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 

 n/t/tntnfic. This word (derived from a = privative, and TrXara'w, to 

 wander, 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 the 

 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. 



Fm. 23. Under-corrected system. 



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

 plano-concave is not sufficiently powerful to neutralise all the 

 >pherical 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 Hattened, 

 while that of the flint has been 

 deepened, which increases the cor- 

 rective power of the Hint, and thus 

 destroys the balance of the com 

 binal ion 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 focu.ssed at F. This is what is known as over-correction. 



FIG. 24. Over-corrected system. 



