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THREE-DIMENSIONAL MOTION PICTURES 133 residual lack of color correction for the intervening wavelengths is called the secondary spectrum of the lens, and in the useful spectral interval reaches a maximum at a certain wavelength. This color error of the astro- photographic objective is for the center of the field the worst error that it has; and however small other errors may be, they will be hidden in the color diffusion circle. The latter, therefore, even if somewhat better tolerated, furnishes a criterion for the measurement of other errors. It is hardly necessary to mention that objectives comprised of thin unspaced glasses* as in telescopes, show considerable astigmatism and curvatur of the image fields. By separation of the elements, curvature of the field and coma can be eliminated. In motion picture projection, the size of the field to be projected remains constant regardless of the focal length of the lens; hence the designers of projection lenses have been under no obligation to make all focal lengths geometrically similar, such as is generally done in the case of ordinary photographic lenses where plate size is proportional to focal length. Likewise, the focal surface characteristics of certain motion picture cam era lenses vary with the focal length. The Taylor type of objective, with three spaced elements and somewhat greater curvatures of the glass surfaces, can be designed with appreciably smaller dif fusion • circles due to astigmatism, but the secondary spectrum increases. The center of a sharp motion picture negative or positive will show distinctly letters formed by lines 0.01 to 0.02 millimeter wide. Letters that are 0.1 millimeter high on the film may be nearly illegible without one's noticing loss of detail when the picture is projected. In a theater with a 100-foot throw and a 25-foot picture,