GENERAL DISCUSSION OF FLUORESCENCE SPECTRA. 



13 



The presence of band 4 in both spectra may be due to the imperfect 

 separation of the two substances. That it is much stronger in spectra 

 obtained from mixtures showing green fluorescence than from those 

 the fluorescence of which is blue-violet would seem to warrant ascribing 

 it to the chrysogen component, a conclusion strengthened by the con- 

 sideration of the placing of the bands. The most probable positions 

 of the crests are given in table 3. The positions of the three bands 

 assigned to anthracene are from photographic measurements by Miss 

 McDowell; 1 those due to chrysogen are from spectrophotometric read- 

 ings made in 1910, 2 combined with more recent observations. 



TABLE 3. Wave-lengths and frequencies of the bands of commercial anthracene. 



It will be noted that the three bands in the blue- violet are members 

 of a series having a frequency interval of about 121; also that the 

 4 bands of greater wave-length form a series with a somewhat greater 

 interval, i. e., about 131. Band 4 is too near to band 5 to belong 

 to the anthracence series, but may, within the rather large errors 

 due to the breadth and vagueness of these bands, be regarded as one 

 of the chrysogen series. 



There are several criteria based on experimentally established facts 

 by which the homogeneity or heterogeneity of a fluorescence band or 

 complex may be determined. 



CRITERIA OF HOMOGENEITY. 



(1) The position and distribution of intensities in a homogeneous 

 band is independent of the mode of excitation. This was established 

 by various observations published several years ago, 3 and subsequent 

 experience strengthens our conviction that it is a general principle and 

 that shifts in position and change of form are to be regarded as indica- 

 tions of heterogeneity due to the presence of more than one lumines- 

 cent substance. 



(2) The distribution of intensities in a homogeneous band is such 

 that the curve has a single well-marked maximum. The slope toward 

 the violet is steeper than that toward the red, like that in the corre- 

 sponding curve of intensities of an incandescent black body. 



1 Miss L. S. McDowell, 1. c. 



2 Nichols, E. L., Proc. Am. Philos. Soc., XLIX, p. 277. 



3 See Nichols and Merritt, Studies in Luminescence, Carnegie [Inst. Wash. Pub. No. 152, pp. 

 24, 38, 144. 



