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130 THREE-DIMENSIONAL MOTION PICTURES cover a standard frame. A second modification is obtained by placing a strong collective element in front of or behind suitably designed Taylor triplets. A third modification, semisymmetrical in type, is based on the old Rudolph Planar which, in turn, was developed after Alvan Clark's lens of 1889. Each half has a strong collective element in front of a compound dispersive element. Another type is derived from a symmetrical lens by introducing into each half a dispersive meniscus turned convex toward the diaphragm. Such lenses having large apertures have been widely used for amateur photog raphy, but they cover only a small field and show con siderable spherical aberration. Another class of objectives deserving mention are those triplets that have been redesigned to increase the opening to //3.5 or more, The quality and suitability of any of these lenses is in part dependent upon the curvatures of their focal sur faces, their spherical aberrations and their sine condition errors. The rate of deterioration of the images with angle is for many of these lenses such as to make the images un satisfactory when the angle is more than 10 or 12 degrees unless the focus is very short. The maximum field satis factorily covered by short focus wide aperture lenses is 22 degrees, (28 millimeters at a focal length of 35 milli meters), and then only with a certain loss of quality which is noticeable on projection if details are to be pictured. When the focal length is increased to 50 millimeters, the standard movietone frame has a horizontal semi- angular field of about 1H/2 degrees, which is about the limit to which most lenses still give sufficiently good marginal definition not to detract from the quality of