32 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 .o5 mm. in thickness may quite obliterate 
Fic. 21.—DisturBInc Errect oF THE Cover-cLass. (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 
