18 



STUDIES IN LUMINESCENCE. 



prism, to explore the spectrum of this solution. The results of his measure- 

 ments are plotted in Fig. 16. His observations extend from 0.61 ^ to 1 .45 /i, 

 at which wave-length the alcohol in which the chlorophyll was dissolved 

 becomes so nearly opaque as to prevent further readings. The absorption 

 band at 0.675 M i s clearly shown; also the band at the extreme edge of the 

 visible red 0.745 M- The latter, however, is of little intensity. In Mr. 

 Coblentz's measurements comparison was made between a cell filled with 

 the chlorophyll solution and the same cell when filled with alcohol in which 

 no chlorophyll had been dissolved. The curve therefore indicates the 

 effect of the chlorophyll and other dissolved matters upon the transparency 

 of the alcohol. It will be noted that between 0.8 /x and 0.9 /x the material 

 in solution has no absorbing power. At the latter wave-length absorption 

 again begins to show itself and increases steadily to 1.45 /x, where the trans- 

 mitting power of the solution is only 60 per cent as great as that of the 

 alcohol itself. 



Au 



.811 



1.2/( 



1.0 /i 



Fig. 16. 



Transmission of chlorophyll in the infra-red. (Measurements by W. W. Coblentz.) 



In determining the fluorescence curve A (Fig. 15), which was obtained 

 by using the green line of the mercury arc (0.546 /j.) for excitation, we were 

 able to trace the fluorescence to wave-length 0.624 M which lies well beyond 

 the ultra side of the absorption band. This had not been found possible in 

 the case of the fluorescence spectra previously described, partly on account 

 of the greater width of the bands and partly for the following reason. The 

 fluorescence spectrum of chlorophyll is doubtless traceable to an unusual 

 distance toward the violet because the maximum lies in the extreme red, 

 in a region of very low luminosity. Fluorescence in this region, to be 

 appreciable, must be of great intensity, and on the side toward the violet 

 the rapidly falling intensity is largely counterbalanced by increase in 

 luminosity. The same is true of the infra side of the fluorescence band 

 of substances like quinine sulphate; so that, although the curves are of 

 the same type as those described above fluorescein, etc. the fluorescence 



