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FOR MANAGERS AND OPERATORS 137
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14, and the photograph, Plate 17, is precisely identical in every way. As the light leaves the front end of the objective you will observe that rays of the two are not entirely intermingled but that the mingling can be traced clear through by the brighter light. This intermingling condition continues out to where the cone projected by the upper pinhole has passed down sufficiently to entirely leave the cone thrown from the lower pinhole, which latter is at the same time passing on its way to the screen. The action of these two cones of rays are typical of those passing through every pinpoint of the film picture.
Each individual point of the film acts as does the pin hole, and dn sending a cone of rays forward exactly like those shown at L, Plate 17, and, since each of the rays contained in each of these cones carries an image of the point of the film through which it passed, it follows that all these rays must be refocused at the screen; it also follows that the actual crossing is as shown in the photograph. I believe this photograph will be of vast interest to operators.
And now let us apply the theory of the "two sets of rays," and see how it works in practice. Operators have long been puzzled as to why the spot on the revolving shutter is round when the shutter is close to the lens, whereas a little further ahead, toward the screen, it becomes rectangular. The two-ray idea is the only theory that seems to account for this, and it is to some extent this fact which has convinced me of its absolutely correctness.
First fix the following firmly in your mind. There are two complete but entirely separate optical systems in the projection machine lens system, which are, in effect, combined into one. Remember that the optical action of the condenser and objective is precisely the same. In fact the condenser is a crude, extremely imperfect objective lens ; therefore, we have in effect two objective lenses joined together, and the object ds to so join these two systems together that there will be a maximum of illumination of the object to be projected, and the rays directed against and through this object by the first system must be so joined to the second system that there will be a minimum loss of light, and no opposition created by the refractive power of one system as against the refractive power of the other system. The second system (the objective) picks up the rays delivered at the aperture by the first system (the condenser) exactly as they are delivered by it, therefore when the distance of the crater from the rear condensing lens is so proportioned with relation to the distance