High magnification non-immersion objectives are designed to be used with air between the front lens and the cover glass.  Most objectives are designed to used with a standard cover glass thickness of 0.17 millimeters (a number 1.5 cover glass).  When air objectives with high numerical apertures (0.8 or greater) are used, variations of just a few micometers in cover glass thickness can cause image degradation.  Abberations get worse as the cover glass thickness increases.  A correction collar is used to compensate for these errors.  The collar allows for adjustment of the positioning of the central lens group within the objective.  This correction is also used with objectives intended to be used with water or glycerin immersion media.