CH. XII] 



BRIGHTNESS OF SCREENS 



465 



details should be plainly visible when the slide is 30 cm. in front of 

 the eyes, and therefore are plainly visible in the screen image as 

 far back as the lantern. 



FIG. 250. DISTRIBUTION OF BRIGHTNESS OF TRANSLUCENT AND REFLECT- 

 ING SCREENS WHEN SEEN FROM DIFFERENT DIRECTIONS 



Reflected Light from Magnesium Oxide and Mirror Screen. 

 Light Transmitted through Ground Glass. 



Note that the mirror screen when seen perpendicularly reflects 7 times 

 as much light as does magnesium oxide, and ground-glass 19 times as much. 



But this great brightness of the mirror screen and the ground-glass is 

 limited to a very narrow angle, while the white magnesium oxide reflects 

 nearly equally throughout the entire hemisphere. 



Brightness of MgO is taken as unity and the figures on the radius indicate 

 the number of times brighter the screen appears than this. 



