SCIOPTICON MANUAL. 23 



of an almost plano-concave flint, and a biconvex crown- 

 glass lens, separated by a ring. It is seen that some 

 marginal rays entering the lens A do not reach the lens 

 J5, because of the length of the tube F F, which effects 

 about the same result as the slight stop D D. 



The curvature of the field is somewhat exaggerated in 

 the diagram, to remind us that an aplanatic lens cannot 

 give a perfectly flat field without a stop. In this gene- 

 ral form all portrait lenses mostly coincide, differing in 

 regard to the focal length of the separate objectives A 

 and B, the distance and size of the same, the position 

 of the stops, &c. 



Considering Fig. 13 as representing a portrait ob- 

 jective, the pencils JS S S proceed from an object com- 

 paratively large and distant, forming a small image in 

 the camera. Considering the diagram as representing 

 a lantern objective, the order is reversed. A small 

 transparency is in place of the curved line, which in its 

 turn becomes the focus of incident rays, projecting upon 

 a screen a comparatively large and distant image where 

 the pencils S S S, if extended, meet in a focus of re- 

 fraction. B is called the back lens in either case, as it 

 is back next to the instrument to which the tube is 

 attached. 



