The sciopticon manual, explaining lantern projection in general, and the sciopticon apparatus in paricular (1877)

26 SCIOPTICON MANUAL. their radiations cross at a very small angle. The rays which meet and cross at each point in the picture j?, become a pencil of diverging rays to be focused on the screen by the objective. We conclude, therefore, that the more concentrated the light, the nearer in each pencil will the marginal rays coincide with the axial ray, and the less will the imperfections of lenses become manifest. THE CONDENSER. The condenser is formed of combined lenses, because the refracting angles would be too great in a single lens of sufficient diameter and short focal distance. The simplest arrangement is where two plano-convex lenses are combined, with their curved surfaces inwards, as in Fig. 14 ; or at p q in Fig. 15. The shorter the focus of the condenser, the shorter, with a given objective, must be its distance from the light; it will collect more light, but it will be in more danger of breakage from the heat. In the Sciopticon the space between the lens q (Fig. 15) and the front of the flame is only about two and a half inches, but the glass 6r, and the air between it and q rising up and out at A, makes it perfectly secure. The condensing lenses in the Sciopticon are usually each 4 inches in diameter; but a 4J inch front condensing lens, p, is used to advantage for slides larger than the standard size, and to show fully the corners of some of the ordinary square transparencies. THE REFLECTOR. The reflector r (Fig. 14) is so adjusted that the light is in the centre of concavity. Each ray is reflected back to its starting-point, and continues in line with