240 



FLUORESCENCE OF THE URANYL SALTS. 



To study the early stages of the cathodo-phosphorescence of calcite, Nichols 

 and Howes 1 devised a vacuum phosphoroscope, outlined in figure 10. The 

 phosphorescent specimen was applied to the periphery of the disk P and 

 excited by means of the cathode discharge from K. The vacuum-tube V was 

 fitted to the ground plate N. The shaft was balanced in an iron tube 115 cm. 

 in length, the mercury rising from the iron reservoir C to the barometric 

 height. The shaft was driven by a pulley, cord, and variable-speed motor, 

 and the revolutions were recorded by the commutating device at the bottom 

 wired to the chronograph. Intensities of phosphorescence were measured 

 with the aid of lamp P, photometer bar S, and Lummer-Brodhun cube T. 



The third type of phosphoroscope, in which the specimen is excited for a 

 definite time and viewed at varying but definite times after excitation, in- 

 cludes the form used by Nichols and Merritt in their extensive studies 2 of the 

 luminescence of sidot blende. 



FIG. 11. 



The specimen was mounted diagonally in a box having two openings with 

 shutters, one to admit excitation, the other to allow the luminescence to be 

 viewed after excitation. The eye of the observer was thus protected from the 

 brilliant luminescence during excitation, but was able to view the phosphor- 

 escence with no fatigue when the shutter of the luminescence window opened. 

 The time when the phosphorescence intensity became equal to that of the 

 comparison field was recorded on the chronograph by means of a key in the 



1 Nichols, Howes, and Wilber, I. c. 



2 Nichols a"nd Merritt, Carnegie Inst. Wash. Pub. No. 152, p. 41. 



