Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

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12 OPTICAL PROJECTION made more convergent so as to do this. We thus learn that for a successful projection, especially of apparatus, the position of the condenser relative to both the light and the object, and to the focussing lens, is almost as important as that of the latter; and may have to be varied materially, according to any of these circumstances. All the parts of the optical system must be so arranged that (1) as many rays as possible may be made to pass through or fall upon the slide or object; and (2) also pass through the projecting lens. But it is further necessary that the rays should not be more converged than will cause them to pass through the lens. For lastly, we can hardly help seeing that the rays proceeding in fig. 7 from the slide s to the projecting lens L, must be considered in a double character. From the focussing or projecting point of view, they must be regarded as bundles of image-forming rays diverging from every point of the slide s to the surface of the lens L. On the other hand, from the illuminating point of view, we must regard them as converging bundles of rays proceeding from the condenser c to the lens L. It is natural to suppose that this dual character might lead to some want of sharpness or definition in the focussed image. In ordinary lantern arrangements, and in exhibiting slides, this is not sensibly the case, because any ray from each point in the slide, to any point on the lens, forms an image on the same spot of the screen; and thus the luminous rays are identical with the chief image-forming rays. But in movable projecting arrangements, the matter is of the greatest im- portance. Thus, the light might be so drawn back from the condensers as for all the rays to pass through the slide, and then actually converge and cross before entering the focussing lens. 1 All the light would then still pass through the lens, and would reach the screen in some form or other ; but such violent crossing would blurr the image and spoil the even 1 I have seen this mistake made repeatedly in physical demonstration, especially with the electric light.