SPECIFIC EXCITING POWER OP DIFFERENT WAVE-LENGTHS. 1 89 



in quality and energy distribution between the Nernst glower and the 

 acetylene flame, but also eliminates any errors which might arise on account 

 of selective transmission or other causes anywhere in the apparatus. 



The center of the slit S' through which the fluorescence was absorbed 

 was 8.2 mm. above the bottom of the cell containing the solution. On 

 the average, therefore, the exciting light had passed through 8.2 mm. of 

 solution before reaching the region whose fluorescence was measured by 

 the spectrophotometer. The light available at this point to produce 

 excitation was therefore less than that falling upon the face of the cell in 

 the ratio of one to e~- 82a . It was therefore necessary to compute this 

 factor e~ ' S2a for each wave-length and for this purpose we determined with 

 considerable accuracy the coefficient of absorption of both eosin and reso- 

 rufin for different wave-lengths. 



COEFFICIENTS OF ABSORPTION. 



The measurements, which were extended throughout the absorption 

 band for both dilute and relatively concentrated solutions of both sub- 

 stances, were made by comparing the intensity of the light transmitted by 

 a cell containing pure alcohol with the transmission, for the same wave- 

 length, when the cell was filled with the solution in question. For the con- 

 centrated solution two cells were used, the thickness of the absorbing layer 

 being 1 cm. in one case and 3 cm. in the other. The dilute solutions were 

 contained in a cell which gave an absorbing layer 29.5 cm. thick. The 

 source of light for transmission was an acetylene flame. The comparison 

 standard was also an acetylene flame, which was mounted so as to slide upon 

 a track as previously described. 



The ratio of the intensity of the light transmitted by the solution to the 

 intensity of the light transmitted by the alcohol gave the percentage trans- 

 mission, from which, with the thickness of the cell, the coefficient of absorp- 

 tion, a, could be computed, a being defined by the expression 



I = he~ ax 



where x is the thickness. 



The results for eosin are shown in Fig. 182 and for resorufin in Fig. 183. 

 Curve I in each case gives the coefficient of absorption for the dilute solution 

 as a function of the wave-length, while curve II gives the coefficient of 

 absorption for the concentrated solution. In Fig. 182 the scale is fifty 

 times greater for curve I than for curve 77, and in Fig. 1 83 the scale for 

 curve I is ten times as large as that for curve II. 



It will be seen that the absorption curve has very nearly the same form 

 for the dilute and concentrated solutions. This is especially true in the 

 case of resorufin, where each little ripple on the curve for the dilute solution 

 is reproduced on the curve for the concentrated solution. In Table 25 

 will be found the values of a for the two solutions, together with the ratio 

 of the two values. It will be seen that the ratio remains nearly constant 

 throughout the whole spectrum. The variation from constancy is most 

 marked at the two ends of the spectrum where the liability to experimental 

 error is greatest. The results in the case of resorufin appear to indicate 

 that the form of the absorption curve is not altered by concentration 



