ON PHOSPHOROSCOPES. 



239 



by a disk below the phosphorescent disk, but on the same shaft. This disk 

 cut the field between two electromagnets M, one brush B on the rim, the 

 other, B', on the shaft delivering the current to the galvanometer. The con- 

 stancy of the magnetic field was maintained by examining the readings of an 

 ammeter in series with the electromagnets and storage cells, and precautions 

 were taken to eliminate thermal electromotive forces at the brushes. In the 

 use of this type of phosphoroscope, as well as the first type, readings of in- 



X 



L 



r 



^ 





FIG. 10. 



tensity are only comparable through a range of speeds for which saturation is 

 obtained. With the red variety of calcite, saturation was found to exist with 

 the iron spark 1 cm. from the disk for all speeds, which gave more than 0.02 

 second decay; hence measurements in which the time interval from the close 

 of excitation to observation was not greater than 0.02 second were rejected. 

 To use this instrument with greater precision it is necessary to take account 

 of the variations in the spark. For this purpose an auxiliary station, with 

 photometer P and lamp C 2 , was arranged (fig. 9), where simultaneous readings 

 of intensity of phosphorescence were taken while the chief observer, with aid 

 of the spectrophotometer H and lamp Ci, measured the intensity of the phos- 

 phorescence throughout the spectrum. 



