94 



W. L. Butler and N. I. Bishop 



actinic light. In the presence of monochromatic actinic light, 

 the intensity of fluorescence excited by the weak, chopped, 

 650-nm beam is given by the curve through the points. The in- 

 tensity of actinic light was adjusted at each wavelength to give 

 the same level of fluorescence as measured by the B.C. voltmeter. 

 This procedure was adopted to make sure that approximately the 

 same energy was absorbed at each wavelength. The intensity of 

 actinic light at 5^0 nm was $0 percent greater than that at 6^0 

 nm because less light was absorbed at 5^0 nm. This method does 

 not insure that precisely equal energy will be absorbed at all 

 actinic wavelengths because the fluorescence yield appears to 

 vary approximately 20 percent with wavelength across the visible 

 spectrum. Adjusting to a constant level of fluorescence is more 

 valid, however, than using a constant intensity of actinic light 

 which would make the red and blue maxima in Fig. 1 more pro- 

 nounced simply because those wavelengths were absorbed more 



140 



RELATIVE 

 FLUORESCENCE 120 

 YIELD 



100 



80- 



400 



500 600 

 WAVELENGTH -nm 



700 



Fig. 1. Relative fluorescence yield of green bean leaf in 

 the presence of actinic light vs. wavelength of 

 actinic light. Betails of measurement in text. 



