Phosphorescence in Calcium Sulpiride. 213 



as far as the eve can recognize colour of low intensity towards 

 the red. It became a matter of curiosity to know if any rays 

 lay beyond the violet : and for this purpose an Iceland-spar 

 prism and quartz lenses to the collimator and camera were 

 brought into requisition, together with extremely sensitive 

 photographic plates. Glass was spread with a layer of this sul- 

 phide and held together by paraffin as a substratum : the phos- 

 phorescence was excited by the electric light, and the tablet 

 held in front of the slit. The exposure lasted one minute, when 

 the phosphorescent tablet was again excited, and used as a 

 source of light as before. After forty such excitations the pho- 

 tographic plate was developed ; and a band (shown in II.; was 

 registered, which absolutely coincided with the band already 

 registered in the visible spectrum. There was no trace of any 

 radiation having wave-lengths in the ultra-violet. TThether 

 there is any radiation below the red is a moot point : but from 

 the gradually increasing brightness of the spectrum in the 

 yellow, it seems probable that there is such. 



The next point to ascertain was the part of the spectrum 

 which excited phosphorescence. A tablet similarly prepared 

 was exposed to the spectrum of the electric light, which 

 showed carbon-bands strongly just above H, and also in the 

 violet near G-. A sensitive photographic plate was placed 

 in contact with the tablet, and allowed to remain in contact 

 40 seconds, after which it was developed. Ill shows the 



H G P E 3 C A 



rJEgXWMl' 1 I'M 7 illlll 



locality of the spectrum by which phosphorescence was ex- 

 cited. This agreed absolutely with the visual observations. 

 The exact locality was fixed by the carbon-bands above alluded 

 to, and also by comparing it with a sensitive photographic 

 plate exposed in the ordinary manner. It will be noticed that 

 the ultra-violet rays do not seem to cause phosphorescence in 

 this case. 



It now remained to register those rays which destroy the 

 phosphorescence. This was effected in the following manner : — 

 A tablet was first excited, and after half a minute exposed to 



