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

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THE PARTS OF A LANTERN 29 These photographic lenses have however the advantage, that very often by unscrewing the double lens at the back, and substituting for it the front lens reversed, or with its convex side to the slide, a very good long-focus single lens is obtained of the (a) kind. The best lantern-makers have lately, taking this photo- graphic lens as a basis, worked out by screen tests improved curves for lantern use only. Such are sent out in the highest class lanterns. These lenses reverse the two single lenses F, c, of the Petzval system (fig. 18), need no stop whatever, and give FIG. 18—Double Combination magnificent definition ; but, as a rule, they are not corrected for photography, and the single lens can rarely be used satis- factorily. It only remains to combine these points with the other excellent qualities already attained ; and there is little doubt that the growing use of enlarging lanterns, or adapta- tion of the lantern to photography direct, with the aid of the new optical glasses now made in Germany, will enable this crown- ing perfection to be obtained, at least in all lenses of 6-inches focus and upwards. With lenses of 4^ inches focus, used chiefly for paraffin oil lanterns in small rooms, to exhibit through a transparent sheet, a perfect image of a slide 3 inches