40 



FLUORESCENCE OF THE URANYL SALTS. 



The uranyl salts, because of their remarkable spectra, afford an 

 unusual opportunity to establish the exact relation between the emis- 

 sion of light during excitation and at various times after excitation 

 has ceased, and it was for this purpose that the first experiments with 

 the new phosphoroscope were undertaken. 



The method, briefly outlined, is as follows : The substance, inclosed 

 in a flat tube of glass BA about 8 cm. long and 2 cm. wide, is viewed 

 through the rapidly revolving sectored disk of the synchrono-phos- 

 phoroscope. It is mounted vertically, with its axis at right angles to 

 the radius of the disk, as shown in figure 29. 



D 

 1 

 I 

 I 



< 



<-_-, H 

 I 



B 



FIQ. 29. 



FIG. 30. 



It is uniformly excited by zinc sparks 120 times a second while 

 hidden by the closed sectors and is visible for 1/240 of a second during 

 the passage of each of the intervening open sectors. 



A phosphorescent substance of slow decay appears under these cir- 

 cumstances to be equally bright from top to bottom, but if one of the 

 uranyl salts, such as the double uranyl-ammonio sulphate, which was 

 the substance selected for detailed study, be used, it appears a very 

 bright green at the bottom of the tube, shading off to bare visibility 

 at the top. 



The rate of decay of this substance and of the other uranyl salts 

 is so rapid that the upper end of the tube, which is seen at the intensity 

 which corresponds approximately to the instant 0.003 second after 

 excitation, has only a small fraction of the brightness of the lower end, 

 which is viewed about 0.0005 second after excitation. 



The particular salt mentioned above was selected because at low 

 temperatures its spectrum is unusually well resolved in groups of com- 

 plexes of narrow, line-like bands, making it possible to detect changes in 

 the individual components. 



To obtain simultaneous observations a pair of right-angled prisms 

 was mounted before the slit of a Hilger spectroscope, as shown in 

 figure 30. 



Light from the lower end of the tube A enters the lower half of the 

 slit. That from the upper end B, after two total reflections, enters 

 the upper half of the slit, and we have two spectra one above the other, 



