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MANAGERS AND PROJECTIONISTS 175
The main point in deciding upon the focal length of the condenser, is to secure a focal length which will establish the crater produced by the amperage to be used at the minimum practical distance from the collector lens, at the same time giving the smallest practical diameter of spot and a maximum distance from the center of the condenser to the aperture. (See page 162.)
The latter, however, is only of importance in cases where, due to the large diameter crater produced by high amperage, ;t is difficult to secure sufficient distance between the center of the condenser and the aperture to confine the divergence of the beam beyond the aperture within limits which will enable the objective to pick up the entire beam, and at the same time keep the spot at the minimum practical working diameter.
In considering the relation of the focal length of the condenser to the size of the spot, to the distance from the center of the condenser to the spot, to the size of the crater, and to the distance of the crater from the center of the condenser, let is be clearly understood that while the office of the condenser is to intercept as much as possible of the light emerging from the crater, and to concentrate it on the aperture, it after all, in so doing acts precisely the same as would a photographic lens. With a condenser of given focal length, and a crater of given diameter, the diameter of the spot will be as many times the diameter of the crater as the distance from the apex of the curved surface of the collector lens to the floor of the crater is contained times into the distance from the apex of the curved surface of the converging, (front) lens to the aperture, when the curved surfaces of the lenses are not to exceed l/16th of an inch apart. For instance: If the distance from the apex of the curved surface of the collector lens to the aperture is 16 inches, the distance from the apex of the curved surface of the collector lens from the floor of the crater 4 inches, and the crater is .5 of an inch in horizontal diameter, then the resultant spot will be 16 -= 4 = 4, and 4 x y2 inch = 2 inches in diameter. This applies equally for both plano-convex and meniscus bi-convex lenses, except that when figuring the meniscus bi-convex, instead of figuring from the curved surface to the floor of the crater we figure the distance from a point l/i of an inch in front of the convex face of the meniscus lens, and the other distance from the center of the bi-convex to the aperture.