98 



one, less under correction is needed, so that if an 

 objective is corrected for a definite thickness of 

 cover, its correction will be disturbed if greater or 

 less thickness be used, and since resolving" power 

 depends, as first condition, upon the proper cor- 

 rection of the two aberrations, it will be entirely 

 lost if the variation from the normal thickness is 

 considerable. 



It will be well to bear this phase in mind, as it 

 has a great influence on the efficiency of the objec- 

 tive and will appear repeatedly in the instructions 

 to be given later on. 



There are several methods by which corrections 

 may be made. In objectives with fixed mountings, 

 such as are ordinarily used, in which the lenses 

 have fixed relations, which correct the spherical 

 aberration for a definite thickness of cover glass, 

 no change can be made in them and therefore 

 cover glass should be used, which is very close in 

 thickness to that which was used as a standard. 

 Or, correction can be made within narrow limits 

 by varying the tube length. 



For a thick cover the tube must be contracted; 

 for a thin cover the tube must be extended. 



Objectives are also constructed in which the dis- 

 tances between lenses may be varied and then are 

 called adjustable. 



For thick covers the lenses are brought together y 

 or the adjustment is closed. 



