PHOSPHORESCENCE SPECTRA. 



53 



the curves of decay of phosphorescent substances. Where the excita- 

 tion is chiefly superficial, as in the case of some powders, the excitation 

 may be nearly of one intensity and the curve made up of well-defined 

 linear processes with sharp inflection-points. We have found this to 

 be the case in many instances. Where, on the other hand, fluorescence 

 is excited within the crystalline mass by rays that have suffered con- 

 siderable loss by absorption, etc., there will be a wide range of intensi- 

 ties of excitation and a curve results with distributed knees and linear 

 processes shortened and sometimes almost obliterated. We observed 

 this particularly where a clear crystal was mounted with faces per- 

 pendicular to the photometer and was excited from behind so that the 

 light emitted by the surface nearest the exciting source passed through 



TABLE 17. Variation of length of processes with excitation (phosphorescence of uranyl 



rubidium nitrate). 



the crystal and was partially absorbed. Excitation occurred within 

 the crystal in diminishing amount with increasing depth and the com- 

 posite phosphorescence reaching the eye under such conditions showed 

 this blending effect to a marked degree. The same crystal when 

 excited from in front gave a curve in which the angles between pro- 

 cesses were made more sharply defined. The effect in question is 

 probably a general one and may well account for the perplexing differ- 

 ences in the curves of decay obtained under slightly varying circum- 

 stances. Thus, one observer will obtain an angular curve, where 

 another, studying the same material, can detect no linear processes. 

 The same observer, indeed, in attempting to repeat his measurements, 

 will often find the above-mentioned change of type under conditions 

 which seem to be identical but in which the same relations as regards 

 superficial and internal excitation are not preserved. 



We found in the study of this effect a crystal one smooth face of which 

 gave the blended curve, while the opposite face, which was rough, gave 

 the angular curve, a change produced and reproducible by merely 

 rotating the crystal through 180. 



