In high power objective lens, the distance between outer lens and cover glass gets very small and the light from the specimen will reflect by total internal reflection. To avoid this and get larger NA for better resolution, special oil is used for the medium between cover glass and the lens. This oil is arranged so it will have the refractive index of 1.516 and this number is exactly the same as those of optical glass. That makes light not to reflect or bend in each contacting surface. You can see the ice in water because the refractive ratio is different between ice and water.
Immersion oil is an interesting material.
If you drop glass piece in cup of immersion oil, the glass will be completely invisible. There was 2 type of standard in microscope. But now, most of the microscope is standardized to 160mm mechanical body tube length. Direction of cover glass thickness to use From specimen to eye-piece is recognized as an optical system. So the cover glass is a part of optical system and the slide glass is not.