correcting the spherical aberration for a definite thick- 

 ness of cover glass, will not permit of any change to 

 correct for other thicknesses. With these, 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, however, also constructed with a 

 mechanical arrangement by which the distances between 

 the lenses may be varied and are then called adjustable. 



For thick covers the lenses are brought together, 

 or the adjustment is closed. 



For thin covers the lenses are separated, or the 

 adjustment is opened. 



In order therefore to obtain the highest efficiency 

 of an objective, a cover glass should be used which has 

 the same thickness as the standard used by the maker 

 of the objective. Covers of standard thickness may be 

 purchased, but the cost will be considerably higher 

 than for the thicknesses regularly listed. 



The best plan is to purchase a cover glass guage 

 by means of which the thickness may be accurately 

 measured. Under this plan we recommend the follow- 

 ing procedure : 



Select standard thickness and use with high and 

 medium power dry objectives. Thicker covers use 



93 



