CH. XII] SIZE OF SCREENS 469 



the actors could not be seen if they were only of the size of average 

 human beings. On the theater stage the action is made more 

 intelligible by the spoken words; but where there is only pan- 

 tomine one must see the details of the action and the facial expres- 

 sion to make the play fully intelligible. 



To enable those seated in the extreme rear seats to see the action 

 on the screen without getting the picture too large for those on the 

 front seats, the width of the picture should be between /^ to Ya of 

 the distance of the farthest seat to the screen. The width of 

 ^6 is on the whole the most satisfactory if the end of the room is 

 large enough to permit a screen of this size. 



639. The size of the screen limited by the room. It some- 

 times happens that the size of the screen which can be used is 

 limited by the size of the wall on which it can be placed. The size 

 of the screen may also be limited by the height of the ceiling above, 

 and the heads of the spectators below. This is true of some lecture 

 halls, and of many of the moving picture theaters which are re- 

 modeled store buildings. If the screen image is limited in size by 

 any of these factors thus requiring a smaller picture than that hav- 

 ing a width of ;^th the distance from the screen to the farthest seat 

 for the magic lantern or >6th the distance for a moving picture, 

 it is necessary to use an objective of longer focus accordingly. 



If the width is limited and one can use any height desired, the 

 calculation is made exactly as in the previous section. 



If the height is limited, then the calculation is made in the same 

 way except that the height of the object instead of its width is 

 taken; that is, for lantern slides the extreme opening of the mat is 

 taken as 7 cm. (2^4 in.), or for moving pictures 23.08 mm. long, 

 17.3 mm. high, 2 %2 in. X 87 /i2s in. (see 57oa). 



For example, in a university lecture room the greatest height of 

 the screen which could be used was 2.9 meters (9.5 ft.), and the 

 room was 14.3 meters (47 ft.) long. The question was: What 

 focus of objective would give this size of screen image with the 

 lantern at the back of the room? 



