Cover Glass Thickness - Most transmitted light objectives are designed to image specimens that are covered by a cover glass (or cover slip). The thickness of these small glass plates is now standardized at 0.17 mm for most applications, although there is often some variation in thickness within a batch of coverslips. For this reason, some of the more advanced objectives have a correction collar adjustment of the internal lens elements to compensate for this variation. Abbreviations for the correction collar adjustment include Corr, w/Corr, and CR, although the presence of a movable, knurled collar and graduated scale is also an indicator of this feature.