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MANAGERS AND PROJECTIONISTS 127
optical terms, would be perpendicular to both surfaces of the lens, hence would pass straight through.
Let us also examine line C, which, too, will meet one surface of the lens, the one farthest from point F, exactly at right angles, because any line drawn from a point of curvature will be exactly perpendicular to the surface at the point it meets it. In Fig. 27 line D is intended to represent the surface of the lens exactly at the point line C meets it.
But, as a matter of fact, if line C be a ray of light it will not continue straight through as shown, but will be refracted by the first surface, because it meets the surface at an angle, and will follow the path indicated by the dotted line. Likewise line G would be perpendicular to the lens surface at E, if it were continued straight through. But it is refracted by the first surface, and refracted very much more than line C because it meets the glass at a much greater angle.
Neither line C or line G is a principal axis, because neither is perpendicular to the plane of the lens as represented by line B.
CONJUGATE FOCI.— Conjugate foci is a term having reference to two points, one being the distance of the optical center of the lens from a light source, or from an object, and the other the distance from the optical center of the lens to the point at which the rays from the light source or object are focused into an image. The conjugate foci points are shown in Fig. 30, in which object X (candle) is one point and the image, Y, the other. Altering the distance of the object from the lens automatically alters the distance of the image. If the candle (Fig. 30) be moved nearer the lens, then image Y will automatically be removed further away, and vice versa. In a projector optical train the conjugate foci points of the condenser are the light source and the image of it which is formed near the "spot," while the conjugate foci points of the projection lens are the film and the screen.
REFRACTION. — The action of lenses is based upon the following:
Light rays travel in straight lines in any transparent medium of even density, but are bent or refracted while passing from a medium of one density to a medium of another density, provided the rays enter the second medium at an angle to its surface.