Handbook of projection for theatre managers and motion picture projectionists ([1922])

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MANAGERS AND PROJECTIONISTS 211 procedure, "but it will be negligible as compared to the error due to spherical aberration. The same plan applies to a plano-convex condenser combination. There are in fact two distinct planes from which measurement should be made, but a single plane located at a Figure 64. position between the two real planes which will vary with the radius of the convex surfaces of the lenses, will serve. This plane will be nearest the surface having shortest radius. Very unscientific, yes, but it is practical, while the scientific method of two measurements is not practical, unless spherical aberration be taken into account, a thing we have yet to see any of the scientific men even attempt to do. There are 2 rules in optics to which attention is directed, viz.: (A) When the object and image are both located at a distance from the lens plane equal to twice its E. F. they will be icqual in size. (B) The relative size of object and image are in proportion to their respective distances from the lens plane. It may be assumed that most opticians are familiar with the wording of these rules, but how many of them have reasoned out what the rules really mean? How many of them realize that rule A means that, no matter where the object and image may actually be, there are always two planes, located a distance equal to 2 times the E. F. of the combination (or the focal length if it be a single lens) on either side of the lens plane, and that these planes are exact duplicates of each other, except that one is inverted. Yet if rule A is correct, this must be true. By this we mean that if a ray passes through or reaches a certain point in the image plane (see Fig. 62) after refraction, its incident