SCIOPTICON MANUAL. 



17 



sequence of this want of coincidence between the foci 

 of the central and marginal rays the picture on the screen, 

 or ground-glass, will appear blurred and ill defined. 



We can conceive of a lens with a gradually lessening 

 degree of convexity towards the margin, causing the 

 foci to coincide, but lenses cannot well be ground in this 

 form. The crystalline lens in the eye is supposed to 

 cause the foci to coincide by an increase of density 

 towards its centre, but such an arrangement of matter 

 would be impracticable in art. Much is gained by re- 

 versing the lens, for spherical aberration is four times as 

 great when the parallel rays enter its plane surface, as 

 when they enter its convex surface. 



Much is gained by a combination of lenses so that the 

 refracting angle may be less in each. Were the mar- 

 ginal rays d d cut off by a stop, the aberration would be 

 less, as we can see by tracing them in the diagram, but 

 the illumination would also be less by so much. 



DISTORTION. 



When we focus with a single lens with a front stop 



C 



Fig. 7. 



sharply on a square, A (Fig. 7) the resulting picture will 

 not appear square, but barrel-shaped, as at B. When we 



