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numerous tables which are pubHshed on the subject. For a given theater, the focus is fixed by the conditions in that theater.
The diameter of the objective should be as large as can be obtained in order to utilize as much of the light passing through the aperture as is possible. When using the arc lamp as an illuminant, experience has shown that it is much easier to obtain an even screen illumination free from shadows and dark corners with a large than with a small objective. With the incandescent lamp the entire area of the objective is utilized to image each point of the picture, hence a large objective will give a more brilhantly illuminated screen than will a small one. See Figs. lo and ii. Objectives are regularly manufactured with free diameter as great as 2^" and equivalent foci of 5.62'' or greater. The objectives studied in the preparation of this article had an equivalent focus of 5.62'^ One had a free diameter of 23^" and the other i^/s" , i-e., photographically speaking they had a soeed of f/2.3 and f/3.6 respectively. It is possible to make an objective of even greater speed than tnis and at least one has been made up on special order with a speed of as great as f/2.0 which worked successfully with motion pictures. The head of some machines will not take these large sized objectives. This is unfortunate as it imposes a limitation upon such machines which will be increasingly felt in the future. In general, the largest commercial objectives of $^4'^ focus or greater have a free diameter of 23^2" S-rid those of shorter focus have a smaller diameter, so that the ratio between diameter and focus remains about the same.
With the exceotion of Fig. 4, in which the diameter of the objective is drawn absurdly large in order to make clear the formation of the image of the condenser, the different optical parts are drawn correctly to scale, and the objective is drawn with 53^^' equivalent focus, 2y&^' free diameter corresponding to the photographs.
APERTURE
The aperture opening (f, g) shown in the drawings is the standard width of 29/3 2 '\
CONDENSER
In a given projection apparatus the dimensions of the objective and the aperture are fixed, and the condenser and light source are the elements which must be varied in order to secure efficiency. While the condenser to be described was developed to meet the needs of the incandescent lamp the optical reasoning applies equally well to any light source.
As illustrated in Fig. 6 the clearance between the surface of the condenser and the filament L must be sufficient to allow room for the bulb. The greatest angle of hght receivable by this type of condenser for efficient operation is 78° or 80°, and the reflector obviously must cover at least as great an angle as the condenser. The larger diameter reflector (R2) is preferred as it is farther away and does not get as hot as does the smaller one. For the largest
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