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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 pantomine one must see the details of the action and the facial expression 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 sometimes 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 remodeled store buildings. If the screen image is limited in size by any of these factors thus requiring a smaller picture than that having 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?