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CH. XI] OPTICS OF MOVING PICTURES 409
current and alternating current instruments. (See Chap. XIII for using these instruments § 662-674).
OPTICS OF MOVING PICTURE PROJECTION
§ 576. For purposes of description the projection of the individual pictures of a film can be considered apart from the mechanism which moves the film.
The projection of the film picture has much in common with that of the ordinary lantern slide but it is somewhat more difficult.
A theoretical treatment of the proper method of lighting the film is found in § 825. Briefly stated it is this: Light from the arc is collected by the condenser so as to illuminate the film. This illumination must be very intense and at the same time must be evenly distributed over the entire area of the film. To secure this result with the ordinary large condensers (4^2 in. in diameter) requires the condenser to be quite a distance away from the film, the crater of the arc to be of considerable size, and the projection objective to be of fairly large diameter.
Fig. 228 shows the optical arrangement most commonly used. Light from the arc is collected by the condenser upon the film at s, passing through the transparent parts of the film, it is bent by the objective in such a way as to form a sharp image of the film s, upon the screen.
Only one picture of the film is seen at a time, the rest being carried in the magazines or covered with shields. The picture to be shown is just in front of the opening of the aperture plate. Optically we are concerned only with the aperture plate and the short section of film behind it. It is this short section of film which must be evenly illuminated and projected upon the screen.
Beyond the film is the objective (fig. 229). The objective should be of good quality as it is the objective which determines the sharpness of the screen picture. Moreover, the objective must not be of too small diameter, for if it is too small there is danger that the screen image will not be evenly lighted although the illumination of the film may be perfectly even. The focal length