What is the purpose of a cover slip?

Cheman

What is the purpose of a cover slip? Why do we sometimes put one on a slide even though we are not going to keep that slide froever and will inevitably just throw it away? Why (eg - with temporary blood films) do when use oil instead? Why do we sometimes use coverslips instead of oil?

Moonbear

The coverslip protects both your specimen and the objective of the microscope from damage. Moisture will ruin the seal around the edge of the lens unless a lens is specially made to focus through water or oil. With a decent microscope, those lenses are incredibly expensive (keep in mind that even those "cheap" microscopes used in teaching labs cost about $5000 apiece, and a good quality microscope used for research or pathology will cost more than a luxury car). Many of the lenses are made with the refractive index of a coverslip factored into where their focal point is, so even if you just want to wet mount something, you still need a coverslip on it. It also keeps the specimen from drying out by preventing evaporation when exposed to the heat of the lamp on the scope.

So, there are lots of reasons for using coverslips other than permanently mounting something to last a long time.