20 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS 



tinct points, X and Y. If there were no cover-glass all the rays would 

 diverge from O, and then the objective would require to be perfectly 

 aplanatic. This word (derived from a = privative, and irAapaw, to 

 \vander, i.e. free from wandering or error) means, as used by opticians, 



FIG. 21. The effect produced- by a cover-glass on the corrections of an 

 object-glass. 



that all the rays passing through a lens system are brought to an identi- 

 cal conjugate focus, as shown in fig. 22. But as affected by the cover- 

 glass the marginal rays diverge, apparently, from a focus, nearer tJie 

 objective than the central rays ; therefore the objective, to meet this 

 condition, must be what is called under -corrected ; a condition pre- 

 sented in fig. 23, so as to focus both these points at once. Here the 



FIG. 22. Aplanatic system. 



FIG. 23. Under-corrected system. 



curvature of the surface of the crown lens being increased, the Hint 

 plano-concave is not sufficiently powerful to neutralise all the 

 spherical aberration of the crown. As a consequence the peripheral 

 rays are brought to a focus at F, while the central rays pass on to 

 F. This is what is meant by * under-correction ' in an object-glass. 



In fig. 24 the reverse condition 

 is presented, for the incident curve 

 of the crown lens has been flattened, 

 while that of the flint has been 

 deepened, which increases the cor- 

 rective power of the flint, and thus 

 destroys the balance of the com- 

 bination in other directions. The rays passing through the periphery 

 of the combination will be brought to a focus F', while the central 

 rays will be focussed at F. This is what is known as over -correction. 



FIG. 24. Over-corrected system. 



