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464 SIZE OF SCREENS CH. XII]
SIZE OF SCREENS AND SCREEN IMAGES
§ 633. The size of screen images which will give the best results in a given case can only be determined by trial. The size should be great enough so that the people sitting on the back seats can see all the details to be shown and still not so large that those sitting near the front will be repelled by the coarseness of the image.
As a result of experiments to determine the best size of screen picture for the average seat in a room the following general rules have been worked out so : —
§ 634. Size of the screen for lantern slides. — The screen image must be large enough so that details are visible to the most distant spectator. For example, in teaching work and in demonstrations at scientific meetings, etc., lantern slides often contain tables of figures and printed sentences. Naturally, the farthest sitter should be able to see the figures and to read the words easily.
This could not be done by those on the back seats if the letters were much smaller than six point. Of course, if the letters on the slide are as large as eight or ten point type (fig. 216), they can be read at a glance.
In long, narrow rooms the magnification necessary to enable the people on the back seats to see the details well will make everything gigantic for those sitting near the screen.
For a well atranged auditorium, if the letters and numerals on the slide are of the size of 6 point type, such as shown in this sentence, and the screen image is from one-fourth to one-, fifth as wide a1-, the distance from the farthest seat in the room to the screen, all in the audience should be able to read the print on the lantern slide with ease.
§ 635 Projection objectives necessary to give the proper screen image with the magic lantern. — If the lantern can be at the extreme rear of the room, and the image of the slide is to be onefourth or one-fifth as wide as the room is long, as stated above (§ 634), a projection objective of 30 cm. (12 in.) focus will give the desired screen image for a properly made lantern slide, no matter what the size of the room. This is because the 30 cm. objective gives an image on the screen, regardless of its distance, which will appear to the observer standing by the lantern, like the same lantern slide held 30 cm. (12 in.) in front of the observer's eyes. If the lantern slide is well made and properly proportioned all the