4 66 



SIZE OF SCREENS 



[CH. XII 



If the letters and numerals and other details on the slide are too 

 small to be seen by the normal eye when held 30 cm. (12 in.) away, 

 then they will not show clearly in the screen image with this objec- 

 tive at the back of the room, although they may be plainly visible 

 to those near the screen. 



As the lantern is frequently not quite at the extreme back of the 

 room, an objective of 25 cm. (10 in.) focus is more commonly used 

 than the one of 30 cm. (12 in.). It makes the image somewhat 

 larger, and for many people is more satisfactory. 



636. Objective to use when the lantern is not at the back of 

 the room. Regardless of the position of the lantern a screen image 

 must be large enough for all in the room to see the details as stated 

 above (633-634). 



FIG. 251. 



SIZES OF SCREENS NECESSARY FOR DIFFERENT 

 SCREEN DISTANCES. 



This shows that the same object and objective will give a screen image of a 

 size directly proportional to the screen distance. 



