14 SCIOPTICON MANUAL. 



emerging from it. As the result of these refractions 

 they meet at /. The converging power of lenses, of 

 uniform substance and density, is in proportion to their 



Fig. 4. 



degree of convexity. For a lens to sharpen the image, 

 the screen must be adjusted to the focal distance. 



STOPS. 



To get on the same plate something like distinct 

 images of objects at various distances, a " stop " is used 

 by the photographer, which, though it necessitates long 

 exposure, secures " depth of focus." This expedient of 

 having a small aperture is also resorted to for lessening 

 the defects or aberrations of lenses, just as the aperture 

 b (Fig. 3) is made small to lessen the greater defect of 

 having no lens. 



Stops are not used in the Sciopticon objective, because 

 all portions of the picture-slide are in the same plane, 

 and because sharpness produced by stops is always at 

 the expense of light. 



CHAPTEE II. 



THE CORRECTIONS REQUIRED BT I.EXSES. 



The corrections required by lenses (as well as every- 

 thing photographic), is well set forth in Dr. Yogel's 

 Handbook of Photography. The use of such diagrams, 

 as are here appropriated, is kindly allowed by the Ameri- 



