A slide was a flat piece of glass used to position samples for study underneath a microscope. In 2266, Doctor Leonard McCoy examined several slides while trying to find a cure for the life prolongation project. He eventually discovered that he had failed to adjust something on the last slide he'd looked at and blamed it on his infection. Upon correcting his error, he found the disease they'd used. (TOS: "Miri" ) In 2374, Kathryn Janeway put a slide into The Doctor's microscope while investigating Seven of Nine's claims that Kovin hurt her. (VOY: "Retrospect")