. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. ole, I consider the bestresults are obtained by a combination of the two methods,adopting an angle of about 105° and truncating the prismsufficiently to have no surplus glass. An erecting prism should not be used unnecessarily, as itsadjustment takes time, and there is considerable loss oflight. 87. Optical Front.—For general use in optical and someother experiments, a special focussing front is desirable, ofconstruction some-what different fromthe ordinary one forexhibiting slides. Ihave fo
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. ole, I consider the bestresults are obtained by a combination of the two methods,adopting an angle of about 105° and truncating the prismsufficiently to have no surplus glass. An erecting prism should not be used unnecessarily, as itsadjustment takes time, and there is considerable loss oflight. 87. Optical Front.—For general use in optical and someother experiments, a special focussing front is desirable, ofconstruction some-what different fromthe ordinary one forexhibiting slides. Ihave found the ar-rangement shownin fig. 94 very con-venient, a b is atube or fitting which fits nicely into the lantern nozzle like theusual one for diagrams, b c is a tube 8 in. diameter whichscrews into a collar at b, and having a rectangular opening ecut through both sides to serve as a slide-stage. The openingshould be 1| inches by 2§ inches, so as to receive easily slidesover 1 inch thick, and 2^ inches wide. These slides are pressedto the back by a flange l, forced against the slide by a spiral. FiU. 94.—Optical Front 172 OPTICAL PROJECTION spring m, and worked by the hand through two studs workingin slots at the top and bottom of b c. Into another collar atc screws the jacket d of the focussing tube, which may eitherslide nicely, or have a rack and pinion e. The lens tube f isabout 2} inches diameter, and carries a combination power oftwo lenses, the back one a plano-convex 2 inches diameter,plane side towards the stage, and 5 or 6 inches focus (both areconvenient), and the front lens rather over \\ diameter, about8 inches focus. This lens h screws into a collar from whichprojects a nozzle, n, If external diameter. The exact diametersare, of course, not material. A rack and pinion has advantagesand disadvantages. The advantages are obvious. On theother hand, a sliding movement may be made very pleasant ifcloth-lined, has more range, and allows of easy withdrawal ofthe po
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