398 Prof. E. Wiedemann on the 



green, after interpolating another glass which reduced the 

 brightness to ^. 



But now we see the comparison-lamp constantly, but the 

 phosphorescent light only during £ of a revolution. The 

 actual brightness of the latter is therefore four times as great 

 as the observed apparent brightness. For the yellowish- 

 green band, the brightness F of the fluorescent light, accord- 

 ing to the previous determination of/, denotes the above- 

 mentioned factor, and "const." a constant peculiar to the 

 photometer, 



-^ 4 x sin 2 16° x const. 



F= 7 • 



Since the disk makes 124 revolutions per second, and ^ of the 

 circle is covered by each opening, 8 = 2000 » further 6 = 2 x 10 3 , 

 whence it follows that /= 0*15, so that 



F = 2*0 const. 



The brightness of the amylacetate lamp is Y : 



Y=34x sin 2 45° x const. =17 const. 



Hence in the yellowish-green the brightness of the fluor- 

 escent light of our uranium plate under these conditions of 

 illumination is 



F _ const. 2 _ n>19 



Y const. 17 



times as great as that of the amylacetate lamp. 



In the spectrum of the phosphorescent light, the bands are 

 so distributed that they occupy about J of the total spectrum 

 between the Fraunhofer-lines C and F. They are, however, 

 not equally bright. If we take account of their difference in 

 brightness, we may assume with considerable probability that 

 they would occupy J of a spectrum that should have every- 

 where a brightness corresponding to that of the green line. 



According to this reasoning, £ x 0*12 = 0*015 of the energy 

 of the light of the amylacetate lamp is included in the phos- 

 phorescent light between C and F. 



We may further assume with sufficient exactness for our 

 purpose, that the distribution of brightness in the part of the 

 spectrum of the amylacetate lamp used and in that of the 

 glowing platinum wire is the same. For the latter calcula- 

 tions, exactly similar to the former ones of § 24, give for the 

 quantity of energy between C and F 0*016 of the total energy. 



The energy of the amylacetate lamp for a region within the 

 visible spectrum (p. 259) is 



E' = 0\L3E, 



