Slip covers are small, thin sheets of glass that are placed over a specimen on a microscope slide.
You may think you are missing some of the slip covers because they are packaged tightly together inside what appears to be a square of tissue paper. Gently open the tissue to remove the slip covers.
As one example, the Micro360 Dual Purpose Microscope (#44125) comes with four slip covers. They are all bundled together inside the unmarked white square on the top left portion of the slide case pictured below.
Updated 12/18/13