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The light of the arc is gathered by the inner condenser (Fig. i) into a parallel bearn, which is then converged to a spot upon the film. The projector lens is focused upon the film and the light passing the film is then projected upon the screen. The condensing lenses and the projecting lens are independent in size and focal lengths from each other. The former are chosen to best correct the light rays and converge them into the most suitably shaped beam. The projection lens is chosen totally according to the throw and size of screen picture. To get a picture lo feet wide at 40 feet throw, we merely choose a projection lens having a focal length in inches in proportion to the width of the film picture (roughly i inch) as 40:10, that is, 4 inches focal length of lens in this example.
When the projection machine is used as a stereopticon for slides, etc., the projection lens must be of different focal length.
The same principle in calculation is used but now we focus the stereopticon lens on a slide 3 inches wide which is located close to the lamp. Since the same size screen picture is required in both cases (10 feet wide at 40 feet throw) the stereopticon lens must have a focal length in proportion to the width of the slide (3 inches) as 40:10 or 12 inches. (See Fig. 2.)
Fig. 3
The Lamp
The entire success of the projection is now up to the lantern and its operation.
The curves in Fig. 3 show the relationship between screen area and the candle power required at the arc, and the current necessary. It is interesting to note that of the total illumination available from the D. C. arc, but 10 percent, gets to the screen, while with A. C. only 5 percent, gets to the screen.
The maximum shown in the curves are no longer maximums but are really minimum values in the better show houses. One hundred am