SPECIFIC EXCITING POWER OF DIFFERENT WAVE-EENGTHS. 



191 



In the case of eosin there is some indication of a secondary maximum in 

 the neighborhood of 0.49 n- The values of a for the two solutions and the 

 ratios for these two values are given in Table 26. It will be noticed that 

 the ratio is not nearly so constant as in the case of resorufin. There is 

 perhaps some slight indication that the secondary band at 0.49 /u changes 

 with the concentration at a different rate from the principal band, but it 

 is doubtful whether the results are sufficiently accurate to give any certainty 

 to such a conclusion. The small values of the ratio on the red side of the 

 band imply a slight shift toward the red in the case of the dilute solution, 

 but the region in which these small values in the ratio occur is the region 

 where the results are most liable to error. 



In each case we have shown by the curve F the distribution of energy 

 in the fluorescence spectrum of the substance in question. The resemblance 

 is noticeable between this curve of energy distribution and the absorption 

 curve. Roughly speaking the one curve is the image of the other. In the 



Table 26. 

 Coefficients of absorption of eosin. 



absorption curve the side toward the red is steep, while the absorption dies 

 away gradually toward the violet. In the fluorescence curve the intensity 

 dies away gradually toward the red and stops abruptly on the violet side. 

 When the mechanism of fluorescence is more fully understood the reason 

 for this peculiar relationship between the two curves, which seems to be 

 a characteristic of the fluorescent substances of this class, will doubtless 

 be clear. 



COMPUTATION OF SPECIFIC EXCITING POWER. 



In the determination of the specific exciting power for eosin the solution 

 used was the same as that employed in determining the absorption curve, 

 for the more concentrated of the two solutions, shown in Fig. 182. The 

 value of a for each wave-length could therefore be read off at once from 

 that curve. 



In the case of resorufin a different solution was used. Since, however, 

 the form of the absorption curve is the same for all concentrations, it was 

 sufficient to determine the absorption for one particular wave-length and 

 then to reduce the values read from the curve for that substance (Fig. 183) 

 in a constant ratio. 



