3* 



ELEMENTS OF APPLIED MICROSCOPY: 



bation which must be allowed for. Reference to Fig. 

 21 will show in a general way how a cover-glass shifts 

 the rays coming from a point, F, and makes them arise 

 apparently from F' and F". In the making of achro- 

 matic objectives this effect of the cover-glass is taken into 

 account; but with each lens, standard results can only 

 be obtained with cover-glasses of a certain thickness. A 

 variation of .05 mm. in thickness may quite obliterate 



Fig. 21. — DiSTtTRBiNG Effect of the Cover-glass. (After Gage.) 



certain fine structures. Adjustable objectives are so made 

 that by turning a ring or collar the. distance between 

 their systems of lenses may be varied, being increased 

 for the thinner cover-glasses. With an ordinary unad- 

 justable objective one may select cover-glasses of the 

 thickness for which it is corrected, the dimension being 

 easily measured by some such apparatus, as is shown in 

 Fig. 22. Or the lens system may be adjusted for cover- 

 glasses other than the standard by altering the position of 

 the draw-tube, changes in the tube-length producing 

 changes similar to those which are provided for in the ad- 

 justable objective. The tube-length should be increased 

 for thin covers and decreased for those thicker than the 



