C. STACY FRENCH 



57 



curves of this pure pigmeut illustrates two well-known facts. First, the 

 fluorescence spectrum looks somewhat like a mirror image of the ab- 

 sorption curve, at least two peaks in this case appear to be reflections 

 of two of the absorption peaks. Second, the major fluorescence peak 

 is universally located near an absorption peak, but at a longer wave- 

 length. The situation that can cause a great deal of trouble in fluores- 

 cence spectroscopy is the overlapping of the absorption and the 

 fluorescence bands. Here we see that the left-hand part of the fluores- 

 cence curve can be very strongly absorbed by the pigment itself 





600 650 700 750 m;i. 



WAVELENGTH 



Fig. 6. Fluorescence spectra in ether of pure chlorophylls a and h of Smith and 

 Benitez compared with the recalculated curves of Zscheile and Harris ( 1943 ) . 



whereas the right-hand part is outside of the absorption region of the 

 pigment. This means that except in extremely dilute solutions the 

 observed curve is very likely to be greatly distorted by internal reab- 

 sorption of the fluorescent light. This difficulty can be avoided, at 

 least in solutions, by using very low concentrations or thin layers. 

 Reabsorption within the sample can be the cause of a great deal of 

 trouble in interpreting the spectra of living organisms which have a 

 high pigment content. 



Chlorophylls a and h 



Since Dr. James H. C. Smith and Mr. Allen Benitez have recently 



