18 SCIOPTICON MANUAL. 



substitute a lens with the stop in the rear, the curves 

 will be reversed, as at C. This property is based on the 

 fact that the marginal rays of the field of view strike 

 the lens under a larger angle than the central rays, and 

 consequently suffer a greater refraction. 



Of the simple form Of lenses, the meniscus, with its 

 concave side to the object, shows it the least. But it is 

 best overcome by a combination of lenses with central 

 stops. 



CURVED FIELD. 



This error is not caused by spherical aberration, for 

 it occurs with all perfectly aplanatic lenses, but by the 

 curve of the image, as is shown by the arrow, Fig. 8. 



Fig. 8. 



"When the ground-glass is placed at a a, only the central 

 part will appear sharp ; when at b 5, only the points of 

 the arrow will appear well defined. This error is avoided 

 by combination of lenses with suitable curves, and by 

 stops. 



