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 



