I9I0.] ON PHOSPHORESCENCE AND FLUORESCENCE. 



273 



observing tube in the side of the box already described. Within 

 this sight tube and in the focus of the eyepiece e a disk of thin 

 plane glass was mounted in the middle of which was fastened, with 

 Canada balsam, a very small rectangular prism. Opposite this a 

 a side tube t was inserted, the outer end of which was covered 

 with a screen of ground glass g. When the screen was illuminated 

 from without an observer at the eyepiece saw a rectangular patch of 

 light — the face of the reflecting prism — surrounded by the field of 

 light due to the phosphorescent surface. The brightness of this 



/Oset.. 



Fig. 5. 



Jioset. 



1^0 



20 



^Oiu. 



Fig. 6. 



patch was varied, in making measurements, by the movement of a 

 small frosted tungsten lamp (boxed in and viewed through a small 

 aperture) which travelled along a photometer bar in the direction 

 of the axis of the side tube. 



To determine the curve of decay with this apparatus the com- 

 parison lamp was set at a selected point on the photometer bar, the 

 shutter was opened for ten seconds to secure saturated exposure of 

 the specimen under observation, and at the instant when the decay- 

 ing phosphorescence had fallen to the degree of intensity which 



