correction collar
<microscopy> An adjustment collar provided on some high-numerical aperture, microscope objective lenses. Rotation of the collar adjusts the height of certain lens elements in the objective lens to compensate for variations in coverslip thickness or immersion media. at high numerical apertures, even a small deviation of the coverslip thickness (by as little as a few micrometres in some cases), or refractive index of the immersion medium from the designated standard, can introduce significant aberrations.