B, which is 1 mm. of the 



more concentrated solution, has 20 times the number of absorbers as has an 



equal depth represented by B' , and a decrease in the denominator of the 



fraction means an increase in its value. 



While the ratios A/B and A'/B' give the transmissions for 1 mm. of a 



x 

 solution of concentration x, and 20 mm. of a solution of concentration 



respectively, provided the slits are narrow; yet in the visible part of the 



