PHOSPHORESCENT SCREEN 81 



only about iV mm. broad. A recess has 

 first been made in the board, just where the 

 slit lies, so that the latter is free on both sides. 

 The two plates being first brought within a 

 small distance of each other, some powdered 

 calcium sulphide is packed in between them, 

 after which they are pressed against each other, 

 and maintained with screws, which keep them 

 in contact with the board. The compressed 

 sulphide remains in the slit, and is exposed to 

 sunlight, after the excess has been removed. 

 An extremely narrow phosphorescent slit is 

 thus obtained. 



