INTIMATE STRUCTURE ON COOLING. 



81 



ing components of the absorption band with fluorescence eliminated. 

 In this photograph each component of the fluorescence has its exact 

 reversal in absorption, with reciprocal relations as to intensity indi- 

 cated in figure 68. The weaker component of fluorescence is coin- 

 cident with the stronger component of the absorption doublet, and 

 vice versa. 



In the reversing region fluorescence and absorption are mutually 

 destructive. Consequently one or both are sometimes invisible; but 

 knowing the intervals, we can locate the reversal. By proper screening 

 the fluorescence may be prevented and the absorption band brought 

 out; and by taking extra precautions to secure a dark background and 

 to increase the excitation the fluorescence may be seen. Thus the com- 

 putation may be confirmed. 



In the study of the double chlorides the matter is further confused 

 because the difference between the fluorescence interval (83+) and 

 that of the absorption interval (70+) is approximately equal to the 

 distance between neighboring bands in the fluorescence groups. An 

 absorption series which comes into coincidence with band C, group 7, 

 will therefore nearly coincide with band B, group 8, etc. Furthermore, 

 the degree of resolution in the absorption spectrum, as has already been 

 mentioned, is often greater than in the fluorescence spectrum, and 

 certain series are observable of which the corresponding fluorescence 

 bands can not be identified. 



