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THREE-DIMENSIONAL MOTION PICTURES 65 This would be the Ideal condition under which an object, occupying one plane perpendicular to the axis of the lens, would be reproduced In an Image plane. In reality many fac tors enter into play, which concur to diminish the trueness of rendition of such image. POSITIVE CONVERGENT LENSES Three distinct kinds of positive convergent lenses may be de fined according to the characteristic form of the lens. The most distinct characteristic of convergent lenses is that they are thicker at the axis than at their edges. Recalling the path followed by rays refracted in convergent lenses this char acteristic proves to be essential to provoke the convergence of the refracted rays. Lens a is called biconvex because both of its surfaces are con vex or bulging outwards. The radii of curvature of the lens illustrated in the figure are equal for both faces of the lens but it is quite evident that the surface of a biconvex lens may have radii of curvature varying in length. An infinite variety of such lenses may therefore be designed each one of which will have its proper power of convergence. Lens b is called a plano-convex lens because one of its faces is convex while the other is a plane perpendicular to the axis. The plane surface may be considered as a portion of a sphere having a radius of an infinite length while the other face may have a radius of any desired finite length. Lens c is called a convergent meniscus because of its cres cent-shaped form. Again the radii of curvature of the two faces may be equal to each other or vary in length at the will of the designer. Since the investigation of the path of the light rays through the different media is fundamental in the designing of optical instruments and, therefore, essential for the understanding of their functions, it has been found necessary and convenient to establish a notation with reference to the points, angles, sur-