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THREE-DIMENSIONAL MOTION PICTURES 101 the 23 X 46 mm. picture area the field covered is prac tically 54 1 /2°- This is not by any means an unheard-of angle in lenses of relative aperture of //4.5 or even //3.5 but no photographic lens appeared to be avail able with sufficient speed and satisfactorily sharp defi nition to cover a picture size 23 X 46 mm. at the same time this size was first attempted. To cover the field with lenses of longer focal length is a task of less diffi culty, but here one guard against a deterioration of general definition due to residual aberrations which be comes the more noticeable the longer the focal length. After the pictures have been taken the problem of projection offers difficulties in illumination and in find ing a projection lens competent to project them with satisfactory definition. It is obvious that if the same amount of light which passes through the aperture of the film gate in an ordinary projector, be spread over a screen area twice as large the illumination of the screen image will be only half as great. If a pair of ordinary 4*/2 inch condensers and high intensity arc be employed in their usual adjustment it will be found impossible to illuminate an area 23 by 46 mm. The Illuminated area in the plane of the film is not large enough. The size of the illuminated area can be in creased, however, by reducing the distance from the arc to the condenser. An adjustment can be found in which the spot at the film gate will be large enough to circum scribe the 23 by 46 mm. rectangle. It is obvious that much light will be intercepted by the film gate, but still the illumination will be greater than we might expect as a result of comparison of screen image size. The old 4 1 /2"inch diameter condensers with the high intensity arc, however, did not exhaust the possibilities of the projection lens in respect to its angular aperture. One obvious means of increasing Illumination, there fore,, lay in employing condensers of larger converging