234 



FLUORESCENCE OF THE URANYL SALTS. 



Laborde, 1 which included a rotating pattern to hide the specimen from the 

 observer during excitation by the spark from an induction coil, and later 

 uncover the specimen. 



Wm. Crookes, 2 in his study of the cathode phosphorescence of yttria, noted 

 that the color at the beginning of decay was different from that observed after 

 the decay had continued for a short period, and accordingly devised a phos- 

 phoroscope to study this change. Figure 4 serves to show that it was of the 

 sectored-disk form and driven by cord and pulley. At a convenient distance 

 was located an induction coil whose primary circuit was alternately made and 

 broken by a commutator near the end of the revolving shaft C. The brushes 

 could be so shifted as to cause the excitation of the phosphorescent substance 

 P when an opaque sector passed between P and the eye. It was possible to 

 change the period of decay by changing the speed or by changing the phase of 

 the brushes relative to the edge of the sector. The phosphorescent substance 

 was mounted in a convenient form of cathode-ray tube and excited by the dis- 

 charge from the secondary of the coil. With the aid of the spectrometer, 

 Crookes discovered that different lines appeared in the spectrum of the 

 phosphorescent yttrium after 0.000875 second than at 0.0035 second. 



FIG. 4. 



Ph. Lenard 3 devised a phosphoroscope which differs from the preceding 

 forms, since no revolving disk is employed. To hide the specimen from view 

 during excitation he used the motion of a screen mounted on the plunger of a 

 Ruhmkorff mercury interrupter. The frequency of the interrupter was de- 

 termined by that of the spring fork on which it was balanced; hence change 

 in the period between excitation and observation was accomplished by chang- 

 ing forks. The discharge of a condenser in parallel with secondary circuit 

 of the coil was thus timed by the interruptions of the primary circuit to occur 

 while the vibrating screen was in front of the specimen. 



De Watteville 4 constructed a machine similar in principle to that of Laborde 

 and Crookes (see fig. 5). The specimen, together with the spark, was mounted 



1 Laborde, Comptes Rendus, vol. 68, p. 1576. 



2 Crookes, Proceedings of the Royal Society, vol. 42, p. 111. 



3 Ph. Lenard, Wiedmann's Annalen, 46, p. 637. 



4 De Watteville, Comptes Rendus, 142, p. 1078. 



1887. 



