io6 



PRACTICAL MICROSCOPY. 



some distance from each side of this focal plane. If, how- 

 ever, the rays are not thus corrected, the outlines of edges 

 of the image will be thick and confused, and the glass is 

 said "to be wanting in definition." 



This fault may be shown diagrammatically in Fig. 95, 

 which is an under-corrected glass, the peripheral rays being 



FIG. 95. 



brought to a focus at ///, between the central focus F and 

 the lens itself. Lenses in their primitive state are very 

 much " under-corrected," and can only be employed when 

 it is possible to cut off the peripheral rays by a diaphragm. 

 An over-corrected lens is shown in Fig. 96, from which it 

 may be seen that the marginal rays are thrown further away 

 from the glass, being brought to a focus at /// respectively, 

 while F represents the focus of the central portion. 



FIG. 96. 



An objective free from both these defects is said to be 

 free from spherical aberration or aplanatic. 



These defects in cheap low-angle objectives are corrected 

 (if the term will apply) by the interposition of a diaphragm 

 behind the back lens cutting off the marginal rays. These 

 rays cannot then enter into the formation of the picture, 

 the result being a dark, not very well defining glass of 



